What's New In Python 3.8
************************

編集者:
   Raymond Hettinger

この記事では 3.7 と比較した Python 3.8 の新機能を解説します。全詳細に
ついては *changelog* をご覧ください。


概要 -- リリースハイライト
==========================


新しい機能
==========


代入式
------

大きな構文の一部として、変数に値を割り当てる新しい構文 ":=" が追加され
ました。 この構文は セイウチの目と牙 に似ているため、「セイウチ演算子
」の愛称で知られています。

以下の例では、代入式により "len()" 関数を二重に呼びだすことを回避して
います:

   if (n := len(a)) > 10:
       print(f"List is too long ({n} elements, expected <= 10)")

同様の利点が、マッチがしたかどうかの判定とサブグループの取得とで2回マ
ッチオブジェクトが必要になる、正規表現によるマッチでも、発揮されます:

   discount = 0.0
   if (mo := re.search(r'(\d+)% discount', advertisement)):
       discount = float(mo.group(1)) / 100.0

この演算子は、生成した値をループ終了の判定とループ本体の両方で使用する
ような while 文においても、有用です:

   # Loop over fixed length blocks
   while (block := f.read(256)) != '':
       process(block)

動機となったもう 1 つのユースケースは、リスト内包表記においてフィルタ
ー条件の中で計算した値が式の本体でも必要になるところから出てきます:

   [clean_name.title() for name in names
    if (clean_name := normalize('NFC', name)) in allowed_names]

セイウチ演算子の使用は、複雑さを減らしたり可読性を向上させる綺麗なケー
スに限るよう努めてください。

完全な説明は **PEP 572** を参照してください。

(Emily Morehouse の貢献による bpo-35224)


位置専用引数
------------

関数の引数が位置引数として指定されねばならずキーワード引数としては指定
できないことを表す、新しい構文 "/" を追加しました。この構文は、
"help()" で Larry Hasting の Argument Clinic でアノテートした C関数を
表示したときと同じ表記です。

以下の例では、引数 *a* と *b* が位置専用、 *c* と *d* が位置引数でもキ
ーワード引数でも可、*e* と *f* はキーワード引数専用です:

   def f(a, b, /, c, d, *, e, f):
       print(a, b, c, d, e, f)

以下は正しい関数呼び出しです:

   f(10, 20, 30, d=40, e=50, f=60)

一方、以下は不正です:

   f(10, b=20, c=30, d=40, e=50, f=60)   # b cannot be a keyword argument
   f(10, 20, 30, 40, 50, f=60)           # e must be a keyword argument

One use case for this notation is that it allows pure Python functions
to fully emulate behaviors of existing C coded functions.  For
example, the built-in "divmod()" function does not accept keyword
arguments:

   def divmod(a, b, /):
       "Emulate the built in divmod() function"
       return (a // b, a % b)

もう一つのユースケースは、引数名が有用ではない場合にキーワード引数とし
ての利用を排除することです。例えば組み込み関数 *len* は "len(obj, /)"
というシグニチャです。これにより、以下のようなぎこちない呼び出しが排除
されます:

   len(obj='hello')  # The "obj" keyword argument impairs readability

引数を位置専用とすることは、のちに引数名を変更する際に呼び出し側のコー
ドを壊してしまう心配がないという利点があります。例えば、"statistics"
モジュールでは、*dist* という引数名が将来変更される可能性があります。
これは、関数が以下のような定義だからこそ可能なことです:

   def quantiles(dist, /, *, n=4, method='exclusive')
       ...

Since the parameters to the left of "/" are not exposed as possible
keywords, the parameters names remain available for use in "**kwargs":

   >>> def f(a, b, /, **kwargs):
   ...     print(a, b, kwargs)
   ...
   >>> f(10, 20, a=1, b=2, c=3)         # a and b are used in two ways
   10 20 {'a': 1, 'b': 2, 'c': 3}

これにより、任意のキーワード引数を受け取る関数やメソッドを定義するのが
、ぐんとシンプルになります。例えば、これは "collections" モジュールか
ら抜き出したコードです：

   class Counter(dict):

       def __init__(self, iterable=None, /, **kwds):
           # Note "iterable" is a possible keyword argument

詳細は **PEP 570** を参照してください。

(Pablo Galindo の貢献による bpo-36540。)


コンパイル済みのバイトコードファイルのキャッシュを併存させるファイルシステム
----------------------------------------------------------------------------

暗黙のバイトコードキャッシュは、デフォルトでは各ソースコードのディレク
トリ内の "__pycache__" サブディレクトリを使用しますが、新たに追加され
た環境変数 "PYTHONPYCACHEPREFIX" （またはコマンドラインオプション "-X"
"pycache_prefix") を指定すると、ソースコードと分離されたディレクトリツ
リーに格納されるようになります。

キャッシュの場所は "sys.pycache_prefix" で参照できます（"__pycache__"
を使用する場合は "None" になります）。

（Carl Meyer の貢献による bpo-33499）


デバッグビルドとリリースビルドのABI 共通化
------------------------------------------

Python はリリースモード、デバッグモード両方のビルドにおいて、同じABI
を用いるようになりました。Unix において、Python がデバッグモードでビル
ドされている場合、リリースモードで、あるいは安定ABI を用いて、ビルドさ
れたC拡張モジュールを読み込むことができます。

リリースビルドとデバッグビルドはABI 互換になりました。"Py_DEBUG" マク
ロを定義することは、ABI の非互換性を引き起こす、"Py_TRACE_REFS" マクロ
の定義を伴いません。"sys.getobjects()" 関数の定義や "PYTHONDUMPREFS"
環境変数を加えるような "Py_TRACE_REFS" マクロは、新しいビルドオプショ
ン "./configure --with-trace-refs" を有効にすることで利用できます。
(Victor Stinner の貢献による bpo-36465)

Unix  においては、Android とCygwin  を除き、C拡張モジュールはlibpython
にリンクされなくなりました。静的リンクされたPython は、Python 共有ライ
ブラリを用いてビルドされたC拡張モジュールを読み込めます。(Victor
Stinner の貢献による bpo-21536.)

Unix において、Python がデバッグモードでビルドされている場合、インポー
ト("import") は、リリースモードで、あるいは安定ABI を用いて、ビルドさ
れたC拡張モジュールを探すようになりました。(Victor Stinner の貢献によ
る bpo-36722.)

Python をアプリケーション内に組み込むためには、新しい "--embed" オプシ
ョンを有効にして "python3-config --libs --embed" とすると、
"-lpython3.8" を有効にできます。(アプリケーションがlibpython にリンク
されます) Python3.8 以前との互換性を保つためには、 "python3-config
--libs --embed" をまず試みてから、失敗した場合 "python3-config --libs"
("--embed" なし) にフォールバックしてください。

Python をアプリケーションに組み込むために、"python-3.8-embed" pkg-
config モジュールが追加されました: "pkg-config python-3.8-embed
--libs" の出力は、"-lpython3.8" を含みます。 Python3.8 以前との互換性
を保つためには、"pkg-config python-X.Y-embed --libs" をまず試みてから
、失敗した場合 "pkg-config python-X.Y --libs" ("--embed" なし) にフォ
ールバックしてください。("X.Y" を使用するPython のバージョンに置き換え
てください)

一方で、"pkg-config python3.8 --libs" は出力に "-lpython3.8" を含まな
くなりました。C拡張モジュールは、libpython にリンクされるべきではあり
ません。 (スクリプトによって処理される、Android とCygwin を除く) この
変更では、意図的に後方互換性をなくしています。(Victor Stinner の貢献に
よる bpo-36721)


*f-string* での "=" を用いたオブジェクトの表現
----------------------------------------------

フォーマット済み文字列リテラル(*f-string*) において "=" 指定子が有効に
なりました。"f'{expr=}'" のようなf-string は、f-string に書かれた式、
イコール記号、式を評価されたものの文字列表現、の順で表示されます。例え
ば:

>>> user = 'eric_idle'
>>> member_since = date(1975, 7, 31)
>>> f'{user=} {member_since=}'
"user='eric_idle' member_since=datetime.date(1975, 7, 31)"

通常の f-string 書式指定子 を用いると、より柔軟に式の結果の表示方法を
変えられます:

   >>> delta = date.today() - member_since
   >>> f'{user=!s}  {delta.days=:,d}'
   'user=eric_idle  delta.days=16,075'

"=" 指定子は式すべてを表示するので、次のような計算も表示できます:

   >>> print(f'{theta=}  {cos(radians(theta))=:.3f}')
   theta=30  cos(radians(theta))=0.866

(Eric V. Smith, Larry Hastings の貢献による bpo-36817)


PEP 578: Pythonランタイムの監査フック
-------------------------------------

このPEP は、監査用フックと信頼された Open のためのフックを追加します。
これらは Python およびネイティブコードから利用できます。純粋に Python
で書かれたフレームワークやアプリケーションはコードに関する追加情報を得
ることができ、システム管理者や埋め込みでの利用者は Python のビルドを常
に監査が有効な状態で利用できます。

詳細は **PEP 578** を参照してください。


PEP 587: Python 初期化設定
--------------------------

**PEP 587** による C API  の改善により、Python 初期化時に、すべての初
期化設定に対するより良い制御と改善されたエラー報告が得られます。

新たな構造体:

* "PyConfig"

* "PyPreConfig"

* "PyStatus"

* "PyWideStringList"

新たな関数:

* "PyConfig_Clear()"

* "PyConfig_InitIsolatedConfig()"

* "PyConfig_InitPythonConfig()"

* "PyConfig_Read()"

* "PyConfig_SetArgv()"

* "PyConfig_SetBytesArgv()"

* "PyConfig_SetBytesString()"

* "PyConfig_SetString()"

* "PyPreConfig_InitIsolatedConfig()"

* "PyPreConfig_InitPythonConfig()"

* "PyStatus_Error()"

* "PyStatus_Exception()"

* "PyStatus_Exit()"

* "PyStatus_IsError()"

* "PyStatus_IsExit()"

* "PyStatus_NoMemory()"

* "PyStatus_Ok()"

* "PyWideStringList_Append()"

* "PyWideStringList_Insert()"

* "Py_BytesMain()"

* "Py_ExitStatusException()"

* "Py_InitializeFromConfig()"

* "Py_PreInitialize()"

* "Py_PreInitializeFromArgs()"

* "Py_PreInitializeFromBytesArgs()"

* "Py_RunMain()"

このPEP は、"_PyRuntimeState.preconfig" ("PyPreConfig" 型) と
"PyInterpreterState.config" ("PyConfig" 型) を内部構造体に加えます。
"PyInterpreterState.config" は新たな参照設定となり、グローバル設定や他
の内部変数を置き換えます。

Python 初期化設定 も参照してください。

詳細は **PEP 587** を参照してください。

(Victor Stinner の貢献による bpo-36763)


PEP 590: Vectorcall: a fast calling protocol for CPython
--------------------------------------------------------

The Vectorcall Protocol is added to the Python/C API. It is meant to
formalize existing optimizations which were already done for various
classes. Any static type implementing a callable can use this
protocol.

これは現在、暫定版です。Python3.9 から公開で使われることを目ざしていま
す。

詳細は **PEP 590** を参照してください。

(Contributed by Jeroen Demeyer, Mark Shannon and Petr Viktorin in
bpo-36974.)


Pickle プロトコルバージョン 5 による帯域外データバッファ
--------------------------------------------------------

"pickle" モジュールが Python のプロセス間での大容量データ転送でマルチ
コアないしマルチマシンでの利点を得るためには、メモリコピーの削減のほか
、データ種別に応じた圧縮などの個別に合わせた最適化が重要です。

"pickle" プロトコルバージョン5 は、**PEP 3118** 互換であるようなデータ
が、メインのpickle ストリームとは別の独立した通信レイヤで転送されるよ
うな、帯域外バッファを提供します。

詳細は **PEP 574** を参照してください。

(Antoine Pitrou の貢献による bpo-36785)


その他の言語変更
================

* 実装上の問題により、"finally" 節での "continue" 文は不正でした。
  Python3.8 では、この制限は取り除かれています。 (Serhiy Storchaka の
  貢献による bpo-32489)

* "bool", "int", "fractions.Fraction" タイプは、"float" や
  "decimal.Decimal" と同様の "as_integer_ratio()" メソッドを持つように
  なりました。このAPI の拡張によって、"numerator, denominator =
  x.as_integer_ratio()" のようなコードがより多くの数値型で動作します。
  (Lisa Roach の貢献による bpo-33073 および Raymond Hettinger の貢献に
  よる bpo-37819)

* "int", "float", "complex" のコンストラクタは、可能ならば
  "__index__()" 特殊メソッドを利用し、その場合は対応する "__int__()",
  "__float__()" や "__complex__()" は使用できません。(Serhiy Storchaka
  の貢献による bpo-20092)

* "regular expressions" での "\N{name}" エスケープの追加:

     >>> notice = 'Copyright © 2019'
     >>> copyright_year_pattern = re.compile(r'\N{copyright sign}\s*(\d{4})')
     >>> int(copyright_year_pattern.search(notice).group(1))
     2019

  (Jonathan Eunice, Serhiy Storchaka の貢献による bpo-30688)

* dict と dictview は、"reversed()" を使って逆順でイテレートすることが
  可能になりました。(Rémi Lapeyre の貢献による bpo-33462)

* 関数内でのキーワード名に関する構文の制限が強化されました。具体的には
  、"f((keyword)=arg)" の表現が許容されなくなりました。キーワード引数
  への代入の際、左側にキーワードそのまま以外の表現を許可する意図は、以
  前からありませんでした。 (Benjamin Peterson の貢献による bpo-34641)

* "yield" 文と "return" 文における一般化されたイテラブルへの展開のため
  に、全体を覆うかっこが不要になりました。これは *yield* と *return*
  の表現を、一般の展開と同様のわかりやすい形式にします:

     >>> def parse(family):
             lastname, *members = family.split()
             return lastname.upper(), *members

     >>> parse('simpsons homer marge bart lisa maggie')
     ('SIMPSONS', 'homer', 'marge', 'bart', 'lisa', 'maggie')

  (David Cuthbert, Jordan Chapman の貢献による bpo-32117)

* "[(10, 20) (30, 40)]" のようなコードでコンマが無い場合、コンパイラは
  "SyntaxWarning" を、分かりやすい形式で表示します。これで、最初のタプ
  ルが呼び出し可能ではないことを "TypeError" で表示するよりもわかりや
  すくなります。(Serhiy Storchaka の貢献による bpo-15248)

* "datetime.date", "datetime.datetime", "datetime.timedelta" オブジェ
  クトのサブクラス間の算術演算は、基底クラスではなくサブクラスのインス
  タンスを返すようになりました。この変更は、"astimezone()" のような、
  "datetime.timedelta" に対する算術演算を直接ないし間接的に使用する実
  装の戻り値の型に影響します。(Paul Ganssle の貢献による bpo-32417)

* Python インタープリタがCtrl-C (SIGINT) によって中断され、
  "KeyboardInterrupt" 例外の結果が捕捉されなかった場合、Python プロセ
  スはSIGINT シグナルか、呼び出したプロセスが、Ctrl-C  によって終了し
  たことを判別できるような適切な終了コードで終了するようになりました。
  POSIX とWindows 上のシェルは、非対話セッション内でスクリプトの終了処
  理を適切に行うために、これを利用できます。(Gregory P. Smith を通して
  Google の貢献による bpo-1054041)

* 先進的なプログラミングにおいて、既存の関数の "types.CodeType" オブジ
  ェクトを更新する必要が生じます。code オブジェクトはイミュータブルな
  ので、既存のオブジェクトがあるにもかかわらず、新しい code オブジェク
  トを生成する必要がありました。パラメータが19もあるので、一から作成す
  るのは非合理的です。新しい "replace()" メソッドを使えば、更新すべき
  パラメータを書くだけで、新しいクローンを作成することができます。

  ここでは、"statistics.mean()" 関数がキーワード引数として *data* を使
  えないように変更する例を見てみましょう:

     >>> from statistics import mean
     >>> mean(data=[10, 20, 90])
     40
     >>> mean.__code__ = mean.__code__.replace(co_posonlyargcount=1)
     >>> mean(data=[10, 20, 90])
     Traceback (most recent call last):
       ...
     TypeError: mean() got some positional-only arguments passed as keyword arguments: 'data'

  (Victor Stinner の貢献による bpo-37032)

* For integers, the three-argument form of the "pow()" function now
  permits the exponent to be negative in the case where the base is
  relatively prime to the modulus. It then computes a modular inverse
  to the base when the exponent is "-1", and a suitable power of that
  inverse for other negative exponents.  For example, to compute the
  modular multiplicative inverse of 38 modulo 137, write:

     >>> pow(38, -1, 137)
     119
     >>> 119 * 38 % 137
     1

  Modular inverses arise in the solution of linear Diophantine
  equations. For example, to find integer solutions for "4258𝑥 + 147𝑦
  = 369", first rewrite as "4258𝑥 ≡ 369 (mod 147)" then solve:

  >>> x = 369 * pow(4258, -1, 147) % 147
  >>> y = (4258 * x - 369) // -147
  >>> 4258 * x + 147 * y
  369

  (Mark Dickinson の貢献による bpo-36027)

* 辞書内包表現は辞書リテラルと同期されたため、まずキーが解釈され、次に
  値が解釈されます:

     >>> # Dict comprehension
     >>> cast = {input('role? '): input('actor? ') for i in range(2)}
     role? King Arthur
     actor? Chapman
     role? Black Knight
     actor? Cleese

     >>> # Dict literal
     >>> cast = {input('role? '): input('actor? ')}
     role? Sir Robin
     actor? Eric Idle

  この実行順の保証は代入式の役に立ち、キーの表現で代入されたものは、値
  の表現で使用することができます:

     >>> names = ['Martin von Löwis', 'Łukasz Langa', 'Walter Dörwald']
     >>> {(n := normalize('NFC', name)).casefold() : n for name in names}
     {'martin von löwis': 'Martin von Löwis',
      'łukasz langa': 'Łukasz Langa',
      'walter dörwald': 'Walter Dörwald'}

  (Jörn Heissler の貢献による bpo-35224)

* The "object.__reduce__()" method can now return a tuple from two to
  six elements long. Formerly, five was the limit.  The new, optional
  sixth element is a callable with a "(obj, state)" signature.  This
  allows the direct control over the state-updating behavior of a
  specific object.  If not *None*, this callable will have priority
  over the object's "__setstate__()" method. (Contributed by Pierre
  Glaser and Olivier Grisel in bpo-35900.)


新たなモジュール
================

* 新しい "importlib.metadata" モジュールは、サードパーティパッケージか
  らのメタデータ読み込みをサポートします。これは暫定版です。例えば、イ
  ンストールされたパッケージのバージョンを得たり、エントリーポイントの
  一覧を得たりできます:

     >>> # Note following example requires that the popular "requests"
     >>> # package has been installed.
     >>>
     >>> from importlib.metadata import version, requires, files
     >>> version('requests')
     '2.22.0'
     >>> list(requires('requests'))
     ['chardet (<3.1.0,>=3.0.2)']
     >>> list(files('requests'))[:5]
     [PackagePath('requests-2.22.0.dist-info/INSTALLER'),
      PackagePath('requests-2.22.0.dist-info/LICENSE'),
      PackagePath('requests-2.22.0.dist-info/METADATA'),
      PackagePath('requests-2.22.0.dist-info/RECORD'),
      PackagePath('requests-2.22.0.dist-info/WHEEL')]

  (Barry Warsaw, Jason R. Coombs の貢献による bpo-34632)


改良されたモジュール
====================


ast
---

AST ノードは "end_lineno" と "end_col_offset" の属性を持つようになりま
した。これは、正確なノードの終端位置を提供します。(これは、"lineno" と
"col_offset" の属性を持つノードにのみ有効です)

新しい "ast.get_source_segment()" 関数は、特定の AST ノードのソースコ
ードを返します。

(Ivan Levkivskyi の貢献による bpo-33416)

"ast.parse()" 関数の新しいフラグが有効になりました:

* "type_comments=True" は、そのAST ノードに関連する、**PEP 484** と
  **PEP 526** に規定されるような型コメントを返します;

* "mode='func_type'" can be used to parse **PEP 484** "signature type
  comments" (returned for function definition AST nodes);

* "feature_version=(3, N)" を使うと、以前の Python 3 バージョンを指定
  できます。例えば、"feature_version=(3, 4)" とすると、"async" と
  "await" キーワードは、予約語ではないとして扱われます。

(Guido van Rossum の貢献による bpo-35766)


asyncio
-------

"asyncio.run()" は暫定から安定API に移動しました。この関数は
*coroutine* の実行と、イベントループを管理しながら自動的に結果を返すこ
とに使用できます。例えば:

   import asyncio

   async def main():
       await asyncio.sleep(0)
       return 42

   asyncio.run(main())

これは、だいたい次と等価です

   import asyncio

   async def main():
       await asyncio.sleep(0)
       return 42

   loop = asyncio.new_event_loop()
   asyncio.set_event_loop(loop)
   try:
       loop.run_until_complete(main())
   finally:
       asyncio.set_event_loop(None)
       loop.close()

実装はこれよりもずっと複雑なものになっています。ですが、
"asyncio.run()" は非同期IOプログラムを動かす際にまず選ぶべき選択肢です
。

(Yury Selivanov の貢献による bpo-32314)

"python -m asyncio" を実行すると、ネイティブ async 対話型インタプリタ
が起動します。高水準キーワード "await" を使うコードの試験を簡単に行え
ます。すべての呼び出しに対して新しいイベントループを作る
"asyncio.run()" を呼び出す必要はありません:

   $ python -m asyncio
   asyncio REPL 3.8.0
   Use "await" directly instead of "asyncio.run()".
   Type "help", "copyright", "credits" or "license" for more information.
   >>> import asyncio
   >>> await asyncio.sleep(10, result='hello')
   hello

(Yury Selivanov の貢献による bpo-37028)

The exception "asyncio.CancelledError" now inherits from
"BaseException" rather than "Exception" and no longer inherits from
"concurrent.futures.CancelledError". (Contributed by Yury Selivanov in
bpo-32528.)

Windows では、イベントループのデフォルトが "ProactorEventLoop" になり
ました。(Victor Stinner の貢献による bpo-34687)

"ProactorEventLoop" はUDP もサポートするようになりました。(Adam Meily,
Andrew Svetlov の貢献による bpo-29883)

"ProactorEventLoop" は "KeyboardInterrupt" ("CTRL+C") によって中断でき
るようになりました。(Vladimir Matveev の貢献による bpo-23057)

"asyncio.Task" 内で、内包されたコルーチンを得られる
"asyncio.Task.get_coro()" メソッドが追加されました。(Alex Grönholm の
貢献による bpo-36999)

"asyncio.create_task()" 関数か "create_task()" の "name" 引数を指定す
るか、もしくはタスクに対して "set_name()" メソッドを呼ぶことによって、
asyncio タスクに名前をつけることができます。タスクの名前は、
"asyncio.Task" の "repr()" 出力で、または "get_name()" メソッドで確認
することができます。(Alex Grönholm の貢献による bpo-34270)

"asyncio.loop.create_connection()" は、Happy Eyeballs をサポートするよ
うになりました。挙動を定義するため、2つの新しいパラメータ:
*happy_eyeballs_delay* と *interleave* が加えられます。Happy Eyeballs
アルゴリズムは、アプリケーションが IPv4 と IPv6 をサポートする場合、そ
の両方を使って接続を保つことによって、アプリケーションの応答性を向上さ
せます。(twisteroid ambassador の貢献による bpo-33530)


builtins
--------

組み込み関数 "compile()" が、"ast.PyCF_ALLOW_TOP_LEVEL_AWAIT" フラグを
受け取るようになりました。このフラグが有効である場合、"compile()" は、
一般には有効な公文ではない、トップレベルの "await", "async for",
"async with" 構文が有効になります。この場合、"CO_COROUTINE" フラグを持
つ非同期コードオブジェクトが返されるでしょう。(Matthias Bussonnier の
貢献による bpo-34616)


collections
-----------

"collections.namedtuple()" の "_asdict()" メソッドは、
"collections.OrderedDict" ではなく "dict" を返すようになりました。
Python 3.7 からは一般に辞書の順番が保証されるため、このような変更が可
能です。"OrderedDict" 特有の機能を使いたい場合は、結果を
"OrderedDict(nt._asdict())" 型にキャストして使用することを推奨します。
(Raymond Hettinger の貢献による bpo-35864)


cProfile
--------

"cProfile.Profile" クラスは、コンテキストマネージャとして使用できるよ
うになりました。このようにしてコードブロックをプロファイルします:

   import cProfile

   with cProfile.Profile() as profiler:
         # code to be profiled
         ...

(Scott Sanderson の貢献による bpo-29235)


csv
---

"csv.DictReader" は、"collections.OrderedDict" ではなく "dict" のイン
スタンスを返すようになりました。この変更によって、順序を保ちながら、よ
り高速にかつメモリの使用量を減らして実行できます。(Michael Selik の貢
献による bpo-34003)


curses
------

ncurses ライブラリの、構造化されたバージョン情報を保持する変数を追加し
ました: "ncurses_version"　 (Serhiy Storchaka の貢献による bpo-31680)


ctypes
------

Windows では、"CDLL" とそのサブクラスが *winmode* 引数を受け取り、根底
の "LoadLibraryEx" 呼び出しのフラグを指定できるようになりました。デフ
ォルトでは、フラグは (フルパスか部分パスが初期DLL を読み込むのに使われ
ていた場合) そのDLL が保存されている場所を含んだ信頼済みの場所と、
"add_dll_directory()" によって加えられた場所からのみ、DLL を読み込むよ
うになっています。(Steve Dower の貢献による bpo-36085)


datetime
--------

ISO の規定に沿った年、週番号、曜日によって、"date" や "datetime" オブ
ジェクトを作成するメソッド "datetime.date.fromisocalendar()" と
"datetime.datetime.fromisocalendar()" が追加されました; これは、各クラ
スの "isocalendar" メソッドの逆を行うものです。(Paul Ganssle の貢献に
よる bpo-36004)


functools
---------

"functools.lru_cache()" は、デコレータを返す関数としての使い方に加えて
、デコレータそのものとしても使えるようになりました。両方の使い方ができ
ます:

   @lru_cache
   def f(x):
       ...

   @lru_cache(maxsize=256)
   def f(x):
       ...

(Raymond Hettinger の貢献による bpo-36772)

インスタンスが存在する間、計算型プロパティをキャッシュする
"functools.cached_property()" デコレータが追加されました:

   import functools
   import statistics

   class Dataset:
      def __init__(self, sequence_of_numbers):
         self.data = sequence_of_numbers

      @functools.cached_property
      def variance(self):
         return statistics.variance(self.data)

(Carl Meyer の貢献による bpo-21145)

*single dispatch* を利用して、メソッドを *ジェネリック関数* に変換する
"functools.singledispatchmethod()" デコレータが追加されました:

   from functools import singledispatchmethod
   from contextlib import suppress

   class TaskManager:

       def __init__(self, tasks):
           self.tasks = list(tasks)

       @singledispatchmethod
       def discard(self, value):
           with suppress(ValueError):
               self.tasks.remove(value)

       @discard.register(list)
       def _(self, tasks):
           targets = set(tasks)
           self.tasks = [x for x in self.tasks if x not in targets]

(Ethan Smith の貢献による bpo-32380)


gc
--

"get_objects()" は、オブジェクトを得る世代を指定するオプションのパラメ
ータ引数 *generation* を受け取るようになりました。(Pablo Galindo の貢
献による bpo-36016)


gettext
-------

"pgettext()" を含む、類似のいくつかの関数が追加されました。 (Franz
Glasner, Éric Araujo, Cheryl Sabella の貢献による bpo-2504)


gzip
----

出力の再現性のために、"gzip.compress()" に *mtime* パラメータが追加さ
れました。 (Guo Ci Teo の貢献による bpo-34898)

不正な、あるいは破損した gzip ファイルのうち、所定の種類のものについて
、"OSError" にかわって "BadGzipFile"  例外が送出されるようになりました
。(Filip Gruszczyński, Michele Orrù, Zackery Spytz の貢献による
bpo-6584)


IDLE と idelelib
----------------

Output over N lines (50 by default) is squeezed down to a button. N
can be changed in the PyShell section of the General page of the
Settings dialog.  Fewer, but possibly extra long, lines can be
squeezed by right clicking on the output.  Squeezed output can be
expanded in place by double-clicking the button or into the clipboard
or a separate window by right-clicking the button.  (Contributed by
Tal Einat in bpo-1529353.)

Add "Run Customized" to the Run menu to run a module with customized
settings. Any command line arguments entered are added to sys.argv.
They also re-appear in the box for the next customized run.  One can
also suppress the normal Shell main module restart.  (Contributed by
Cheryl Sabella, Terry Jan Reedy, and others in bpo-5680 and
bpo-37627.)

Added optional line numbers for IDLE editor windows. Windows open
without line numbers unless set otherwise in the General tab of the
configuration dialog.  Line numbers for an existing window are shown
and hidden in the Options menu. (Contributed by Tal Einat and
Saimadhav Heblikar in bpo-17535.)

OS native encoding is now used for converting between Python strings
and Tcl objects. This allows IDLE to work with emoji and other non-BMP
characters. These characters can be displayed or copied and pasted to
or from the clipboard.  Converting strings from Tcl to Python and back
now never fails. (Many people worked on this for eight years but the
problem was finally solved by Serhiy Storchaka in bpo-13153.)

New in 3.8.1:

Add option to toggle cursor blink off.  (Contributed by Zackery Spytz
in bpo-4603.)

Escape key now closes IDLE completion windows.  (Contributed by Johnny
Najera in bpo-38944.)

上で挙げた変更は 3.7 のメンテナンスリリースにバックポートされました。

Add keywords to module name completion list.  (Contributed by Terry J.
Reedy in bpo-37765.)


inspect
-------

"inspect.getdoc()" 関数は、"__slots__" 属性に値が docstrings であるよ
うな "dict" がある場合、docstrings の内容を探索できるようになりました
。これは、既に "property()"、"classmethod()"、"staticmethod()" に現在
あるような、ドキュメント生成の選択肢を提供します:

   class AudioClip:
       __slots__ = {'bit_rate': 'expressed in kilohertz to one decimal place',
                    'duration': 'in seconds, rounded up to an integer'}
       def __init__(self, bit_rate, duration):
           self.bit_rate = round(bit_rate / 1000.0, 1)
           self.duration = ceil(duration)

(Raymond Hettinger の貢献による bpo-36326)


io
--

In development mode ("-X" "env") and in debug build, the "io.IOBase"
finalizer now logs the exception if the "close()" method fails. The
exception is ignored silently by default in release build.
(Contributed by Victor Stinner in bpo-18748.)


itertools
---------

"itertools.accumulate()" 関数に、初期値を定める、オプションの
*initial* キーワード引数が追加されました:

   >>> from itertools import accumulate
   >>> list(accumulate([10, 5, 30, 15], initial=1000))
   [1000, 1010, 1015, 1045, 1060]

(Lisa Roach の貢献による bpo-34659)


json.tool
---------

一行ごとに別々の JSON オブジェクトへ解析をする "--json-lines" オプショ
ンが追加されました。(Weipeng Hong の貢献による bpo-31553)


logging
-------

"logging.basicConfig()" に *force* キーワード引数が追加されました。こ
れが真になっている場合、ルートのロガーに取り付けられたハンドラは全て取
り除かれ、他の引数によって指定された設定が有効になる前に閉じられます。

This solves a long-standing problem.  Once a logger or *basicConfig()*
had been called, subsequent calls to *basicConfig()* were silently
ignored. This made it difficult to update, experiment with, or teach
the various logging configuration options using the interactive prompt
or a Jupyter notebook.

(Suggested by Raymond Hettinger, implemented by Dong-hee Na, and
reviewed by Vinay Sajip in bpo-33897.)


math
----

二点間のユークリッド距離を計算する "math.dist()" 関数が追加されました
。(Raymond Hettinger の貢献による bpo-33089)

"math.hypot()" 関数は、3次元以上を扱えるように拡張されました。以前は、
二次元の場合のみに対応していました。(Raymond Hettinger の貢献による
bpo-33089)

"sum()" に類似する関数として、初期値(デフォルトは1) に対してイテレート
可能なオブジェクトの数を掛けていく "math.prod()" 関数が追加されました:

   >>> prior = 0.8
   >>> likelihoods = [0.625, 0.84, 0.30]
   >>> math.prod(likelihoods, start=prior)
   0.126

(Pablo Galindo の貢献による bpo-35606)

2つの組み合わせの関数 "math.perm()" と "math.comb()" が追加されました:

   >>> math.perm(10, 3)    # Permutations of 10 things taken 3 at a time
   720
   >>> math.comb(10, 3)    # Combinations of 10 things taken 3 at a time
   120

(Yash Aggarwal, Keller Fuchs, Serhiy Storchaka, Raymond Hettinger の貢
献による bpo-37128, bpo-37178, bpo-35431)

浮動小数点数を介することなく、正確な整数の二乗根を計算する関数
"math.isqrt()" が追加されました。この関数は、巨大な整数にも使用できま
す。"floor(sqrt(n))" より高速ですが、"math.sqrt()" より少し遅いです:

   >>> r = 650320427
   >>> s = r ** 2
   >>> isqrt(s - 1)         # correct
   650320426
   >>> floor(sqrt(s - 1))   # incorrect
   650320427

(Mark Dickinson の貢献による bpo-36887)

"math.factorial()" 関数は、引数に int、またはそれに類似するもの以外を
受け取らなくなりました。(Pablo Galindo の貢献による bpo-33083)


mmap
----

The "mmap.mmap" class now has an "madvise()" method to access the
"madvise()" system call. (Contributed by Zackery Spytz in bpo-32941.)


multiprocessing
---------------

Added new "multiprocessing.shared_memory" module. (Contributed by
Davin Potts in bpo-35813.)

On macOS, the *spawn* start method is now used by default.
(Contributed by Victor Stinner in bpo-33725.)


os
--

Added new function "add_dll_directory()" on Windows for providing
additional search paths for native dependencies when importing
extension modules or loading DLLs using "ctypes". (Contributed by
Steve Dower in bpo-36085.)

A new "os.memfd_create()" function was added to wrap the
"memfd_create()" syscall. (Contributed by Zackery Spytz and Christian
Heimes in bpo-26836.)

On Windows, much of the manual logic for handling reparse points
(including symlinks and directory junctions) has been delegated to the
operating system. Specifically, "os.stat()" will now traverse anything
supported by the operating system, while "os.lstat()" will only open
reparse points that identify as "name surrogates" while others are
opened as for "os.stat()". In all cases, "stat_result.st_mode" will
only have "S_IFLNK" set for symbolic links and not other kinds of
reparse points. To identify other kinds of reparse point, check the
new "stat_result.st_reparse_tag" attribute.

On Windows, "os.readlink()" is now able to read directory junctions.
Note that "islink()" will return "False" for directory junctions, and
so code that checks "islink" first will continue to treat junctions as
directories, while code that handles errors from "os.readlink()" may
now treat junctions as links.

(Contributed by Steve Dower in bpo-37834.)


os.path
-------

"os.path" functions that return a boolean result like "exists()",
"lexists()", "isdir()", "isfile()", "islink()", and "ismount()" now
return "False" instead of raising "ValueError" or its subclasses
"UnicodeEncodeError" and "UnicodeDecodeError" for paths that contain
characters or bytes unrepresentable at the OS level. (Contributed by
Serhiy Storchaka in bpo-33721.)

"expanduser()" on Windows now prefers the "USERPROFILE" environment
variable and does not use "HOME", which is not normally set for
regular user accounts. (Contributed by Anthony Sottile in bpo-36264.)

"isdir()" on Windows no longer returns "True" for a link to a non-
existent directory.

"realpath()" on Windows now resolves reparse points, including
symlinks and directory junctions.

(Contributed by Steve Dower in bpo-37834.)


pathlib
-------

"pathlib.Path" methods that return a boolean result like "exists()",
"is_dir()", "is_file()", "is_mount()", "is_symlink()",
"is_block_device()", "is_char_device()", "is_fifo()", "is_socket()"
now return "False" instead of raising "ValueError" or its subclass
"UnicodeEncodeError" for paths that contain characters unrepresentable
at the OS level. (Contributed by Serhiy Storchaka in bpo-33721.)

Added "pathlib.Path.link_to()" which creates a hard link pointing to a
path. (Contributed by Joannah Nanjekye in bpo-26978)


pickle
------

"pickle" extensions subclassing the C-optimized "Pickler" can now
override the pickling logic of functions and classes by defining the
special "reducer_override()" method. (Contributed by Pierre Glaser and
Olivier Grisel in bpo-35900.)


plistlib
--------

Added new "plistlib.UID" and enabled support for reading and writing
NSKeyedArchiver-encoded binary plists. (Contributed by Jon Janzen in
bpo-26707.)


pprint
------

The "pprint" module added a *sort_dicts* parameter to several
functions. By default, those functions continue to sort dictionaries
before rendering or printing.  However, if *sort_dicts* is set to
false, the dictionaries retain the order that keys were inserted.
This can be useful for comparison to JSON inputs during debugging.

In addition, there is a convenience new function, "pprint.pp()" that
is like "pprint.pprint()" but with *sort_dicts* defaulting to "False":

   >>> from pprint import pprint, pp
   >>> d = dict(source='input.txt', operation='filter', destination='output.txt')
   >>> pp(d, width=40)                  # Original order
   {'source': 'input.txt',
    'operation': 'filter',
    'destination': 'output.txt'}
   >>> pprint(d, width=40)              # Keys sorted alphabetically
   {'destination': 'output.txt',
    'operation': 'filter',
    'source': 'input.txt'}

(Contributed by Rémi Lapeyre in bpo-30670.)


py_compile
----------

"py_compile.compile()" now supports silent mode. (Contributed by
Joannah Nanjekye in bpo-22640.)


shlex
-----

The new "shlex.join()" function acts as the inverse of
"shlex.split()". (Contributed by Bo Bayles in bpo-32102.)


shutil
------

"shutil.copytree()" now accepts a new "dirs_exist_ok" keyword
argument. (Contributed by Josh Bronson in bpo-20849.)

"shutil.make_archive()" now defaults to the modern pax (POSIX.1-2001)
format for new archives to improve portability and standards
conformance, inherited from the corresponding change to the "tarfile"
module. (Contributed by C.A.M. Gerlach in bpo-30661.)

"shutil.rmtree()" on Windows now removes directory junctions without
recursively removing their contents first. (Contributed by Steve Dower
in bpo-37834.)


socket
------

Added "create_server()" and "has_dualstack_ipv6()" convenience
functions to automate the necessary tasks usually involved when
creating a server socket, including accepting both IPv4 and IPv6
connections on the same socket.  (Contributed by Giampaolo Rodolà in
bpo-17561.)

The "socket.if_nameindex()", "socket.if_nametoindex()", and
"socket.if_indextoname()" functions have been implemented on Windows.
(Contributed by Zackery Spytz in bpo-37007.)


ssl
---

Added "post_handshake_auth" to enable and
"verify_client_post_handshake()" to initiate TLS 1.3 post-handshake
authentication. (Contributed by Christian Heimes in bpo-34670.)


statistics
----------

Added "statistics.fmean()" as a faster, floating point variant of
"statistics.mean()".  (Contributed by Raymond Hettinger and Steven
D'Aprano in bpo-35904.)

Added "statistics.geometric_mean()" (Contributed by Raymond Hettinger
in bpo-27181.)

Added "statistics.multimode()" that returns a list of the most common
values. (Contributed by Raymond Hettinger in bpo-35892.)

Added "statistics.quantiles()" that divides data or a distribution in
to equiprobable intervals (e.g. quartiles, deciles, or percentiles).
(Contributed by Raymond Hettinger in bpo-36546.)

Added "statistics.NormalDist", a tool for creating and manipulating
normal distributions of a random variable. (Contributed by Raymond
Hettinger in bpo-36018.)

   >>> temperature_feb = NormalDist.from_samples([4, 12, -3, 2, 7, 14])
   >>> temperature_feb.mean
   6.0
   >>> temperature_feb.stdev
   6.356099432828281

   >>> temperature_feb.cdf(3)            # Chance of being under 3 degrees
   0.3184678262814532
   >>> # Relative chance of being 7 degrees versus 10 degrees
   >>> temperature_feb.pdf(7) / temperature_feb.pdf(10)
   1.2039930378537762

   >>> el_niño = NormalDist(4, 2.5)
   >>> temperature_feb += el_niño        # Add in a climate effect
   >>> temperature_feb
   NormalDist(mu=10.0, sigma=6.830080526611674)

   >>> temperature_feb * (9/5) + 32      # Convert to Fahrenheit
   NormalDist(mu=50.0, sigma=12.294144947901014)
   >>> temperature_feb.samples(3)        # Generate random samples
   [7.672102882379219, 12.000027119750287, 4.647488369766392]


sys
---

Add new "sys.unraisablehook()" function which can be overridden to
control how "unraisable exceptions" are handled. It is called when an
exception has occurred but there is no way for Python to handle it.
For example, when a destructor raises an exception or during garbage
collection ("gc.collect()"). (Contributed by Victor Stinner in
bpo-36829.)


tarfile
-------

The "tarfile" module now defaults to the modern pax (POSIX.1-2001)
format for new archives, instead of the previous GNU-specific one.
This improves cross-platform portability with a consistent encoding
(UTF-8) in a standardized and extensible format, and offers several
other benefits. (Contributed by C.A.M. Gerlach in bpo-36268.)


threading
---------

Add a new "threading.excepthook()" function which handles uncaught
"threading.Thread.run()" exception. It can be overridden to control
how uncaught "threading.Thread.run()" exceptions are handled.
(Contributed by Victor Stinner in bpo-1230540.)

Add a new "threading.get_native_id()" function and a "native_id"
attribute to the "threading.Thread" class. These return the native
integral Thread ID of the current thread assigned by the kernel. This
feature is only available on certain platforms, see "get_native_id"
for more information. (Contributed by Jake Tesler in bpo-36084.)


tokenize
--------

The "tokenize" module now implicitly emits a "NEWLINE" token when
provided with input that does not have a trailing new line.  This
behavior now matches what the C tokenizer does internally.
(Contributed by Ammar Askar in bpo-33899.)


tkinter
-------

Added methods "selection_from()", "selection_present()",
"selection_range()" and "selection_to()" in the "tkinter.Spinbox"
class. (Contributed by Juliette Monsel in bpo-34829.)

Added method "moveto()" in the "tkinter.Canvas" class. (Contributed by
Juliette Monsel in bpo-23831.)

The "tkinter.PhotoImage" class now has "transparency_get()" and
"transparency_set()" methods.  (Contributed by Zackery Spytz in
bpo-25451.)


time
----

Added new clock "CLOCK_UPTIME_RAW" for macOS 10.12. (Contributed by
Joannah Nanjekye in bpo-35702.)


typing
------

The "typing" module incorporates several new features:

* A dictionary type with per-key types.  See **PEP 589** and
  "typing.TypedDict". TypedDict uses only string keys.  By default,
  every key is required to be present. Specify "total=False" to allow
  keys to be optional:

     class Location(TypedDict, total=False):
         lat_long: tuple
         grid_square: str
         xy_coordinate: tuple

* Literal types.  See **PEP 586** and "typing.Literal". Literal types
  indicate that a parameter or return value is constrained to one or
  more specific literal values:

     def get_status(port: int) -> Literal['connected', 'disconnected']:
         ...

* "Final" variables, functions, methods and classes.  See **PEP 591**,
  "typing.Final" and "typing.final()". The final qualifier instructs a
  static type checker to restrict subclassing, overriding, or
  reassignment:

     pi: Final[float] = 3.1415926536

* Protocol definitions.  See **PEP 544**, "typing.Protocol" and
  "typing.runtime_checkable()".  Simple ABCs like "typing.SupportsInt"
  are now "Protocol" subclasses.

* New protocol class "typing.SupportsIndex".

* New functions "typing.get_origin()" and "typing.get_args()".


unicodedata
-----------

The "unicodedata" module has been upgraded to use the Unicode 12.1.0
release.

New function "is_normalized()" can be used to verify a string is in a
specific normal form, often much faster than by actually normalizing
the string.  (Contributed by Max Belanger, David Euresti, and Greg
Price in bpo-32285 and bpo-37966).


unittest
--------

Added "AsyncMock" to support an asynchronous version of "Mock".
Appropriate new assert functions for testing have been added as well.
(Contributed by Lisa Roach in bpo-26467).

Added "addModuleCleanup()" and "addClassCleanup()" to unittest to
support cleanups for "setUpModule()" and "setUpClass()". (Contributed
by Lisa Roach in bpo-24412.)

Several mock assert functions now also print a list of actual calls
upon failure. (Contributed by Petter Strandmark in bpo-35047.)

"unittest" module gained support for coroutines to be used as test
cases with "unittest.IsolatedAsyncioTestCase". (Contributed by Andrew
Svetlov in bpo-32972.)

以下はプログラム例です:

   import unittest


   class TestRequest(unittest.IsolatedAsyncioTestCase):

       async def asyncSetUp(self):
           self.connection = await AsyncConnection()

       async def test_get(self):
           response = await self.connection.get("https://example.com")
           self.assertEqual(response.status_code, 200)

       async def asyncTearDown(self):
           await self.connection.close()


   if __name__ == "__main__":
       unittest.main()


venv
----

"venv" now includes an "Activate.ps1" script on all platforms for
activating virtual environments under PowerShell Core 6.1.
(Contributed by Brett Cannon in bpo-32718.)


weakref
-------

The proxy objects returned by "weakref.proxy()" now support the matrix
multiplication operators "@" and "@=" in addition to the other numeric
operators. (Contributed by Mark Dickinson in bpo-36669.)


xml
---

As mitigation against DTD and external entity retrieval, the
"xml.dom.minidom" and "xml.sax" modules no longer process external
entities by default. (Contributed by Christian Heimes in bpo-17239.)

The ".find*()" methods in the "xml.etree.ElementTree" module support
wildcard searches like "{*}tag" which ignores the namespace and
"{namespace}*" which returns all tags in the given namespace.
(Contributed by Stefan Behnel in bpo-28238.)

The "xml.etree.ElementTree" module provides a new function
"–xml.etree.ElementTree.canonicalize()" that implements C14N 2.0.
(Contributed by Stefan Behnel in bpo-13611.)

The target object of "xml.etree.ElementTree.XMLParser" can receive
namespace declaration events through the new callback methods
"start_ns()" and "end_ns()".  Additionally, the
"xml.etree.ElementTree.TreeBuilder" target can be configured to
process events about comments and processing instructions to include
them in the generated tree. (Contributed by Stefan Behnel in bpo-36676
and bpo-36673.)


xmlrpc
------

"xmlrpc.client.ServerProxy" now supports an optional *headers* keyword
argument for a sequence of HTTP headers to be sent with each request.
Among other things, this makes it possible to upgrade from default
basic authentication to faster session authentication. (Contributed by
Cédric Krier in bpo-35153.)


最適化
======

* The "subprocess" module can now use the "os.posix_spawn()" function
  in some cases for better performance. Currently, it is only used on
  macOS and Linux (using glibc 2.24 or newer) if all these conditions
  are met:

  * *close_fds* is false;

  * *preexec_fn*, *pass_fds*, *cwd* and *start_new_session* parameters
    are not set;

  * the *executable* path contains a directory.

  (Contributed by Joannah Nanjekye and Victor Stinner in bpo-35537.)

* "shutil.copyfile()", "shutil.copy()", "shutil.copy2()",
  "shutil.copytree()" and "shutil.move()" use platform-specific "fast-
  copy" syscalls on Linux and macOS in order to copy the file more
  efficiently. "fast-copy" means that the copying operation occurs
  within the kernel, avoiding the use of userspace buffers in Python
  as in ""outfd.write(infd.read())"". On Windows "shutil.copyfile()"
  uses a bigger default buffer size (1 MiB instead of 16 KiB) and a
  "memoryview()"-based variant of "shutil.copyfileobj()" is used. The
  speedup for copying a 512 MiB file within the same partition is
  about +26% on Linux, +50% on macOS and +40% on Windows. Also, much
  less CPU cycles are consumed. See プラットフォーム依存の効率的なコピ
  ー操作 section. (Contributed by Giampaolo Rodolà in bpo-33671.)

* "shutil.copytree()" uses "os.scandir()" function and all copy
  functions depending from it use cached "os.stat()" values. The
  speedup for copying a directory with 8000 files is around +9% on
  Linux, +20% on Windows and +30% on a Windows SMB share. Also the
  number of "os.stat()" syscalls is reduced by 38% making
  "shutil.copytree()" especially faster on network filesystems.
  (Contributed by Giampaolo Rodolà in bpo-33695.)

* The default protocol in the "pickle" module is now Protocol 4, first
  introduced in Python 3.4.  It offers better performance and smaller
  size compared to Protocol 3 available since Python 3.0.

* Removed one "Py_ssize_t" member from "PyGC_Head".  All GC tracked
  objects (e.g. tuple, list, dict) size is reduced 4 or 8 bytes.
  (Contributed by Inada Naoki in bpo-33597.)

* "uuid.UUID" now uses "__slots__" to reduce its memory footprint.
  (Contributed by Wouter Bolsterlee and Tal Einat in bpo-30977)

* Improved performance of "operator.itemgetter()" by 33%.  Optimized
  argument handling and added a fast path for the common case of a
  single non-negative integer index into a tuple (which is the typical
  use case in the standard library).  (Contributed by Raymond
  Hettinger in bpo-35664.)

* Sped-up field lookups in "collections.namedtuple()".  They are now
  more than two times faster, making them the fastest form of instance
  variable lookup in Python. (Contributed by Raymond Hettinger, Pablo
  Galindo, and Joe Jevnik, Serhiy Storchaka in bpo-32492.)

* The "list" constructor does not overallocate the internal item
  buffer if the input iterable has a known length (the input
  implements "__len__"). This makes the created list 12% smaller on
  average. (Contributed by Raymond Hettinger and Pablo Galindo in
  bpo-33234.)

* Doubled the speed of class variable writes.  When a non-dunder
  attribute was updated, there was an unnecessary call to update
  slots. (Contributed by Stefan Behnel, Pablo Galindo Salgado, Raymond
  Hettinger, Neil Schemenauer, and Serhiy Storchaka in bpo-36012.)

* Reduced an overhead of converting arguments passed to many builtin
  functions and methods.  This sped up calling some simple builtin
  functions and methods up to 20--50%.  (Contributed by Serhiy
  Storchaka in bpo-23867, bpo-35582 and bpo-36127.)

* "LOAD_GLOBAL" instruction now uses new "per opcode cache" mechanism.
  It is about 40% faster now.  (Contributed by Yury Selivanov and
  Inada Naoki in bpo-26219.)


ビルドならびに C API の変更
===========================

* Default "sys.abiflags" became an empty string: the "m" flag for
  pymalloc became useless (builds with and without pymalloc are ABI
  compatible) and so has been removed. (Contributed by Victor Stinner
  in bpo-36707.)

  Example of changes:

  * Only "python3.8" program is installed, "python3.8m" program is
    gone.

  * Only "python3.8-config" script is installed, "python3.8m-config"
    script is gone.

  * The "m" flag has been removed from the suffix of dynamic library
    filenames: extension modules in the standard library as well as
    those produced and installed by third-party packages, like those
    downloaded from PyPI. On Linux, for example, the Python 3.7 suffix
    ".cpython-37m-x86_64-linux-gnu.so" became ".cpython-38-x86_64
    -linux-gnu.so" in Python 3.8.

* The header files have been reorganized to better separate the
  different kinds of APIs:

  * "Include/*.h" should be the portable public stable C API.

  * "Include/cpython/*.h" should be the unstable C API specific to
    CPython; public API, with some private API prefixed by "_Py" or
    "_PY".

  * "Include/internal/*.h" is the private internal C API very specific
    to CPython. This API comes with no backward compatibility warranty
    and should not be used outside CPython. It is only exposed for
    very specific needs like debuggers and profiles which has to
    access to CPython internals without calling functions. This API is
    now installed by "make install".

  (Contributed by Victor Stinner in bpo-35134 and bpo-35081, work
  initiated by Eric Snow in Python 3.7.)

* Some macros have been converted to static inline functions:
  parameter types and return type are well defined, they don't have
  issues specific to macros, variables have a local scopes. Examples:

  * "Py_INCREF()", "Py_DECREF()"

  * "Py_XINCREF()", "Py_XDECREF()"

  * "PyObject_INIT()", "PyObject_INIT_VAR()"

  * Private functions: "_PyObject_GC_TRACK()",
    "_PyObject_GC_UNTRACK()", "_Py_Dealloc()"

  (Contributed by Victor Stinner in bpo-35059.)

* The "PyByteArray_Init()" and "PyByteArray_Fini()" functions have
  been removed. They did nothing since Python 2.7.4 and Python 3.2.0,
  were excluded from the limited API (stable ABI), and were not
  documented. (Contributed by Victor Stinner in bpo-35713.)

* The result of "PyExceptionClass_Name()" is now of type "const char
  *" rather of "char *". (Contributed by Serhiy Storchaka in
  bpo-33818.)

* The duality of "Modules/Setup.dist" and "Modules/Setup" has been
  removed.  Previously, when updating the CPython source tree, one had
  to manually copy "Modules/Setup.dist" (inside the source tree) to
  "Modules/Setup" (inside the build tree) in order to reflect any
  changes upstream.  This was of a small benefit to packagers at the
  expense of a frequent annoyance to developers following CPython
  development, as forgetting to copy the file could produce build
  failures.

  Now the build system always reads from "Modules/Setup" inside the
  source tree.  People who want to customize that file are encouraged
  to maintain their changes in a git fork of CPython or as patch
  files, as they would do for any other change to the source tree.

  (Contributed by Antoine Pitrou in bpo-32430.)

* Functions that convert Python number to C integer like
  "PyLong_AsLong()" and argument parsing functions like
  "PyArg_ParseTuple()" with integer converting format units like "'i'"
  will now use the "__index__()" special method instead of
  "__int__()", if available.  The deprecation warning will be emitted
  for objects with the "__int__()" method but without the
  "__index__()" method (like "Decimal" and "Fraction").
  "PyNumber_Check()" will now return "1" for objects implementing
  "__index__()". "PyNumber_Long()", "PyNumber_Float()" and
  "PyFloat_AsDouble()" also now use the "__index__()" method if
  available. (Contributed by Serhiy Storchaka in bpo-36048 and
  bpo-20092.)

* Heap-allocated type objects will now increase their reference count
  in "PyObject_Init()" (and its parallel macro "PyObject_INIT")
  instead of in "PyType_GenericAlloc()". Types that modify instance
  allocation or deallocation may need to be adjusted. (Contributed by
  Eddie Elizondo in bpo-35810.)

* The new function "PyCode_NewWithPosOnlyArgs()" allows to create code
  objects like "PyCode_New()", but with an extra *posonlyargcount*
  parameter for indicating the number of positional-only arguments.
  (Contributed by Pablo Galindo in bpo-37221.)

* "Py_SetPath()" now sets "sys.executable" to the program full path
  ("Py_GetProgramFullPath()") rather than to the program name
  ("Py_GetProgramName()"). (Contributed by Victor Stinner in
  bpo-38234.)


非推奨
======

* The distutils "bdist_wininst" command is now deprecated, use
  "bdist_wheel" (wheel packages) instead. (Contributed by Victor
  Stinner in bpo-37481.)

* Deprecated methods "getchildren()" and "getiterator()" in the
  "ElementTree" module now emit a "DeprecationWarning" instead of
  "PendingDeprecationWarning". They will be removed in Python 3.9.
  (Contributed by Serhiy Storchaka in bpo-29209.)

* Passing an object that is not an instance of
  "concurrent.futures.ThreadPoolExecutor" to
  "loop.set_default_executor()" is deprecated and will be prohibited
  in Python 3.9. (Contributed by Elvis Pranskevichus in bpo-34075.)

* The "__getitem__()" methods of "xml.dom.pulldom.DOMEventStream",
  "wsgiref.util.FileWrapper" and "fileinput.FileInput" have been
  deprecated.

  Implementations of these methods have been ignoring their *index*
  parameter, and returning the next item instead. (Contributed by
  Berker Peksag in bpo-9372.)

* The "typing.NamedTuple" class has deprecated the "_field_types"
  attribute in favor of the "__annotations__" attribute which has the
  same information. (Contributed by Raymond Hettinger in bpo-36320.)

* "ast" classes "Num", "Str", "Bytes", "NameConstant" and "Ellipsis"
  are considered deprecated and will be removed in future Python
  versions. "Constant" should be used instead. (Contributed by Serhiy
  Storchaka in bpo-32892.)

* "ast.NodeVisitor" methods "visit_Num()", "visit_Str()",
  "visit_Bytes()", "visit_NameConstant()" and "visit_Ellipsis()" are
  deprecated now and will not be called in future Python versions. Add
  the "visit_Constant()" method to handle all constant nodes.
  (Contributed by Serhiy Storchaka in bpo-36917.)

* The "asyncio.coroutine()" *decorator* is deprecated and will be
  removed in version 3.10.  Instead of "@asyncio.coroutine", use
  "async def" instead. (Contributed by Andrew Svetlov in bpo-36921.)

* In "asyncio", the explicit passing of a *loop* argument has been
  deprecated and will be removed in version 3.10 for the following:
  "asyncio.sleep()", "asyncio.gather()", "asyncio.shield()",
  "asyncio.wait_for()", "asyncio.wait()", "asyncio.as_completed()",
  "asyncio.Task", "asyncio.Lock", "asyncio.Event",
  "asyncio.Condition", "asyncio.Semaphore",
  "asyncio.BoundedSemaphore", "asyncio.Queue",
  "asyncio.create_subprocess_exec()", and
  "asyncio.create_subprocess_shell()".

* The explicit passing of coroutine objects to "asyncio.wait()" has
  been deprecated and will be removed in version 3.11. (Contributed by
  Yury Selivanov in bpo-34790.)

* The following functions and methods are deprecated in the "gettext"
  module: "lgettext()", "ldgettext()", "lngettext()" and
  "ldngettext()". They return encoded bytes, and it's possible that
  you will get unexpected Unicode-related exceptions if there are
  encoding problems with the translated strings. It's much better to
  use alternatives which return Unicode strings in Python 3. These
  functions have been broken for a long time.

  Function "bind_textdomain_codeset()", methods "output_charset()" and
  "set_output_charset()", and the *codeset* parameter of functions
  "translation()" and "install()" are also deprecated, since they are
  only used for the "l*gettext()" functions. (Contributed by Serhiy
  Storchaka in bpo-33710.)

* The "isAlive()" method of "threading.Thread" has been deprecated.
  (Contributed by Dong-hee Na in bpo-35283.)

* Many builtin and extension functions that take integer arguments
  will now emit a deprecation warning for "Decimal"s, "Fraction"s and
  any other objects that can be converted to integers only with a loss
  (e.g. that have the "__int__()" method but do not have the
  "__index__()" method).  In future version they will be errors.
  (Contributed by Serhiy Storchaka in bpo-36048.)

* Deprecated passing the following arguments as keyword arguments:

  * *func* in "functools.partialmethod()", "weakref.finalize()",
    "profile.Profile.runcall()", "cProfile.Profile.runcall()",
    "bdb.Bdb.runcall()", "trace.Trace.runfunc()" and
    "curses.wrapper()".

  * *function* in "unittest.TestCase.addCleanup()".

  * *fn* in the "submit()" method of
    "concurrent.futures.ThreadPoolExecutor" and
    "concurrent.futures.ProcessPoolExecutor".

  * *callback* in "contextlib.ExitStack.callback()",
    "contextlib.AsyncExitStack.callback()" and
    "contextlib.AsyncExitStack.push_async_callback()".

  * *c* and *typeid* in the "create()" method of
    "multiprocessing.managers.Server" and
    "multiprocessing.managers.SharedMemoryServer".

  * *obj* in "weakref.finalize()".

  In future releases of Python, they will be positional-only.
  (Contributed by Serhiy Storchaka in bpo-36492.)


API と機能の削除
================

The following features and APIs have been removed from Python 3.8:

* Starting with Python 3.3, importing ABCs from "collections" was
  deprecated, and importing should be done from "collections.abc".
  Being able to import from collections was marked for removal in 3.8,
  but has been delayed to 3.9. (See bpo-36952.)

* The "macpath" module, deprecated in Python 3.7, has been removed.
  (Contributed by Victor Stinner in bpo-35471.)

* The function "platform.popen()" has been removed, after having been
  deprecated since Python 3.3: use "os.popen()" instead. (Contributed
  by Victor Stinner in bpo-35345.)

* The function "time.clock()" has been removed, after having been
  deprecated since Python 3.3: use "time.perf_counter()" or
  "time.process_time()" instead, depending on your requirements, to
  have well-defined behavior. (Contributed by Matthias Bussonnier in
  bpo-36895.)

* The "pyvenv" script has been removed in favor of "python3.8 -m venv"
  to help eliminate confusion as to what Python interpreter the
  "pyvenv" script is tied to. (Contributed by Brett Cannon in
  bpo-25427.)

* "parse_qs", "parse_qsl", and "escape" are removed from the "cgi"
  module.  They are deprecated in Python 3.2 or older. They should be
  imported from the "urllib.parse" and "html" modules instead.

* "filemode" function is removed from the "tarfile" module. It is not
  documented and deprecated since Python 3.3.

* The "XMLParser" constructor no longer accepts the *html* argument.
  It never had an effect and was deprecated in Python 3.4. All other
  parameters are now keyword-only. (Contributed by Serhiy Storchaka in
  bpo-29209.)

* Removed the "doctype()" method of "XMLParser". (Contributed by
  Serhiy Storchaka in bpo-29209.)

* "unicode_internal" codec is removed. (Contributed by Inada Naoki in
  bpo-36297.)

* The "Cache" and "Statement" objects of the "sqlite3" module are not
  exposed to the user. (Contributed by Aviv Palivoda in bpo-30262.)

* The "bufsize" keyword argument of "fileinput.input()" and
  "fileinput.FileInput()" which was ignored and deprecated since
  Python 3.6 has been removed. bpo-36952 (Contributed by Matthias
  Bussonnier.)

* The functions "sys.set_coroutine_wrapper()" and
  "sys.get_coroutine_wrapper()" deprecated in Python 3.7 have been
  removed; bpo-36933 (Contributed by Matthias Bussonnier.)


Python 3.8 への移植
===================

このセクションでは前述の変更とバグフィックスにより必要となるかもしれな
いコードの変更を列挙します:


Python の挙動の変更
-------------------

* Yield expressions (both "yield" and "yield from" clauses) are now
  disallowed in comprehensions and generator expressions (aside from
  the iterable expression in the leftmost "for" clause). (Contributed
  by Serhiy Storchaka in bpo-10544.)

* The compiler now produces a "SyntaxWarning" when identity checks
  ("is" and "is not") are used with certain types of literals (e.g.
  strings, numbers).  These can often work by accident in CPython, but
  are not guaranteed by the language spec.  The warning advises users
  to use equality tests ("==" and "!=") instead. (Contributed by
  Serhiy Storchaka in bpo-34850.)

* The CPython interpreter can swallow exceptions in some
  circumstances. In Python 3.8 this happens in fewer cases.  In
  particular, exceptions raised when getting the attribute from the
  type dictionary are no longer ignored. (Contributed by Serhiy
  Storchaka in bpo-35459.)

* Removed "__str__" implementations from builtin types "bool", "int",
  "float", "complex" and few classes from the standard library.  They
  now inherit "__str__()" from "object". As result, defining the
  "__repr__()" method in the subclass of these classes will affect
  their string representation. (Contributed by Serhiy Storchaka in
  bpo-36793.)

* On AIX, "sys.platform" doesn't contain the major version anymore. It
  is always "'aix'", instead of "'aix3'" .. "'aix7'".  Since older
  Python versions include the version number, so it is recommended to
  always use "sys.platform.startswith('aix')". (Contributed by M. Felt
  in bpo-36588.)

* "PyEval_AcquireLock()" and "PyEval_AcquireThread()" now terminate
  the current thread if called while the interpreter is finalizing,
  making them consistent with "PyEval_RestoreThread()",
  "Py_END_ALLOW_THREADS()", and "PyGILState_Ensure()". If this
  behavior is not desired, guard the call by checking
  "_Py_IsFinalizing()" or "sys.is_finalizing()". (Contributed by
  Joannah Nanjekye in bpo-36475.)


Python API の変更
-----------------

* The "os.getcwdb()" function now uses the UTF-8 encoding on Windows,
  rather than the ANSI code page: see **PEP 529** for the rationale.
  The function is no longer deprecated on Windows. (Contributed by
  Victor Stinner in bpo-37412.)

* "subprocess.Popen" can now use "os.posix_spawn()" in some cases for
  better performance. On Windows Subsystem for Linux and QEMU User
  Emulation, the "Popen" constructor using "os.posix_spawn()" no
  longer raises an exception on errors like "missing program".
  Instead the child process fails with a non-zero "returncode".
  (Contributed by Joannah Nanjekye and Victor Stinner in bpo-35537.)

* The *preexec_fn* argument of * "subprocess.Popen" is no longer
  compatible with subinterpreters. The use of the parameter in a
  subinterpreter now raises "RuntimeError". (Contributed by Eric Snow
  in bpo-34651, modified by Christian Heimes in bpo-37951.)

* The "imap.IMAP4.logout()" method no longer silently ignores
  arbitrary exceptions. (Contributed by Victor Stinner in bpo-36348.)

* The function "platform.popen()" has been removed, after having been
  deprecated since Python 3.3: use "os.popen()" instead. (Contributed
  by Victor Stinner in bpo-35345.)

* The "statistics.mode()" function no longer raises an exception when
  given multimodal data.  Instead, it returns the first mode
  encountered in the input data.  (Contributed by Raymond Hettinger in
  bpo-35892.)

* The "selection()" method of the "tkinter.ttk.Treeview" class no
  longer takes arguments.  Using it with arguments for changing the
  selection was deprecated in Python 3.6.  Use specialized methods
  like "selection_set()" for changing the selection.  (Contributed by
  Serhiy Storchaka in bpo-31508.)

* The "writexml()", "toxml()" and "toprettyxml()" methods of
  "xml.dom.minidom", and the "write()" method of "xml.etree", now
  preserve the attribute order specified by the user. (Contributed by
  Diego Rojas and Raymond Hettinger in bpo-34160.)

* A "dbm.dumb" database opened with flags "'r'" is now read-only.
  "dbm.dumb.open()" with flags "'r'" and "'w'" no longer creates a
  database if it does not exist. (Contributed by Serhiy Storchaka in
  bpo-32749.)

* The "doctype()" method defined in a subclass of "XMLParser" will no
  longer be called and will emit a "RuntimeWarning" instead of a
  "DeprecationWarning". Define the "doctype()" method on a target for
  handling an XML doctype declaration. (Contributed by Serhiy
  Storchaka in bpo-29209.)

* A "RuntimeError" is now raised when the custom metaclass doesn't
  provide the "__classcell__" entry in the namespace passed to
  "type.__new__".  A "DeprecationWarning" was emitted in Python 3.6--
  3.7.  (Contributed by Serhiy Storchaka in bpo-23722.)

* The "cProfile.Profile" class can now be used as a context manager.
  (Contributed by Scott Sanderson in bpo-29235.)

* "shutil.copyfile()", "shutil.copy()", "shutil.copy2()",
  "shutil.copytree()" and "shutil.move()" use platform-specific "fast-
  copy" syscalls (see プラットフォーム依存の効率的なコピー操作
  section).

* "shutil.copyfile()" default buffer size on Windows was changed from
  16 KiB to 1 MiB.

* The "PyGC_Head" struct has changed completely.  All code that
  touched the struct member should be rewritten.  (See bpo-33597.)

* The "PyInterpreterState" struct has been moved into the "internal"
  header files (specifically Include/internal/pycore_pystate.h).  An
  opaque "PyInterpreterState" is still available as part of the public
  API (and stable ABI).  The docs indicate that none of the struct's
  fields are public, so we hope no one has been using them.  However,
  if you do rely on one or more of those private fields and have no
  alternative then please open a BPO issue.  We'll work on helping you
  adjust (possibly including adding accessor functions to the public
  API).  (See bpo-35886.)

* The "mmap.flush()" method now returns "None" on success and raises
  an exception on error under all platforms.  Previously, its behavior
  was platform-dependent: a nonzero value was returned on success;
  zero was returned on error under Windows.  A zero value was returned
  on success; an exception was raised on error under Unix.
  (Contributed by Berker Peksag in bpo-2122.)

* "xml.dom.minidom" and "xml.sax" modules no longer process external
  entities by default. (Contributed by Christian Heimes in bpo-17239.)

* Deleting a key from a read-only "dbm" database ("dbm.dumb",
  "dbm.gnu" or "dbm.ndbm") raises "error" ("dbm.dumb.error",
  "dbm.gnu.error" or "dbm.ndbm.error") instead of "KeyError".
  (Contributed by Xiang Zhang in bpo-33106.)

* Simplified AST for literals.  All constants will be represented as
  "ast.Constant" instances.  Instantiating old classes "Num", "Str",
  "Bytes", "NameConstant" and "Ellipsis" will return an instance of
  "Constant". (Contributed by Serhiy Storchaka in bpo-32892.)

* "expanduser()" on Windows now prefers the "USERPROFILE" environment
  variable and does not use "HOME", which is not normally set for
  regular user accounts. (Contributed by Anthony Sottile in
  bpo-36264.)

* The exception "asyncio.CancelledError" now inherits from
  "BaseException" rather than "Exception" and no longer inherits from
  "concurrent.futures.CancelledError". (Contributed by Yury Selivanov
  in bpo-32528.)

* The function "asyncio.wait_for()" now correctly waits for
  cancellation when using an instance of "asyncio.Task". Previously,
  upon reaching *timeout*, it was cancelled and immediately returned.
  (Contributed by Elvis Pranskevichus in bpo-32751.)

* The function "asyncio.BaseTransport.get_extra_info()" now returns a
  safe to use socket object when 'socket' is passed to the *name*
  parameter. (Contributed by Yury Selivanov in bpo-37027.)

* "asyncio.BufferedProtocol" has graduated to the stable API.

* DLL dependencies for extension modules and DLLs loaded with "ctypes"
  on Windows are now resolved more securely. Only the system paths,
  the directory containing the DLL or PYD file, and directories added
  with "add_dll_directory()" are searched for load-time dependencies.
  Specifically, "PATH" and the current working directory are no longer
  used, and modifications to these will no longer have any effect on
  normal DLL resolution. If your application relies on these
  mechanisms, you should check for "add_dll_directory()" and if it
  exists, use it to add your DLLs directory while loading your
  library. Note that Windows 7 users will need to ensure that Windows
  Update KB2533623 has been installed (this is also verified by the
  installer). (Contributed by Steve Dower in bpo-36085.)

* The header files and functions related to pgen have been removed
  after its replacement by a pure Python implementation. (Contributed
  by Pablo Galindo in bpo-36623.)

* "types.CodeType" has a new parameter in the second position of the
  constructor (*posonlyargcount*) to support positional-only arguments
  defined in **PEP 570**. The first argument (*argcount*) now
  represents the total number of positional arguments (including
  positional-only arguments). The new "replace()" method of
  "types.CodeType" can be used to make the code future-proof.

* The parameter "digestmod" for "hmac.new()" no longer uses the MD5
  digest by default.


C API の変更
------------

* The "PyCompilerFlags" structure got a new *cf_feature_version*
  field. It should be initialized to "PY_MINOR_VERSION". The field is
  ignored by default, and is used if and only if "PyCF_ONLY_AST" flag
  is set in *cf_flags*. (Contributed by Guido van Rossum in
  bpo-35766.)

* The "PyEval_ReInitThreads()" function has been removed from the C
  API. It should not be called explicitly: use
  "PyOS_AfterFork_Child()" instead. (Contributed by Victor Stinner in
  bpo-36728.)

* On Unix, C extensions are no longer linked to libpython except on
  Android and Cygwin. When Python is embedded, "libpython" must not be
  loaded with "RTLD_LOCAL", but "RTLD_GLOBAL" instead. Previously,
  using "RTLD_LOCAL", it was already not possible to load C extensions
  which were not linked to "libpython", like C extensions of the
  standard library built by the "*shared*" section of "Modules/Setup".
  (Contributed by Victor Stinner in bpo-21536.)

* Use of "#" variants of formats in parsing or building value (e.g.
  "PyArg_ParseTuple()", "Py_BuildValue()", "PyObject_CallFunction()",
  etc.) without "PY_SSIZE_T_CLEAN" defined raises "DeprecationWarning"
  now. It will be removed in 3.10 or 4.0.  Read 引数の解釈と値の構築
  for detail. (Contributed by Inada Naoki in bpo-36381.)

* Instances of heap-allocated types (such as those created with
  "PyType_FromSpec()") hold a reference to their type object.
  Increasing the reference count of these type objects has been moved
  from "PyType_GenericAlloc()" to the more low-level functions,
  "PyObject_Init()" and "PyObject_INIT()". This makes types created
  through "PyType_FromSpec()" behave like other classes in managed
  code.

  Statically allocated types are not affected.

  For the vast majority of cases, there should be no side effect.
  However, types that manually increase the reference count after
  allocating an instance (perhaps to work around the bug) may now
  become immortal. To avoid this, these classes need to call Py_DECREF
  on the type object during instance deallocation.

  To correctly port these types into 3.8, please apply the following
  changes:

  * Remove "Py_INCREF" on the type object after allocating an instance
    - if any. This may happen after calling "PyObject_New()",
    "PyObject_NewVar()", "PyObject_GC_New()", "PyObject_GC_NewVar()",
    or any other custom allocator that uses "PyObject_Init()" or
    "PyObject_INIT()".

    例:

       static foo_struct *
       foo_new(PyObject *type) {
           foo_struct *foo = PyObject_GC_New(foo_struct, (PyTypeObject *) type);
           if (foo == NULL)
               return NULL;
       #if PY_VERSION_HEX < 0x03080000
           // Workaround for Python issue 35810; no longer necessary in Python 3.8
           PY_INCREF(type)
       #endif
           return foo;
       }

  * Ensure that all custom "tp_dealloc" functions of heap-allocated
    types decrease the type's reference count.

    例:

       static void
       foo_dealloc(foo_struct *instance) {
           PyObject *type = Py_TYPE(instance);
           PyObject_GC_Del(instance);
       #if PY_VERSION_HEX >= 0x03080000
           // This was not needed before Python 3.8 (Python issue 35810)
           Py_DECREF(type);
       #endif
       }

  (Contributed by Eddie Elizondo in bpo-35810.)

* The "Py_DEPRECATED()" macro has been implemented for MSVC. The macro
  now must be placed before the symbol name.

  例:

     Py_DEPRECATED(3.8) PyAPI_FUNC(int) Py_OldFunction(void);

  (Contributed by Zackery Spytz in bpo-33407.)

* The interpreter does not pretend to support binary compatibility of
  extension types across feature releases, anymore.  A "PyTypeObject"
  exported by a third-party extension module is supposed to have all
  the slots expected in the current Python version, including
  "tp_finalize" ("Py_TPFLAGS_HAVE_FINALIZE" is not checked anymore
  before reading "tp_finalize").

  (Contributed by Antoine Pitrou in bpo-32388.)

* The functions "PyNode_AddChild()" and "PyParser_AddToken()" now
  accept two additional "int" arguments *end_lineno* and
  *end_col_offset*.

* The "libpython38.a" file to allow MinGW tools to link directly
  against "python38.dll" is no longer included in the regular Windows
  distribution. If you require this file, it may be generated with the
  "gendef" and "dlltool" tools, which are part of the MinGW binutils
  package:

     gendef - python38.dll > tmp.def
     dlltool --dllname python38.dll --def tmp.def --output-lib libpython38.a

  The location of an installed "pythonXY.dll" will depend on the
  installation options and the version and language of Windows. See
  Windows で Python を使う for more information. The resulting library
  should be placed in the same directory as "pythonXY.lib", which is
  generally the "libs" directory under your Python installation.

  (Contributed by Steve Dower in bpo-37351.)


CPython バイトコードの変更
--------------------------

* The interpreter loop  has been simplified by moving the logic of
  unrolling the stack of blocks into the compiler.  The compiler emits
  now explicit instructions for adjusting the stack of values and
  calling the cleaning-up code for "break", "continue" and "return".

  Removed opcodes "BREAK_LOOP", "CONTINUE_LOOP", "SETUP_LOOP" and
  "SETUP_EXCEPT".  Added new opcodes "ROT_FOUR", "BEGIN_FINALLY",
  "CALL_FINALLY" and "POP_FINALLY".  Changed the behavior of
  "END_FINALLY" and "WITH_CLEANUP_START".

  (Contributed by Mark Shannon, Antoine Pitrou and Serhiy Storchaka in
  bpo-17611.)

* Added new opcode "END_ASYNC_FOR" for handling exceptions raised when
  awaiting a next item in an "async for" loop. (Contributed by Serhiy
  Storchaka in bpo-33041.)

* The "MAP_ADD" now expects the value as the first element in the
  stack and the key as the second element. This change was made so the
  key is always evaluated before the value in dictionary
  comprehensions, as proposed by **PEP 572**. (Contributed by Jörn
  Heissler in bpo-35224.)


Demos and Tools
---------------

Added a benchmark script for timing various ways to access variables:
"Tools/scripts/var_access_benchmark.py". (Contributed by Raymond
Hettinger in bpo-35884.)

Here's a summary of performance improvements since Python 3.3:

   Python version                       3.3     3.4     3.5     3.6     3.7     3.8
   --------------                       ---     ---     ---     ---     ---     ---

   Variable and attribute read access:
       read_local                       4.0     7.1     7.1     5.4     5.1     3.9
       read_nonlocal                    5.3     7.1     8.1     5.8     5.4     4.4
       read_global                     13.3    15.5    19.0    14.3    13.6     7.6
       read_builtin                    20.0    21.1    21.6    18.5    19.0     7.5
       read_classvar_from_class        20.5    25.6    26.5    20.7    19.5    18.4
       read_classvar_from_instance     18.5    22.8    23.5    18.8    17.1    16.4
       read_instancevar                26.8    32.4    33.1    28.0    26.3    25.4
       read_instancevar_slots          23.7    27.8    31.3    20.8    20.8    20.2
       read_namedtuple                 68.5    73.8    57.5    45.0    46.8    18.4
       read_boundmethod                29.8    37.6    37.9    29.6    26.9    27.7

   Variable and attribute write access:
       write_local                      4.6     8.7     9.3     5.5     5.3     4.3
       write_nonlocal                   7.3    10.5    11.1     5.6     5.5     4.7
       write_global                    15.9    19.7    21.2    18.0    18.0    15.8
       write_classvar                  81.9    92.9    96.0   104.6   102.1    39.2
       write_instancevar               36.4    44.6    45.8    40.0    38.9    35.5
       write_instancevar_slots         28.7    35.6    36.1    27.3    26.6    25.7

   Data structure read access:
       read_list                       19.2    24.2    24.5    20.8    20.8    19.0
       read_deque                      19.9    24.7    25.5    20.2    20.6    19.8
       read_dict                       19.7    24.3    25.7    22.3    23.0    21.0
       read_strdict                    17.9    22.6    24.3    19.5    21.2    18.9

   Data structure write access:
       write_list                      21.2    27.1    28.5    22.5    21.6    20.0
       write_deque                     23.8    28.7    30.1    22.7    21.8    23.5
       write_dict                      25.9    31.4    33.3    29.3    29.2    24.7
       write_strdict                   22.9    28.4    29.9    27.5    25.2    23.1

   Stack (or queue) operations:
       list_append_pop                144.2    93.4   112.7    75.4    74.2    50.8
       deque_append_pop                30.4    43.5    57.0    49.4    49.2    42.5
       deque_append_popleft            30.8    43.7    57.3    49.7    49.7    42.8

   Timing loop:
       loop_overhead                    0.3     0.5     0.6     0.4     0.3     0.3

The benchmarks were measured on an Intel® Core™ i7-4960HQ processor
running the macOS 64-bit builds found at python.org. The benchmark
script displays timings in nanoseconds.


Python3.8.1 での重要な変更点
============================

セキュリティ上の重大な懸念により、
"asyncio.loop.create_datagram_endpoint()" での *reuse_address* 引数は
無効になりました。これはソケットオプション *SO_REUSEADDR`* のUDP にお
ける挙動が原因です。詳しくは、"loop.create_datagram_endpoint()" のドキ
ュメントを参照してください。(Kyle Stanley, Antoine Pitrou, Yury
Selivanov による貢献 bpo-37228)


Python3.8.8 での重要な変更点
============================

Earlier Python versions allowed using both ";" and "&" as query
parameter separators in "urllib.parse.parse_qs()" and
"urllib.parse.parse_qsl()".  Due to security concerns, and to conform
with newer W3C recommendations, this has been changed to allow only a
single separator key, with "&" as the default.  This change also
affects "cgi.parse()" and "cgi.parse_multipart()" as they use the
affected functions internally. For more details, please see their
respective documentation. (Contributed by Adam Goldschmidt, Senthil
Kumaran and Ken Jin in bpo-42967.)


Notable changes in Python 3.8.12
================================

Starting with Python 3.8.12 the "ipaddress" module no longer accepts
any leading zeros in IPv4 address strings. Leading zeros are ambiguous
and interpreted as octal notation by some libraries. For example the
legacy function "socket.inet_aton()" treats leading zeros as octal
notation. glibc implementation of modern "inet_pton()" does not accept
any leading zeros.

(Originally contributed by Christian Heimes in bpo-36384, and
backported to 3.8 by Achraf Merzouki.)
