8. 複合文 (compound statement)

複合文には、他の文 (のグループ) が入ります; 複合文は、中に入っている他の文の実行の制御に何らかのやり方で影響を及ぼします。一般的には、複合文は複数行にまたがって書かれますが、全部の文を一行に連ねた単純な書き方もあります。

if 、 while 、および for 文は、伝統的な制御フロー構成を実現します。 try は例外処理および/または一連の文に対するクリーンアップコードを指定します。それに対して、 with 文はコードのかたまりの前後でコードの初期化と終了処理を実行できるようにします。関数とクラス定義もまた、構文的には複合文です。

複合文は、一つ以上の '節 (clause)' からなります。節は、ヘッダと 'スイート (suite)' からなります。一つの複合文を成す各節のヘッダは、全て同じインデントレベルに置かれます。各節のヘッダは一意に識別するキーワードで始まり、コロンで終わります。スイートは、節によって制御される文の集まりです。スイートは、ヘッダがある行のコロンの後にセミコロンで区切って置かれた一つ以上の単純文、または、ヘッダに続く行で一つ多くインデントされた文の集まりです。後者の形式のスイートに限り、さらに複合文をネストできます; 以下の文は、 else 節がどちらの if 節に属するかがはっきりしないなどの理由から不正になります:

if test1: if test2: print(x)

また、このコンテキスト中では、セミコロンによる結合はコロンより強いです。従って、以下の例では、 print() の呼び出しはは全て実行されるか、全く実行されないかのどちらかです:

if x < y < z: print(x); print(y); print(z)

まとめると、以下のようになります:

compound_stmt ::=  if_stmt
                   | while_stmt
                   | for_stmt
                   | try_stmt
                   | with_stmt
                   | match_stmt
                   | funcdef
                   | classdef
                   | async_with_stmt
                   | async_for_stmt
                   | async_funcdef
suite         ::=  stmt_list NEWLINE | NEWLINE INDENT statement+ DEDENT
statement     ::=  stmt_list NEWLINE | compound_stmt
stmt_list     ::=  simple_stmt (";" simple_stmt)* [";"]

なお、文は常に NEWLINE か、その後に DEDENT が続いたもので終了します。また、オプションの継続節は必ず、文を開始できない予約語で始まるので、曖昧さは存在しません。 (Python では、 'ぶら下がり (dangling) else' 問題は、ネストされた if 文をインデントさせることで解決されます)。

以下の節における文法規則の記述方式は、明確さのために、各節を別々の行に書くようにしています。

8.1. if 文

if 文は、条件分岐を実行するために使われます:

if_stmt ::=  "if" assignment_expression ":" suite
             ("elif" assignment_expression ":" suite)*
             ["else" ":" suite]

if 文は、式を一つ一つ評価してゆき、真になるまで続けて、真になった節のスイートだけを選択します (真: true と偽: false の定義については、 ブール演算 (boolean operation) 節を参照してください); 次に、選択したスイートを実行します (そして、 if 文の他の部分は、実行や評価をされません)。全ての式が偽になった場合、 else 節があれば、そのスイートが実行されます。

8.2. while 文

while 文は、式の値が真である間、実行を繰り返すために使われます:

while_stmt ::=  "while" assignment_expression ":" suite
                ["else" ":" suite]

while 文は式を繰り返し真偽評価し、真であれば最初のスイートを実行します。式が偽であれば (最初から偽になっていることもありえます)、 else 節がある場合にはそれを実行し、ループを終了します。

最初のスイート内で break 文が実行されると、 else 節のスイートを実行することなくループを終了します。 continue 文が最初のスイート内で実行されると、スイート内にある残りの文の実行をスキップして、式の真偽評価に戻ります。

8.3. for 文

for 文は、シーケンス (文字列、タプルまたはリスト) や、その他の反復可能なオブジェクト (iterable object) 内の要素に渡って反復処理を行うために使われます:

for_stmt ::=  "for" target_list "in" starred_list ":" suite
              ["else" ":" suite]

The starred_list expression is evaluated once; it should yield an iterable object. An iterator is created for that iterable. The first item provided by the iterator is then assigned to the target list using the standard rules for assignments (see 代入文 (assignment statement)), and the suite is executed. This repeats for each item provided by the iterator. When the iterator is exhausted, the suite in the else clause, if present, is executed, and the loop terminates.

最初のスイートの中で break 文が実行されると、 else 節のスイートを実行することなくループを終了します。 continue 文が最初のスイート内で実行されると、スイート内にある残りの文の実行をスキップして、次の要素の処理に移るか、これ以上次の要素が無い場合は else 節の処理に移ります。

for ループはターゲットリスト内の変数への代入を行います。 これにより、for ループ内も含めて、それ以前の全ての代入は上書きされます:

for i in range(10):
    print(i)
    i = 5             # this will not affect the for-loop
                      # because i will be overwritten with the next
                      # index in the range

Names in the target list are not deleted when the loop is finished, but if the sequence is empty, they will not have been assigned to at all by the loop. Hint: the built-in type range() represents immutable arithmetic sequences of integers. For instance, iterating range(3) successively yields 0, 1, and then 2.

バージョン 3.11 で変更: Starred elements are now allowed in the expression list.

8.4. try 文

The try statement specifies exception handlers and/or cleanup code for a group of statements:

try_stmt  ::=  try1_stmt | try2_stmt | try3_stmt
try1_stmt ::=  "try" ":" suite
               ("except" [expression ["as" identifier]] ":" suite)+
               ["else" ":" suite]
               ["finally" ":" suite]
try2_stmt ::=  "try" ":" suite
               ("except" "*" expression ["as" identifier] ":" suite)+
               ["else" ":" suite]
               ["finally" ":" suite]
try3_stmt ::=  "try" ":" suite
               "finally" ":" suite

例外に関するその他の情報は 例外 節にあります。また、 raise 文の使用による例外の生成に関する情報は、 raise 文 節にあります。

8.4.1. except clause

The except clause(s) specify one or more exception handlers. When no exception occurs in the try clause, no exception handler is executed. When an exception occurs in the try suite, a search for an exception handler is started. This search inspects the except clauses in turn until one is found that matches the exception. An expression-less except clause, if present, must be last; it matches any exception. For an except clause with an expression, that expression is evaluated, and the clause matches the exception if the resulting object is "compatible" with the exception. An object is compatible with an exception if the object is the class or a non-virtual base class of the exception object, or a tuple containing an item that is the class or a non-virtual base class of the exception object.

If no except clause matches the exception, the search for an exception handler continues in the surrounding code and on the invocation stack. 1

If the evaluation of an expression in the header of an except clause raises an exception, the original search for a handler is canceled and a search starts for the new exception in the surrounding code and on the call stack (it is treated as if the entire try statement raised the exception).

When a matching except clause is found, the exception is assigned to the target specified after the as keyword in that except clause, if present, and the except clause's suite is executed. All except clauses must have an executable block. When the end of this block is reached, execution continues normally after the entire try statement. (This means that if two nested handlers exist for the same exception, and the exception occurs in the try clause of the inner handler, the outer handler will not handle the exception.)

When an exception has been assigned using as target, it is cleared at the end of the except clause. This is as if

except E as N:
    foo

が、以下のコードに翻訳されたかのようなものです:

except E as N:
    try:
        foo
    finally:
        del N

This means the exception must be assigned to a different name to be able to refer to it after the except clause. Exceptions are cleared because with the traceback attached to them, they form a reference cycle with the stack frame, keeping all locals in that frame alive until the next garbage collection occurs.

Before an except clause's suite is executed, details about the exception are stored in the sys module and can be accessed via sys.exc_info(). sys.exc_info() returns a 3-tuple consisting of the exception class, the exception instance and a traceback object (see section 標準型の階層) identifying the point in the program where the exception occurred. The details about the exception accessed via sys.exc_info() are restored to their previous values when leaving an exception handler:

>>> print(sys.exc_info())
(None, None, None)
>>> try:
...     raise TypeError
... except:
...     print(sys.exc_info())
...     try:
...          raise ValueError
...     except:
...         print(sys.exc_info())
...     print(sys.exc_info())
...
(<class 'TypeError'>, TypeError(), <traceback object at 0x10efad080>)
(<class 'ValueError'>, ValueError(), <traceback object at 0x10efad040>)
(<class 'TypeError'>, TypeError(), <traceback object at 0x10efad080>)
>>> print(sys.exc_info())
(None, None, None)

8.4.2. except* clause

The except* clause(s) are used for handling ExceptionGroups. The exception type for matching is interpreted as in the case of except, but in the case of exception groups we can have partial matches when the type matches some of the exceptions in the group. This means that multiple except* clauses can execute, each handling part of the exception group. Each clause executes at most once and handles an exception group of all matching exceptions. Each exception in the group is handled by at most one except* clause, the first that matches it.

>>> try:
...     raise ExceptionGroup("eg",
...         [ValueError(1), TypeError(2), OSError(3), OSError(4)])
... except* TypeError as e:
...     print(f'caught {type(e)} with nested {e.exceptions}')
... except* OSError as e:
...     print(f'caught {type(e)} with nested {e.exceptions}')
...
caught <class 'ExceptionGroup'> with nested (TypeError(2),)
caught <class 'ExceptionGroup'> with nested (OSError(3), OSError(4))
  + Exception Group Traceback (most recent call last):
  |   File "<stdin>", line 2, in <module>
  | ExceptionGroup: eg
  +-+---------------- 1 ----------------
    | ValueError: 1
    +------------------------------------

Any remaining exceptions that were not handled by any except* clause are re-raised at the end, combined into an exception group along with all exceptions that were raised from within except* clauses.

If the raised exception is not an exception group and its type matches one of the except* clauses, it is caught and wrapped by an exception group with an empty message string.

>>> try:
...     raise BlockingIOError
... except* BlockingIOError as e:
...     print(repr(e))
...
ExceptionGroup('', (BlockingIOError()))

An except* clause must have a matching type, and this type cannot be a subclass of BaseExceptionGroup. It is not possible to mix except and except* in the same try. break, continue and return cannot appear in an except* clause.

8.4.3. else clause

オプションの else 節は、コントロールフローが try スイートを抜け、例外が送出されず、 return 文、 continue 文、 break 文のいずれもが実行されなかった場合に実行されます。 else 節で起きた例外は、手前にある except 節では処理されません。

8.4.4. finally clause

If finally is present, it specifies a 'cleanup' handler. The try clause is executed, including any except and else clauses. If an exception occurs in any of the clauses and is not handled, the exception is temporarily saved. The finally clause is executed. If there is a saved exception it is re-raised at the end of the finally clause. If the finally clause raises another exception, the saved exception is set as the context of the new exception. If the finally clause executes a return, break or continue statement, the saved exception is discarded:

>>> def f():
...     try:
...         1/0
...     finally:
...         return 42
...
>>> f()
42

The exception information is not available to the program during execution of the finally clause.

When a return, break or continue statement is executed in the try suite of a try...finally statement, the finally clause is also executed 'on the way out.'

The return value of a function is determined by the last return statement executed. Since the finally clause always executes, a return statement executed in the finally clause will always be the last one executed:

>>> def foo():
...     try:
...         return 'try'
...     finally:
...         return 'finally'
...
>>> foo()
'finally'

バージョン 3.8 で変更: Prior to Python 3.8, a continue statement was illegal in the finally clause due to a problem with the implementation.

8.5. with 文

with 文は、ブロックの実行を、コンテキストマネージャによって定義されたメソッドでラップするために使われます (with文とコンテキストマネージャ セクションを参照してください)。これにより、よくある try...except...finally 利用パターンをカプセル化して便利に再利用することができます。

with_stmt          ::=  "with" ( "(" with_stmt_contents ","? ")" | with_stmt_contents ) ":" suite
with_stmt_contents ::=  with_item ("," with_item)*
with_item          ::=  expression ["as" target]

一つの "要素" を持つ with 文の実行は以下のように進行します:

1. The context expression (the expression given in the with_item) is evaluated to obtain a context manager.

2. コンテキストマネージャの __enter__() メソッドが、後で使うためにロードされます。

3. コンテキストマネージャの __exit__() メソッドが、後で使うためにロードされます。

4. コンテキストマネージャの __enter__() メソッドが呼ばれます。

5. with 文にターゲットが含まれていたら、それに __enter__() からの戻り値が代入されます。

   注釈

   The with statement guarantees that if the __enter__() method returns without an error, then __exit__() will always be called. Thus, if an error occurs during the assignment to the target list, it will be treated the same as an error occurring within the suite would be. See step 7 below.

6. スイートが実行されます。

7. コンテキストマネージャの __exit__() メソッドが呼ばれます。スイートが例外によって終了されたのなら、その例外の型、値、トレースバックが __exit__() に引数として渡されます。そうでなければ、 3 つの None 引数が与えられます。

   スイートが例外により終了され、 __exit__() メソッドからの戻り値が偽（false）ならば、例外が再送出されます。この戻り値が真（true）ならば例外は抑制され、実行は with 文の次の文から続きます。

   もしそのスイートが例外でない何らかの理由で終了した場合、その __exit__() からの戻り値は無視されて、実行は発生した終了の種類に応じた通常の位置から継続します。

以下のコード:

with EXPRESSION as TARGET:
    SUITE

これは次と等価です:

manager = (EXPRESSION)
enter = type(manager).__enter__
exit = type(manager).__exit__
value = enter(manager)
hit_except = False

try:
    TARGET = value
    SUITE
except:
    hit_except = True
    if not exit(manager, *sys.exc_info()):
        raise
finally:
    if not hit_except:
        exit(manager, None, None, None)

複数の要素があるとき、コンテキストマネージャは複数の with 文がネストされたかのように進行します:

with A() as a, B() as b:
    SUITE

これは次と等価です:

with A() as a:
    with B() as b:
        SUITE

You can also write multi-item context managers in multiple lines if the items are surrounded by parentheses. For example:

with (
    A() as a,
    B() as b,
):
    SUITE

バージョン 3.1 で変更: 複数のコンテキスト式をサポートしました。

バージョン 3.10 で変更: Support for using grouping parentheses to break the statement in multiple lines.

参考

PEP 343 - "with" ステートメント

Python の with 文の仕様、背景、および例が記載されています。

8.6. The match statement

バージョン 3.10 で追加.

The match statement is used for pattern matching. Syntax:

match_stmt   ::=  'match' subject_expr ":" NEWLINE INDENT case_block+ DEDENT
subject_expr ::=  star_named_expression "," star_named_expressions?
                  | named_expression
case_block   ::=  'case' patterns [guard] ":" block

注釈

This section uses single quotes to denote soft keywords.

Pattern matching takes a pattern as input (following case) and a subject value (following match). The pattern (which may contain subpatterns) is matched against the subject value. The outcomes are:

• A match success or failure (also termed a pattern success or failure).

• Possible binding of matched values to a name. The prerequisites for this are further discussed below.

The match and case keywords are soft keywords.

参考

• PEP 634 -- Structural Pattern Matching: Specification

• PEP 636 -- Structural Pattern Matching: Tutorial

8.6.1. 概要

Here's an overview of the logical flow of a match statement:

1. The subject expression subject_expr is evaluated and a resulting subject value obtained. If the subject expression contains a comma, a tuple is constructed using the standard rules.

2. Each pattern in a case_block is attempted to match with the subject value. The specific rules for success or failure are described below. The match attempt can also bind some or all of the standalone names within the pattern. The precise pattern binding rules vary per pattern type and are specified below. Name bindings made during a successful pattern match outlive the executed block and can be used after the match statement.

   注釈

   During failed pattern matches, some subpatterns may succeed. Do not rely on bindings being made for a failed match. Conversely, do not rely on variables remaining unchanged after a failed match. The exact behavior is dependent on implementation and may vary. This is an intentional decision made to allow different implementations to add optimizations.

3. If the pattern succeeds, the corresponding guard (if present) is evaluated. In this case all name bindings are guaranteed to have happened.

   • If the guard evaluates as true or is missing, the block inside case_block is executed.

   • Otherwise, the next case_block is attempted as described above.

   • If there are no further case blocks, the match statement is completed.

注釈

Users should generally never rely on a pattern being evaluated. Depending on implementation, the interpreter may cache values or use other optimizations which skip repeated evaluations.

A sample match statement:

>>> flag = False
>>> match (100, 200):
...    case (100, 300):  # Mismatch: 200 != 300
...        print('Case 1')
...    case (100, 200) if flag:  # Successful match, but guard fails
...        print('Case 2')
...    case (100, y):  # Matches and binds y to 200
...        print(f'Case 3, y: {y}')
...    case _:  # Pattern not attempted
...        print('Case 4, I match anything!')
...
Case 3, y: 200

In this case, if flag is a guard. Read more about that in the next section.

8.6.2. Guards

guard ::=  "if" named_expression

A guard (which is part of the case) must succeed for code inside the case block to execute. It takes the form: if followed by an expression.

The logical flow of a case block with a guard follows:

1. Check that the pattern in the case block succeeded. If the pattern failed, the guard is not evaluated and the next case block is checked.

2. If the pattern succeeded, evaluate the guard.

   • If the guard condition evaluates as true, the case block is selected.

   • If the guard condition evaluates as false, the case block is not selected.

   • If the guard raises an exception during evaluation, the exception bubbles up.

Guards are allowed to have side effects as they are expressions. Guard evaluation must proceed from the first to the last case block, one at a time, skipping case blocks whose pattern(s) don't all succeed. (I.e., guard evaluation must happen in order.) Guard evaluation must stop once a case block is selected.

8.6.3. Irrefutable Case Blocks

An irrefutable case block is a match-all case block. A match statement may have at most one irrefutable case block, and it must be last.

A case block is considered irrefutable if it has no guard and its pattern is irrefutable. A pattern is considered irrefutable if we can prove from its syntax alone that it will always succeed. Only the following patterns are irrefutable:

• AS Patterns whose left-hand side is irrefutable

• OR Patterns containing at least one irrefutable pattern

• Capture Patterns

• Wildcard Patterns

• parenthesized irrefutable patterns

8.6.4. Patterns

注釈

This section uses grammar notations beyond standard EBNF:

• the notation SEP.RULE+ is shorthand for RULE (SEP RULE)*

• the notation !RULE is shorthand for a negative lookahead assertion

The top-level syntax for patterns is:

patterns       ::=  open_sequence_pattern | pattern
pattern        ::=  as_pattern | or_pattern
closed_pattern ::=  | literal_pattern
                    | capture_pattern
                    | wildcard_pattern
                    | value_pattern
                    | group_pattern
                    | sequence_pattern
                    | mapping_pattern
                    | class_pattern

The descriptions below will include a description "in simple terms" of what a pattern does for illustration purposes (credits to Raymond Hettinger for a document that inspired most of the descriptions). Note that these descriptions are purely for illustration purposes and may not reflect the underlying implementation. Furthermore, they do not cover all valid forms.

8.6.4.1. OR Patterns

An OR pattern is two or more patterns separated by vertical bars |. Syntax:

or_pattern ::=  "|".closed_pattern+

Only the final subpattern may be irrefutable, and each subpattern must bind the same set of names to avoid ambiguity.

An OR pattern matches each of its subpatterns in turn to the subject value, until one succeeds. The OR pattern is then considered successful. Otherwise, if none of the subpatterns succeed, the OR pattern fails.

In simple terms, P1 | P2 | ... will try to match P1, if it fails it will try to match P2, succeeding immediately if any succeeds, failing otherwise.

8.6.4.2. AS Patterns

An AS pattern matches an OR pattern on the left of the as keyword against a subject. Syntax:

as_pattern ::=  or_pattern "as" capture_pattern

If the OR pattern fails, the AS pattern fails. Otherwise, the AS pattern binds the subject to the name on the right of the as keyword and succeeds. capture_pattern cannot be a a _.

In simple terms P as NAME will match with P, and on success it will set NAME = <subject>.

8.6.4.3. Literal Patterns

A literal pattern corresponds to most literals in Python. Syntax:

literal_pattern ::=  signed_number
                     | signed_number "+" NUMBER
                     | signed_number "-" NUMBER
                     | strings
                     | "None"
                     | "True"
                     | "False"
                     | signed_number: NUMBER | "-" NUMBER

The rule strings and the token NUMBER are defined in the standard Python grammar. Triple-quoted strings are supported. Raw strings and byte strings are supported. フォーマット済み文字列リテラル are not supported.

The forms signed_number '+' NUMBER and signed_number '-' NUMBER are for expressing complex numbers; they require a real number on the left and an imaginary number on the right. E.g. 3 + 4j.

In simple terms, LITERAL will succeed only if <subject> == LITERAL. For the singletons None, True and False, the is operator is used.

8.6.4.4. Capture Patterns

A capture pattern binds the subject value to a name. Syntax:

capture_pattern ::=  !'_' NAME

A single underscore _ is not a capture pattern (this is what !'_' expresses). It is instead treated as a wildcard_pattern.

In a given pattern, a given name can only be bound once. E.g. case x, x: ... is invalid while case [x] | x: ... is allowed.

Capture patterns always succeed. The binding follows scoping rules established by the assignment expression operator in PEP 572; the name becomes a local variable in the closest containing function scope unless there's an applicable global or nonlocal statement.

In simple terms NAME will always succeed and it will set NAME = <subject>.

8.6.4.5. Wildcard Patterns

A wildcard pattern always succeeds (matches anything) and binds no name. Syntax:

wildcard_pattern ::=  '_'

_ is a soft keyword within any pattern, but only within patterns. It is an identifier, as usual, even within match subject expressions, guards, and case blocks.

In simple terms, _ will always succeed.

8.6.4.6. Value Patterns

A value pattern represents a named value in Python. Syntax:

value_pattern ::=  attr
attr          ::=  name_or_attr "." NAME
name_or_attr  ::=  attr | NAME

The dotted name in the pattern is looked up using standard Python name resolution rules. The pattern succeeds if the value found compares equal to the subject value (using the == equality operator).

In simple terms NAME1.NAME2 will succeed only if <subject> == NAME1.NAME2

注釈

If the same value occurs multiple times in the same match statement, the interpreter may cache the first value found and reuse it rather than repeat the same lookup. This cache is strictly tied to a given execution of a given match statement.

8.6.4.7. Group Patterns

A group pattern allows users to add parentheses around patterns to emphasize the intended grouping. Otherwise, it has no additional syntax. Syntax:

group_pattern ::=  "(" pattern ")"

In simple terms (P) has the same effect as P.

8.6.4.8. Sequence Patterns

A sequence pattern contains several subpatterns to be matched against sequence elements. The syntax is similar to the unpacking of a list or tuple.

sequence_pattern       ::=  "[" [maybe_sequence_pattern] "]"
                            | "(" [open_sequence_pattern] ")"
open_sequence_pattern  ::=  maybe_star_pattern "," [maybe_sequence_pattern]
maybe_sequence_pattern ::=  ",".maybe_star_pattern+ ","?
maybe_star_pattern     ::=  star_pattern | pattern
star_pattern           ::=  "*" (capture_pattern | wildcard_pattern)

There is no difference if parentheses or square brackets are used for sequence patterns (i.e. (...) vs [...] ).

注釈

A single pattern enclosed in parentheses without a trailing comma (e.g. (3 | 4)) is a group pattern. While a single pattern enclosed in square brackets (e.g. [3 | 4]) is still a sequence pattern.

At most one star subpattern may be in a sequence pattern. The star subpattern may occur in any position. If no star subpattern is present, the sequence pattern is a fixed-length sequence pattern; otherwise it is a variable-length sequence pattern.

The following is the logical flow for matching a sequence pattern against a subject value:

1. If the subject value is not a sequence 2, the sequence pattern fails.

2. If the subject value is an instance of str, bytes or bytearray the sequence pattern fails.

3. The subsequent steps depend on whether the sequence pattern is fixed or variable-length.

   If the sequence pattern is fixed-length:

   1. If the length of the subject sequence is not equal to the number of subpatterns, the sequence pattern fails

   2. Subpatterns in the sequence pattern are matched to their corresponding items in the subject sequence from left to right. Matching stops as soon as a subpattern fails. If all subpatterns succeed in matching their corresponding item, the sequence pattern succeeds.

   Otherwise, if the sequence pattern is variable-length:

   1. If the length of the subject sequence is less than the number of non-star subpatterns, the sequence pattern fails.

   2. The leading non-star subpatterns are matched to their corresponding items as for fixed-length sequences.

   3. If the previous step succeeds, the star subpattern matches a list formed of the remaining subject items, excluding the remaining items corresponding to non-star subpatterns following the star subpattern.

   4. Remaining non-star subpatterns are matched to their corresponding subject items, as for a fixed-length sequence.

   注釈

   The length of the subject sequence is obtained via len() (i.e. via the __len__() protocol). This length may be cached by the interpreter in a similar manner as value patterns.

In simple terms [P1, P2, P3, ... , P<N>] matches only if all the following happens:

• check <subject> is a sequence

• len(subject) == <N>

• P1 matches <subject>[0] (note that this match can also bind names)

• P2 matches <subject>[1] (note that this match can also bind names)

• ... and so on for the corresponding pattern/element.

8.6.4.9. Mapping Patterns

A mapping pattern contains one or more key-value patterns. The syntax is similar to the construction of a dictionary. Syntax:

mapping_pattern     ::=  "{" [items_pattern] "}"
items_pattern       ::=  ",".key_value_pattern+ ","?
key_value_pattern   ::=  (literal_pattern | value_pattern) ":" pattern
                         | double_star_pattern
double_star_pattern ::=  "**" capture_pattern

At most one double star pattern may be in a mapping pattern. The double star pattern must be the last subpattern in the mapping pattern.

Duplicate keys in mapping patterns are disallowed. Duplicate literal keys will raise a SyntaxError. Two keys that otherwise have the same value will raise a ValueError at runtime.

The following is the logical flow for matching a mapping pattern against a subject value:

1. If the subject value is not a mapping 3,the mapping pattern fails.

2. If every key given in the mapping pattern is present in the subject mapping, and the pattern for each key matches the corresponding item of the subject mapping, the mapping pattern succeeds.

3. If duplicate keys are detected in the mapping pattern, the pattern is considered invalid. A SyntaxError is raised for duplicate literal values; or a ValueError for named keys of the same value.

注釈

Key-value pairs are matched using the two-argument form of the mapping subject's get() method. Matched key-value pairs must already be present in the mapping, and not created on-the-fly via __missing__() or __getitem__().

In simple terms {KEY1: P1, KEY2: P2, ... } matches only if all the following happens:

• check <subject> is a mapping

• KEY1 in <subject>

• P1 matches <subject>[KEY1]

• ... and so on for the corresponding KEY/pattern pair.

8.6.4.10. Class Patterns

A class pattern represents a class and its positional and keyword arguments (if any). Syntax:

class_pattern       ::=  name_or_attr "(" [pattern_arguments ","?] ")"
pattern_arguments   ::=  positional_patterns ["," keyword_patterns]
                         | keyword_patterns
positional_patterns ::=  ",".pattern+
keyword_patterns    ::=  ",".keyword_pattern+
keyword_pattern     ::=  NAME "=" pattern

The same keyword should not be repeated in class patterns.

The following is the logical flow for matching a class pattern against a subject value:

1. If name_or_attr is not an instance of the builtin type , raise TypeError.

2. If the subject value is not an instance of name_or_attr (tested via isinstance()), the class pattern fails.

3. If no pattern arguments are present, the pattern succeeds. Otherwise, the subsequent steps depend on whether keyword or positional argument patterns are present.

   For a number of built-in types (specified below), a single positional subpattern is accepted which will match the entire subject; for these types keyword patterns also work as for other types.

   If only keyword patterns are present, they are processed as follows, one by one:

   I. The keyword is looked up as an attribute on the subject.

   • If this raises an exception other than AttributeError, the exception bubbles up.

   • If this raises AttributeError, the class pattern has failed.

   • Else, the subpattern associated with the keyword pattern is matched against the subject's attribute value. If this fails, the class pattern fails; if this succeeds, the match proceeds to the next keyword.

   II. If all keyword patterns succeed, the class pattern succeeds.

   If any positional patterns are present, they are converted to keyword patterns using the __match_args__ attribute on the class name_or_attr before matching:

   I. The equivalent of getattr(cls, "__match_args__", ()) is called.

   • If this raises an exception, the exception bubbles up.

   • If the returned value is not a tuple, the conversion fails and TypeError is raised.

   • If there are more positional patterns than len(cls.__match_args__), TypeError is raised.

   • Otherwise, positional pattern i is converted to a keyword pattern using __match_args__[i] as the keyword. __match_args__[i] must be a string; if not TypeError is raised.

   • If there are duplicate keywords, TypeError is raised.

   参考

   クラスパターンマッチの位置引数のカスタマイズ

   II. Once all positional patterns have been converted to keyword patterns,

   the match proceeds as if there were only keyword patterns.

   For the following built-in types the handling of positional subpatterns is different:

   • bool

   • bytearray

   • bytes

   • dict

   • float

   • frozenset

   • int

   • list

   • set

   • str

   • tuple

   These classes accept a single positional argument, and the pattern there is matched against the whole object rather than an attribute. For example int(0|1) matches the value 0, but not the value 0.0.

In simple terms CLS(P1, attr=P2) matches only if the following happens:

• isinstance(<subject>, CLS)

• convert P1 to a keyword pattern using CLS.__match_args__

• For each keyword argument attr=P2:

  • hasattr(<subject>, "attr")

  • P2 matches <subject>.attr

• ... and so on for the corresponding keyword argument/pattern pair.

参考

• PEP 634 -- Structural Pattern Matching: Specification

• PEP 636 -- Structural Pattern Matching: Tutorial

8.7. 関数定義

関数定義は、ユーザ定義関数オブジェクトを定義します (標準型の階層 節参照):

funcdef                   ::=  [decorators] "def" funcname "(" [parameter_list] ")"
                               ["->" expression] ":" suite
decorators                ::=  decorator+
decorator                 ::=  "@" assignment_expression NEWLINE
parameter_list            ::=  defparameter ("," defparameter)* "," "/" ["," [parameter_list_no_posonly]]
                                 | parameter_list_no_posonly
parameter_list_no_posonly ::=  defparameter ("," defparameter)* ["," [parameter_list_starargs]]
                               | parameter_list_starargs
parameter_list_starargs   ::=  "*" [parameter] ("," defparameter)* ["," ["**" parameter [","]]]
                               | "**" parameter [","]
parameter                 ::=  identifier [":" expression]
defparameter              ::=  parameter ["=" expression]
funcname                  ::=  identifier

関数定義は実行可能な文です。関数定義を実行すると、現在のローカルな名前空間内で関数名を関数オブジェクト (関数の実行可能コードをくるむラッパー) に束縛します。この関数オブジェクトには、関数が呼び出された際に使われるグローバルな名前空間として、現在のグローバルな名前空間への参照が入っています。

関数定義は関数本体を実行しません; 関数本体は関数が呼び出された時にのみ実行されます。 4

関数定義は一つ以上の デコレータ 式でラップできます。デコレータ式は関数を定義するとき、関数定義の入っているスコープで評価されます。その結果は、関数オブジェクトを唯一の引数にとる呼び出し可能オブジェクトでなければなりません。関数オブジェクトの代わりに、返された値が関数名に束縛されます。複数のデコレータはネストして適用されます。例えば、以下のようなコード:

@f1(arg)
@f2
def func(): pass

は、だいたい次と等価です

def func(): pass
func = f1(arg)(f2(func))

ただし、前者のコードでは元々の関数を func という名前へ一時的に束縛することはない、というところを除きます。

バージョン 3.9 で変更: Functions may be decorated with any valid assignment_expression. Previously, the grammar was much more restrictive; see PEP 614 for details.

1 つ以上の 仮引数 が parameter = expression の形を取っているとき、関数は "デフォルト引数値" を持つと言います。デフォルト値を持つ仮引数では、呼び出し時にそれに対応する 実引数 は省略でき、その場合は仮引数のデフォルト値が使われます。ある引数がデフォルト値を持っている場合、それ以降 "*" が出てくるまでの引数は全てデフォルト値を持っていなければなりません -- これは文法定義では表現されていない構文的制限です。

Default parameter values are evaluated from left to right when the function definition is executed. This means that the expression is evaluated once, when the function is defined, and that the same "pre-computed" value is used for each call. This is especially important to understand when a default parameter value is a mutable object, such as a list or a dictionary: if the function modifies the object (e.g. by appending an item to a list), the default parameter value is in effect modified. This is generally not what was intended. A way around this is to use None as the default, and explicitly test for it in the body of the function, e.g.:

def whats_on_the_telly(penguin=None):
    if penguin is None:
        penguin = []
    penguin.append("property of the zoo")
    return penguin

Function call semantics are described in more detail in section 呼び出し (call). A function call always assigns values to all parameters mentioned in the parameter list, either from positional arguments, from keyword arguments, or from default values. If the form "*identifier" is present, it is initialized to a tuple receiving any excess positional parameters, defaulting to the empty tuple. If the form "**identifier" is present, it is initialized to a new ordered mapping receiving any excess keyword arguments, defaulting to a new empty mapping of the same type. Parameters after "*" or "*identifier" are keyword-only parameters and may only be passed by keyword arguments. Parameters before "/" are positional-only parameters and may only be passed by positional arguments.

バージョン 3.8 で変更: The / function parameter syntax may be used to indicate positional-only parameters. See PEP 570 for details.

引数には、引数名に続けて ": expression" 形式の アノテーション を付けられます。 *identifier や **identifier の形式でも、すべての引数にはアノテーションをつけられます。 関数には、引数リストの後に "-> expression" 形式の "return" アノテーションをつけられます。これらのアノテーションは、任意の有効な Python の式が使えます。 アノテーションがあっても、関数の意味論は変わりません。 アノテーションの値は、関数オブジェクトの __annotations__ 属性の、引数名をキーとする値として得られます。 __future__ の annotations インポートを使った場合は、アノテーションは実行時には文字列として保持され、これにより評価の遅延が可能になっています。 そうでない場合は、アノテーションは関数定義が実行されたときに評価されます。 このケースでは、アノテーションはソースコードに現れたのとは違う順序で評価されることがあります。

式を即時に使用するために、無名関数 (名前に束縛されていない関数) を作成することもできます。 これは ラムダ (lambda) の節で解説されているラムダ式を使います。 ラムダ式は簡略化された関数定義の簡略表現に過ぎないことに注意してください; "def" 文で定義された関数もラムダ式で作成された関数のように、引数として渡せたり、他の名前に割り当てることができます。 複数の式とアノテーションが実行できるので、 "def" 形式の方がより強力です。

プログラマへのメモ: 関数は第一級オブジェクトです。関数定義内で実行された "def" 文は、返り値や引数として渡せるローカル関数を定義します。ネストした関数内で使われる自由変数は、 def を含んでいる関数のローカル変数にアクセスできます。詳細は 名前づけと束縛 (naming and binding) 節を参照してください。

参考

PEP 3107 - Function Annotations

関数アノテーションの元の仕様書。

PEP 484 - 型ヒント

アノテーションの標準的な意味付けである型ヒントの定義。

PEP 526 - Syntax for Variable Annotations

クラス変数とインスタンス変数を含む変数に型ヒントが宣言できる機能

PEP 563 - アノテーションの遅延評価

実行時にアノテーションを貪欲評価するのではなく文字列形式で保持することによる、アノテーションにおける前方参照のサポート

8.8. クラス定義

クラス定義は、クラスオブジェクトを定義します (標準型の階層 節参照):

classdef    ::=  [decorators] "class" classname [inheritance] ":" suite
inheritance ::=  "(" [argument_list] ")"
classname   ::=  identifier

クラス定義は実行可能な文です。継承リストは通常、基底クラスリストを与えます (より高度な使い方は、 メタクラス を参照してください)。ですから、リストのそれぞれの要素の評価はサブクラス化しても良いクラスであるべきです。継承リストのないクラスは、デフォルトで、基底クラス object を継承するので:

class Foo:
    pass

は、以下と同等です

class Foo(object):
    pass

次にクラスのスイートが、新たな実行フレーム (名前づけと束縛 (naming and binding) を参照してください) 内で、新たに作られたローカル名前空間と元々のグローバル名前空間を使って実行されます (通常、このスイートには主に関数定義が含まれます)。クラスのスイートが実行し終えると、実行フレームは破棄されますが、ローカルな名前空間は保存されます。5 次に、継承リストを基底クラスに、保存されたローカル名前空間を属性値辞書に、それぞれ使ってクラスオブジェクトが生成されます。最後に、もとのローカル名前空間において、クラス名がこのクラスオブジェクトに束縛されます。

クラス本体で属性が定義された順序は新しいクラスの __dict__ に保持されます。 この性質が期待できるのは、クラスが作られた直後かつ定義構文を使って定義されたクラスであるときのみです。

クラス作成は、 メタクラス を利用して大幅にカスタマイズできます。

関数をデコレートするのと同じように、クラスもデコレートすることが出来ます、

@f1(arg)
@f2
class Foo: pass

は、だいたい次と等価です

class Foo: pass
Foo = f1(arg)(f2(Foo))

デコレータ式の評価規則は関数デコレータと同じです。結果はクラス名に束縛されます。

バージョン 3.9 で変更: Classes may be decorated with any valid assignment_expression. Previously, the grammar was much more restrictive; see PEP 614 for details.

プログラマのための注釈: クラス定義内で定義された変数はクラス属性であり、全てのインスタンス間で共有されます。インスタンス属性は、メソッドの中で self.name = value とすることで設定できます。クラス属性もインスタンス属性も "self.name" 表記でアクセスでき、この表記でアクセスしたとき、インスタンス属性は同名のクラス属性を隠蔽します。クラス属性は、インスタンス属性のデフォルト値として使えますが、そこにミュータブルな値を使うと予期せぬ結果につながります。 記述子 を使うと、詳細な実装が異なるインスタンス変数を作成できます。

参考

PEP 3115 - Metaclasses in Python 3000

メタクラスの宣言を現在の文法と、メタクラス付きのクラスがどのように構築されるかの意味論を変更した提案

PEP 3129 - クラスデコレータ

クラスデコレータを追加した提案。 関数デコレータとメソッドデコレータは PEP 318 で導入されました。

8.9. コルーチン

バージョン 3.5 で追加.

8.9.1. コルーチン関数定義

async_funcdef ::=  [decorators] "async" "def" funcname "(" [parameter_list] ")"
                   ["->" expression] ":" suite

Execution of Python coroutines can be suspended and resumed at many points (see coroutine). await expressions, async for and async with can only be used in the body of a coroutine function.

Functions defined with async def syntax are always coroutine functions, even if they do not contain await or async keywords.

コルーチン関数の本体の中で yield from 式を使用すると SyntaxError になります。

コルーチン関数の例:

async def func(param1, param2):
    do_stuff()
    await some_coroutine()

バージョン 3.7 で変更: await and async are now keywords; previously they were only treated as such inside the body of a coroutine function.

8.9.2. async for 文

async_for_stmt ::=  "async" for_stmt

An asynchronous iterable provides an __aiter__ method that directly returns an asynchronous iterator, which can call asynchronous code in its __anext__ method.

The async for statement allows convenient iteration over asynchronous iterables.

以下のコード:

async for TARGET in ITER:
    SUITE
else:
    SUITE2

は意味論的に以下と等価です:

iter = (ITER)
iter = type(iter).__aiter__(iter)
running = True

while running:
    try:
        TARGET = await type(iter).__anext__(iter)
    except StopAsyncIteration:
        running = False
    else:
        SUITE
else:
    SUITE2

See also __aiter__() and __anext__() for details.

コルーチン関数の本体の外で async for 文を使用すると SyntaxError になります。

8.9.3. async with 文

async_with_stmt ::=  "async" with_stmt

asynchronous context manager は、 enter メソッドと exit メソッド内部で実行を一時停止できる context manager です。

以下のコード:

async with EXPRESSION as TARGET:
    SUITE

これは次と等価です:

manager = (EXPRESSION)
aenter = type(manager).__aenter__
aexit = type(manager).__aexit__
value = await aenter(manager)
hit_except = False

try:
    TARGET = value
    SUITE
except:
    hit_except = True
    if not await aexit(manager, *sys.exc_info()):
        raise
finally:
    if not hit_except:
        await aexit(manager, None, None, None)

See also __aenter__() and __aexit__() for details.

コルーチン関数の本体の外で async with 文を使用すると SyntaxError になります。

参考

PEP 492 - async 構文および await 構文付きのコルーチン

コルーチンを Python のまともな独り立ちした概念にし、サポートする構文を追加した提案。

脚注

1

例外は、別の例外を送出するような finally 節が無い場合にのみ呼び出しスタックへ伝わります。新しい例外によって、古い例外は失われます。

2

In pattern matching, a sequence is defined as one of the following:

• a class that inherits from collections.abc.Sequence

• a Python class that has been registered as collections.abc.Sequence

• a builtin class that has its (CPython) Py_TPFLAGS_SEQUENCE bit set

• a class that inherits from any of the above

The following standard library classes are sequences:

• array.array

• collections.deque

• list

• memoryview

• range

• tuple

注釈

Subject values of type str, bytes, and bytearray do not match sequence patterns.

3

In pattern matching, a mapping is defined as one of the following:

• a class that inherits from collections.abc.Mapping

• a Python class that has been registered as collections.abc.Mapping

• a builtin class that has its (CPython) Py_TPFLAGS_MAPPING bit set

• a class that inherits from any of the above

The standard library classes dict and types.MappingProxyType are mappings.

4

関数の本体の最初の文として現われる文字列リテラルは、その関数の __doc__ 属性に変換され、その関数の ドキュメンテーション文字列 になります。

5

クラスの本体の最初の文として現われる文字列リテラルは、その名前空間の __doc__ 要素となり、そのクラスの ドキュメンテーション文字列 になります。