# 8. 복합문(Compound statements)

복합문은 다른 문장들(의 그룹들)을 포함합니다; 어떤 방법으로 그 다른 문장들의 실행에 영향을 주거나 제어합니다. 간단하게 표현할 때, 전체 복합문을 한 줄로 쓸 수 있기는 하지만, 일반적으로 복합문은 여러 줄에 걸칩니다.

`if`, `while`, `for` 문장은 전통적인 제어 흐름 구조를 구현합니다. 문장들의 그룹에 대해 `try` 는 예외 처리기나 정리(cleanup) 코드 또는 그 둘 모두를 지정하는 반면, `with` 문은 코드 블록 주변으로 초기화와 파이널리제이션 코드를 실행할 수 있도록 합니다. 함수와 클래스 정의 또한 문법적으로 복합문입니다.

복합문은 하나나 그 이상의 ‘절’로 구성됩니다. 절은 헤더와 ‘스위트(suite)’로 구성됩니다. 특정 복합문의 절 헤더들은 모두 같은 들여쓰기 수준을 갖습니다. 각 절 헤더는 특별하게 식별되는 키워드로 시작하고 콜론으로 끝납니다. 스위트는 절에 의해 제어되는 문장들의 그룹입니다. 스위트는 헤더의 콜론 뒤에서 같은 줄에 세미콜론으로 분리된 하나나 그 이상의 단순문일 수 있습니다. 또는 그다음 줄에 들여쓰기 된 하나나 그 이상의 문장들일 수도 있습니다. 오직 후자의 형태만 중첩된 복합문을 포함할 수 있습니다; 다음과 같은 것은 올바르지 않은데, 대체로 뒤따르는 `else` 절이 있다면 어떤 `if` 절에 속하는지 명확하지 않기 때문입니다.

    if test1: if test2: print(x)

또한, 이 문맥에서 세미콜론이 콜론보다 더 강하게 결합해서, 다음과 같은 예에서, `print()` 호출들은 모두 실행되거나 어느 하나도 실행되지 않습니다는 것에 주의해야 합니다:

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

요약하면:

    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)* [";"]

문장들이 항상 `NEWLINE` 으로 끝나고 `DEDENT` 가 그 뒤를 따를 수 있음에 주목해야 합니다. 또한, 생략 가능한 연결 절들이 항상 문장을 시작시킬 수 없는 키워드로 시작하기 때문에, 모호함이 없다는 것도 주목하세요 (파이썬에서는 중첩된 `if` 문이 들여쓰기 되는 것을 요구함으로써 ‘매달린(dangling) `else`’ 문제를 해결합니다).

명확함을 위해 다음에 오는 절들에서 나오는 문법 규칙들은 각 절을 별도의 줄에 놓도록 포매팅합니다.

## 8.1. `if` 문

`if` 문은 조건부 실행에 사용됩니다:

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

참이 되는 것을 발견할 때까지 표현식들의 값을 하나씩 차례대로 구해서 정확히 하나의 스위트를 선택합니다 (참과 거짓의 정의는 논리 연산(Boolean operations) 섹션을 보세요); 그런 다음 그 스위트를 실행합니다 (그리고는 `if` 문의 다른 어떤 부분도 실행되거나 값이 구해지지 않습니다). 모든 표현식들이 거짓이면 `else` 절의 스위트가 (있다면) 실행됩니다.

## 8.2. `while` 문

`while` 문은 표현식이 참인 동안 실행을 반복하는 데 사용됩니다:

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

이것은 표현식을 반복적으로 검사하고, 참이면, 첫 번째 스위트를 실행합니다; 표현식이 거짓이면 (처음부터 거짓일 수도 있습니다) `else` 절의 스위트가 (있다면) 실행되고 루프를 종료합니다.

첫 번째 스위트에서 실행되는 `break` 문은 `else` 절을 실행하지 않고 루프를 종료합니다. 첫 번째 스위트에서 실행되는 `continue` 문은 스위트의 나머지 부분을 건너뛰고 표현식의 검사로 돌아갑니다.

## 8.3. `for` 문

`for` 문은 (문자열, 튜플, 리스트 같은) 시퀀스 나 다른 이터러블 객체의 요소들을 이터레이트하는데 사용됩니다:

    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 대입문), 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.

첫 번째 스위트에서 실행되는 `break` 문은 `else` 절을 실행하지 않고 루프를 종료합니다. 첫 번째 스위트에서 실행되는 `continue` 문은 스위트의 나머지 부분을 건너뛰고 다음 항목으로 넘어가거나, 다음 항목이 없으면 `else` 절로 갑니다.

for-루프는 타깃 목록의 변수들에 대입합니다. for-루프의 스위트에서 이루어진 것들도 포함해서, 그 변수에 앞서 대입된 값들을 모두 덮어씁니다:

    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.

버전 3.11에서 변경: Starred elements are now allowed in the expression list.

## 8.4. `try` 문

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

예외에 관한 추가의 정보는 예외 섹션에서 찾을 수 있고, 예외를 일으키기 위해 `raise` 문을 사용하는 것에 관한 정보는 raise 문 섹션에서 찾을 수 있습니다.

### 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

가 이렇게 변환되는 것과 같습니다

    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, the exception is stored in the `sys` module, where it can be accessed from within the body of the `except` clause by calling `sys.exception()`. When leaving an exception handler, the exception stored in the `sys` module is reset to its previous value:

    >>> print(sys.exception())
    None
    >>> try:
    ...     raise TypeError
    ... except:
    ...     print(repr(sys.exception()))
    ...     try:
    ...          raise ValueError
    ...     except:
    ...         print(repr(sys.exception()))
    ...     print(repr(sys.exception()))
    ...
    TypeError()
    ValueError()
    TypeError()
    >>> print(sys.exception())
    None

### 8.4.2. `except*` clause

The `except*` clause(s) are used for handling `ExceptionGroup`s. 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.

From version 3.11.4, when the entire `ExceptionGroup` is handled and only one exception is raised from an `except*` clause, this exception is no longer wrapped to form a new `ExceptionGroup`.

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

생략 가능한 `else` 절은 제어 흐름이 `try` 스위트를 빠져나가고, 예외가 발생하지 않았고, `return`, `continue` 또는 `break` 문이 실행되지 않으면 실행됩니다. `else` 절에서 발생하는 예외는 앞에 나오는 `except` 절에서 처리되지 않습니다.

### 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'

버전 3.8에서 변경: Prior to Python 3.8, a `continue` statement was illegal in the `finally` clause due to a problem with the implementation.

## 8.5. `with` 문

`with` 문은 블록의 실행을 컨텍스트 관리자 (with 문 컨텍스트 관리자 섹션을 보세요) 가 정의한 메서드들로 감싸는 데 사용됩니다. 이것은 흔한 `try`…`except`…`finally` 사용 패턴을 편리하게 재사용할 수 있도록 캡슐화할 수 있도록 합니다.

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

하나의 “item” 을 사용하는 `with` 문의 실행은 다음과 같이 진행됩니다:

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

2. The context manager’s `__enter__()` is loaded for later use.

3. The context manager’s `__exit__()` is loaded for later use.

4. The context manager’s `__enter__()` method is invoked.

5. If a target was included in the `with` statement, the return value from `__enter__()` is assigned to it.

   참고

   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. 스위트가 실행됩니다.

7. The context manager’s `__exit__()` method is invoked. If an exception caused the suite to be exited, its type, value, and traceback are passed as arguments to `__exit__()`. Otherwise, three `None` arguments are supplied.

   If the suite was exited due to an exception, and the return value from the `__exit__()` method was false, the exception is reraised. If the return value was true, the exception is suppressed, and execution continues with the statement following the `with` statement.

   If the suite was exited for any reason other than an exception, the return value from `__exit__()` is ignored, and execution proceeds at the normal location for the kind of exit that was taken.

다음과 같은 코드는:

    with EXPRESSION as TARGET:
        SUITE

의미상으로 다음과 동등합니다:

    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)

하나 보다 많은 항목을 주면, 컨텍스트 관리자는 `with` 문이 중첩된 것처럼 진행합니다:

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

의미상으로 다음과 동등합니다:

    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

버전 3.1에서 변경: 다중 컨텍스트 표현식의 지원

버전 3.10에서 변경: Support for using grouping parentheses to break the statement in multiple lines.

더 보기

PEP 343 - “with” 문

  파이썬 `with` 문의 규격, 배경, 예.

## 8.6. The `match` statement

버전 3.10에 추가.

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

참고

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.

더 보기

- PEP 634 – Structural Pattern Matching: Specification

- PEP 636 – Structural Pattern Matching: Tutorial

### 8.6.1. Overview

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.

   참고

   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.

참고

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

참고

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 `_`.

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. f-strings 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, `guard`s, 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`

참고

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 `[...]` ).

참고

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.

   참고

   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.

참고

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.

   > 더 보기

   > Customizing positional arguments in class pattern matching

   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.

더 보기

- PEP 634 – Structural Pattern Matching: Specification

- PEP 636 – Structural Pattern Matching: Tutorial

## 8.7. 함수 정의

함수 정의는 사용자 정의 함수 객체 (표준형 계층 섹션을 보세요) 를 정의합니다:

    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

함수 정의는 실행할 수 있는 문장입니다. 실행하면 현재 지역 이름 공간의 함수 이름을 함수 객체 (함수의 실행 가능한 코드를 둘러싼 래퍼(wrapper)). 이 함수 객체는 현재의 이름 공간에 대한 참조를 포함하는데, 함수가 호출될 때 전역 이름 공간으로 사용됩니다.

함수 정의는 함수의 바디를 실행하지 않습니다. 함수가 호출될 때 실행됩니다. [4]

함수 정의는 하나나 그 이상의 데코레이터 표현식으로 감싸질 수 있습니다. 데코레이터 표현식은 함수가 정의될 때, 함수 정의를 포함하는 스코프에서 값을 구합니다. 그 결과는 콜러블이어야 하는데, 함수 객체만을 인자로 사용해서 호출됩니다. 반환 값이 함수 객체 대신 함수의 이름에 연결됩니다. 여러 개의 데코레이터는 중첩되는 방식으로 적용됩니다. 예를 들어, 다음과 같은 코드

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

는 대략 다음과 동등합니다

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

원래의 함수가 임시로 이름 `func` 에 연결되지 않는다는 점만 다릅니다.

버전 3.9에서 변경: Functions may be decorated with any valid `assignment_expression`. Previously, the grammar was much more restrictive; see PEP 614 for details.

하나나 그 이상의 매개변수 들이 parameter `=` expression 형태를 가질 때, 함수가 “기본 매개변수 값”을 갖는다고 말합니다. 기본값이 있는 매개변수의 경우, 호출할 때 대응하는 인자 를 생략할 수 있고, 그럴 때 매개변수의 기본값이 적용됩니다. 만약 매개변수가 기본값을 가지면, “`*`” 까지 그 뒤를 따르는 모든 매개변수도 기본값을 가져야 합니다 — 이것은 문법 규칙에서 표현되지 않는 문법적 제약입니다.

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 호출. 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.

버전 3.8에서 변경: The `/` function parameter syntax may be used to indicate positional-only parameters. See PEP 570 for details.

Parameters may have an annotation of the form “`: expression`” following the parameter name. Any parameter may have an annotation, even those of the form `*identifier` or `**identifier`. Functions may have “return” annotation of the form “`-> expression`” after the parameter list. These annotations can be any valid Python expression. The presence of annotations does not change the semantics of a function. The annotation values are available as values of a dictionary keyed by the parameters’ names in the `__annotations__` attribute of the function object. If the `annotations` import from `__future__` is used, annotations are preserved as strings at runtime which enables postponed evaluation. Otherwise, they are evaluated when the function definition is executed. In this case annotations may be evaluated in a different order than they appear in the source code.

표현식에서 즉시 사용하기 위해, 이름 없는 함수(이름에 연결되지 않은 함수)를 만드는 것도 가능합니다. 이것은 람다 표현식을 사용하는데, 람다(Lambdas) 섹션에서 설명합니다. 람다 표현식은 단순화된 함수 정의를 위한 줄임 표현에 지나지 않는다는 것에 주의하세요; “`def`” 문장에서 정의된 함수는 람다 표현식으로 정의된 함수처럼 전달되거나 다른 이름에 대입될 수 있습니다. 여러 개의 문장을 실행하는 것과 어노테이션을 허락하기 때문에, “`def`” 형태가 사실 더 강력합니다.

프로그래머 유의 사항: 함수는 퍼스트 클래스(first-class) 객체다. 함수 정의 안에서 실행되는 “`def`” 문은 돌려주거나 전달할 수 있는 지역 함수를 정의합니다. 중첩된 함수에서 사용되는 자유 변수들은 그 def 를 포함하는 함수의 지역 변수들을 액세스할 수 있습니다. 더 자세한 내용은 이름과 연결(binding) 섹션을 보세요.

더 보기

PEP 3107 - 함수 어노테이션

  함수 어노테이션의 최초 규격.

PEP 484 - 형 힌트

  어노테이션에 대한 표준 의미 정의: 형 힌트.

PEP 526 - 변수 어노테이션 문법

  Ability to type hint variable declarations, including class variables and instance variables.

PEP 563 - 어노테이션의 지연된 평가

  즉시 평가하는 대신 실행시간에 어노테이션을 문자열 형식으로 보존하여 어노테이션 내에서의 전방 참조를 지원합니다.

PEP 318 - Decorators for Functions and Methods

  Function and method decorators were introduced. Class decorators were introduced in PEP 3129.

## 8.8. 클래스 정의

클래스 정의는 클래스 객체(표준형 계층 섹션을 보세요)를 정의합니다:

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

클래스 정의는 실행 가능한 문장입니다. 계승(inheritance) 목록은 보통 베이스 클래스들의 목록을 제공하는데 (더 고급 사용에 대해서는 메타 클래스 를 보세요), 목록의 각 항목은 값을 구할 때 서브 클래싱을 허락하는 클래스 객체가 되어야 합니다. 계승 목록이 없는 클래스는, 기본적으로, 베이스 클래스 `object` 를 계승합니다; 그래서

    class Foo:
        pass

는 다음과 동등합니다

    class Foo(object):
        pass

클래스의 스위트는 새로 만들어진 지역 이름 공간과 원래의 전역 이름 공간을 사용하는 새 실행 프레임 (이름과 연결(binding) 을 보세요)에서 실행됩니다. (보통, 스위트는 대부분 함수 정의들을 포함합니다.) 클래스의 스위트가 실행을 마치면, 실행 프레임은 파기하지만, 그것의 지역 이름 공간은 보존합니다. [5] 그런 다음, 계승 목록을 베이스 클래스들로, 보존된 지역 이름 공간을 어트리뷰트 딕셔너리로 사용해서 새 클래스 객체를 만듭니다. 클래스의 이름은 원래의 지역 이름 공간에서 이 클래스 객체와 연결됩니다.

The order in which attributes are defined in the class body is preserved in the new class’s `__dict__`. Note that this is reliable only right after the class is created and only for classes that were defined using the definition syntax.

클래스 생성은 메타 클래스 를 사용해서 심하게 커스터마이즈할 수 있습니다.

클래스 역시 함수를 데코레이팅할 때처럼 테코레이트할 수 있습니다,

    @f1(arg)
    @f2
    class Foo: pass

는 대략 다음과 동등합니다

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

데코레이터 표현식의 값을 구하는 규칙은 함수 데코레이터와 같습니다. 그런 다음 그 결과가 클래스 이름에 연결됩니다.

버전 3.9에서 변경: Classes may be decorated with any valid `assignment_expression`. Previously, the grammar was much more restrictive; see PEP 614 for details.

프로그래머 유의 사항: 클래스 정의에서 정의되는 변수들은 클래스 어트리뷰트입니다; 이것들은 인스턴스 간에 공유됩니다. 인스턴스 어트리뷰트는 메서드에서 `self.name = value` 로 설정될 수 있습니다. 클래스와 인스턴스 어트리뷰트 모두 “`self.name`” 표기법으로 액세스할 수 있고, 이런 식으로 액세스할 때 인스턴스 어트리뷰트는 같은 이름의 클래스 어트리뷰트를 가립니다. 클래스 어트리뷰트는 인스턴스 어트리뷰트의 기본값으로 사용될 수 있지만, 가변 값을 사용하는 것은 예상하지 않은 결과를 줄 수 있습니다. 디스크립터 를 다른 구현 상세를 갖는 인스턴스 변수를 만드는데 사용할 수 있습니다.

더 보기

PEP 3115 - 파이썬 3000의 메타 클래스

  메타 클래스 선언을 현재 문법으로 변경하고, 메타 클래스가 있는 클래스를 구성하는 방법의 의미를 변경하는 제안.

PEP 3129 - 클래스 데코레이터

  클래스 데코레이터를 추가하는 제안. 함수와 메서드 데코레이터는 PEP 318에서 도입되었습니다.

## 8.9. 코루틴

버전 3.5에 추가.

### 8.9.1. 코루틴 함수 정의

    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.

`async def` 문법으로 정의된 함수는 항상 코루틴 함수인데, `await` 나 `async` 키워드를 포함하지 않는 경우도 그렇습니다.

코루틴 함수의 바디 안에서 `yield from` 표현식을 사용하는 것은 `SyntaxError` 입니다.

코루틴 함수의 예:

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

버전 3.7에서 변경: `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` 문

    async_for_stmt ::=  "async" for_stmt

비동기 이터러블 은 비동기 이터레이터 를 직접 반환하는 `__aiter__` 메서드를 제공하고, 비동기 이터레이터는 자신의 `__anext__` 메서드에서 비동기 코드를 호출할 수 있습니다.

`async for` 문은 비동기 이터러블에 대한 편리한 이터레이션을 허락합니다.

다음과 같은 코드는:

    async for TARGET in ITER:
        SUITE
    else:
        SUITE2

의미상으로 다음과 동등합니다:

    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.

코루틴 함수의 바디 밖에서 `async for` 문을 사용하는 것은 `SyntaxError` 입니다.

### 8.9.3. `async with` 문

    async_with_stmt ::=  "async" with_stmt

비동기 컨텍스트 관리자 는 enter 와 exit 메서드에서 실행을 일시 중지할 수 있는 컨텍스트 관리자 입니다.

다음과 같은 코드는:

    async with EXPRESSION as TARGET:
        SUITE

의미상으로 다음과 동등합니다:

    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.

코루틴 함수의 바디 밖에서 `async with` 문을 사용하는 것은 `SyntaxError` 입니다.

더 보기

PEP 492 - async 와 await 문법을 사용하는 코루틴

  코루틴을 파이썬에서 적절한 독립적인 개념으로 만들고, 문법 지원을 추가한 제안.

각주

[1]

다른 예외를 일으키는 `finally` 절이 있지 않은 한 예외는 호출 스택으로 퍼집니다. 그 새 예외는 예전의 것을 잃어버리게 만듭니다.

[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`

참고

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]

A string literal appearing as the first statement in the function body is transformed into the function’s `__doc__` attribute and therefore the function’s docstring.

[5]

A string literal appearing as the first statement in the class body is transformed into the namespace’s `__doc__` item and therefore the class’s docstring.