What's New In Python 3.8

編集者

Raymond Hettinger

This article explains the new features in Python 3.8, compared to 3.7. Python 3.8 was released on October 14, 2019. For full details, see the changelog.

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

There is a new function parameter syntax / to indicate that some function parameters must be specified positionally and cannot be used as keyword arguments. This is the same notation shown by help() for C functions annotated with Larry Hastings' Argument Clinic tool.

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。

Release builds and debug builds are now ABI compatible: defining the Py_DEBUG macro no longer implies the Py_TRACE_REFS macro, which introduces the only ABI incompatibility. The Py_TRACE_REFS macro, which adds the sys.getobjects() function and the PYTHONDUMPREFS environment variable, can be set using the new ./configure --with-trace-refs build option. (Contributed by Victor Stinner in 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:

新たな関数:

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:

    (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

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.)