8. Compound statements复合语句

Compound statements contain (groups of) other statements; they affect or control the execution of those other statements in some way. 复合语句包含（一组）其他语句；它们以某种方式影响或控制其他语句的执行。In general, compound statements span multiple lines, although in simple incarnations a whole compound statement may be contained in one line.一般来说，复合语句跨越多行，尽管在简单的情况下，一个完整的复合语句可能包含在一行中。

The if, while and for statements implement traditional control flow constructs. if、while和for语句实现了传统的控制流结构。try specifies exception handlers and/or cleanup code for a group of statements, while the with statement allows the execution of initialization and finalization code around a block of code. try为一组语句指定异常处理程序和/或清理代码，而with语句允许围绕一块代码执行初始化和终结代码。Function and class definitions are also syntactically compound statements.函数和类定义也是语法复合语句。

A compound statement consists of one or more ‘clauses.’ 复合语句由一个或多个“子句”组成。A clause consists of a header and a ‘suite.’ 子句由标题和“suite”组成。The clause headers of a particular compound statement are all at the same indentation level. Each clause header begins with a uniquely identifying keyword and ends with a colon. 特定复合语句的子句头都处于相同的缩进级别。每个子句头以唯一标识的关键字开头，以冒号结尾。A suite is a group of statements controlled by a clause. 套件是由子句控制的一组语句。A suite can be one or more semicolon-separated simple statements on the same line as the header, following the header’s colon, or it can be one or more indented statements on subsequent lines. 套件可以是一个或多个分号分隔的简单语句，位于标题的冒号之后，位于标题的同一行，也可以是后续行中的一个或多个缩进语句。Only the latter form of a suite can contain nested compound statements; the following is illegal, mostly because it wouldn’t be clear to which if clause a following else clause would belong:只有套件的后一种形式可以包含嵌套的复合语句；以下是非法的，主要是因为不清楚以下else子句属于哪一条if子句：

if test1: if test2: print(x)

Also note that the semicolon binds tighter than the colon in this context, so that in the following example, either all or none of the print() calls are executed:还要注意，在这个上下文中，分号比冒号绑定得更紧，因此在下面的示例中，要么执行全部print()调用，要么不执行任何print()调用：

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

Summarizing:总结：

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)* [";"]

Note that statements always end in a NEWLINE possibly followed by a DEDENT. 注意，语句总是以NEWLINE结尾，后面可能跟一个DEDENT。Also note that optional continuation clauses always begin with a keyword that cannot start a statement, thus there are no ambiguities (the ‘dangling else’ problem is solved in Python by requiring nested if statements to be indented).还要注意的是，可选的continuation子句总是以一个不能启动语句的关键字开头，因此没有歧义（在Python中，通过要求嵌套的if语句缩进来解决“悬空else”问题）。

The formatting of the grammar rules in the following sections places each clause on a separate line for clarity.为了清晰起见，以下章节中语法规则的格式将每个子句放在单独的一行。

8.1. The if statementif语句

The if statement is used for conditional execution:if语句用于条件执行：

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

It selects exactly one of the suites by evaluating the expressions one by one until one is found to be true (see section Boolean operations for the definition of true and false); then that suite is executed (and no other part of the if statement is executed or evaluated). 它通过逐个计算表达式，直到找到一个为真（有关真和假的定义，请参阅布尔运算一节），精确选择一个套件；然后执行该套件（不执行或计算if语句的其他部分）。If all expressions are false, the suite of the else clause, if present, is executed.如果所有表达式都为false，则执行else子句套件（如果存在）。

8.2. The while statementwhile语句

The while statement is used for repeated execution as long as an expression is true:while语句用于重复执行，只要表达式为true：

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

This repeatedly tests the expression and, if it is true, executes the first suite; if the expression is false (which may be the first time it is tested) the suite of the else clause, if present, is executed and the loop terminates.这会重复测试表达式，如果是真的，则执行第一个套件；如果表达式为false（可能是第一次测试），则执行else子句套件（如果存在），循环终止。

A break statement executed in the first suite terminates the loop without executing the else clause’s suite. 在第一个套件中执行的break语句终止循环，而不执行else子句的套件。A continue statement executed in the first suite skips the rest of the suite and goes back to testing the expression.在第一个套件中执行的continue语句跳过套件的其余部分，并返回测试表达式。

8.3. The for statementfor语句

The for statement is used to iterate over the elements of a sequence (such as a string, tuple or list) or other iterable object:for语句用于迭代序列（如字符串、元组或列表）或其他可迭代对象的元素：

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

The expression list is evaluated once; it should yield an iterable object. 表达式列表计算一次；它应该产生一个可移植的对象。An iterator is created for the result of the expression_list. 将为expression_list的结果创建一个迭代器。The suite is then executed once for each item provided by the iterator, in the order returned by the iterator. 然后，按照迭代器返回的顺序，对迭代器提供的每个项目执行一次套件。Each item in turn is assigned to the target list using the standard rules for assignments (see Assignment statements), and then the suite is executed. 使用标准分配规则（请参见分配语句）将每个项目依次分配到目标列表，然后执行套件。When the items are exhausted (which is immediately when the sequence is empty or an iterator raises a StopIteration exception), the suite in the else clause, if present, is executed, and the loop terminates.当项目耗尽时（即序列为空或迭代器引发StopIteration异常时），执行else子句中的套件（如果存在），循环终止。

A break statement executed in the first suite terminates the loop without executing the else clause’s suite. 在第一个套件中执行的break语句终止循环，而不执行else子句的套件。A continue statement executed in the first suite skips the rest of the suite and continues with the next item, or with the else clause if there is no next item.在第一个套件中执行的continue语句跳过套件的其余部分并继续执行下一项，如果没有下一项，则继续执行else子句。

The for-loop makes assignments to the variables in the target list. for循环对目标列表中的变量进行赋值。This overwrites all previous assignments to those variables including those made in the suite of the for-loop:这将覆盖以前对这些变量的所有赋值，包括在for循环套件中进行的赋值：

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. 循环完成后，目标列表中的名称不会被删除，但如果序列为空，则循环根本不会为其分配名称。Hint: the built-in function range() returns an iterator of integers suitable to emulate the effect of Pascal’s for i := a to b do; e.g., list(range(3)) returns the list [0, 1, 2].提示：内置函数range()返回一个整数迭代器，该迭代器适合模拟Pascal的for i := a to b do的影响；例如，list(range(3))返回列表[0, 1, 2]。

8.4. The try statementtry语句

The try statement specifies exception handlers and/or cleanup code for a group of statements:try语句为一组语句指定异常处理程序和/或清理代码：

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

The except clause(s) specify one or more exception handlers. except子句指定一个或多个异常处理程序。When no exception occurs in the try clause, no exception handler is executed. 如果try子句中没有发生异常，则不会执行异常处理程序。When an exception occurs in the try suite, a search for an exception handler is started. 当try套件中发生异常时，将开始搜索异常处理程序。This search inspects the except clauses in turn until one is found that matches the exception. 此搜索将依次检查except子句，直到找到与异常匹配的子句。An expression-less except clause, if present, must be last; it matches any exception. 缺少表达式的except子句（如果存在）必须是最后一个；它匹配任何异常。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. 对于带有表达式的except子句，将计算该表达式，如果生成的对象与异常“兼容”，则该子句与异常匹配。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.如果对象是异常对象的类或非虚拟基类，或包含异常对象的类或非虚拟基类的项的元组，则该对象与异常兼容。

If no except clause matches the exception, the search for an exception handler continues in the surrounding code and on the invocation stack. 如果没有与异常匹配的except子句，则在周围的代码和调用堆栈中继续搜索异常处理程序。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).如果对except子句标头中的表达式求值引发异常，则取消对处理程序的原始搜索，并开始在周围代码和调用堆栈中搜索新异常（视为整个try语句引发异常）。

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. 当找到匹配的except子句时，将异常分配给该except子句中as关键字之后指定的目标（如果存在），并执行except子句的套件。All except clauses must have an executable block. 所有except子句都必须具有可执行块。When the end of this block is reached, execution continues normally after the entire try statement. 当到达此块的末尾时，在整个try语句之后，执行将正常继续。(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.)（这意味着，如果同一异常存在两个嵌套处理程序，并且该异常发生在内部处理程序的try子句中，则外部处理程序将不会处理该异常。）

When an exception has been assigned using as target, it is cleared at the end of the except clause. 使用as target分配异常后，将在except子句末尾清除该异常。This is as if这就好像

except E as N:
    foo

was translated to已翻译为

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. 这意味着必须将异常指定给其他名称，才能在except子句之后引用它。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.异常会被清除，因为通过附加到它们的回溯，它们与堆栈帧形成一个引用循环，使该帧中的所有局部变量保持活动状态，直到下一次垃圾收集发生。

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(). 在执行except子句的套件之前，有关异常的详细信息存储在sys模块中，可以通过sys.exc_info()访问。sys.exc_info() returns a 3-tuple consisting of the exception class, the exception instance and a traceback object (see section The standard type hierarchy) identifying the point in the program where the exception occurred. 返回一个三元组，由异常类、异常实例和回溯对象（请参阅标准类型层次结构一节）组成，用于标识程序中发生异常的点。The details about the exception accessed via sys.exc_info() are restored to their previous values when leaving an exception handler:离开异常处理程序时，有关通过sys.exc_info()访问的异常的详细信息将恢复为以前的值：

>>> 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)

The optional else clause is executed if the control flow leaves the try suite, no exception was raised, and no return, continue, or break statement was executed. 如果控制流离开try套件，没有引发异常，并且没有执行return、continue或break语句，则会执行可选的else子句。Exceptions in the else clause are not handled by the preceding except clauses.else子句中的例外情况不由上述except条款处理。

If finally is present, it specifies a ‘cleanup’ handler. 如果finally存在，则指定“清理”处理程序。The try clause is executed, including any except and else clauses. 执行try子句，包括任何except和else子句。If an exception occurs in any of the clauses and is not handled, the exception is temporarily saved. 如果任何子句中发生异常且未处理，则会临时保存该异常。The finally clause is executed. 执行finally子句。If there is a saved exception it is re-raised at the end of the finally clause. 如果存在保存的异常，将在finally子句末尾重新引发该异常。If the finally clause raises another exception, the saved exception is set as the context of the new exception. 如果finally子句引发另一个异常，则将保存的异常设置为新异常的上下文。If the finally clause executes a return, break or continue statement, the saved exception is discarded:如果finally子句执行return、break或continue语句，则丢弃保存的异常：

>>> 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.在执行finally子句期间，程序无法获得异常信息。

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.’当在try…finally语句的try套件中执行return、break或continue语句时，finally子句也会“在退出时”执行

The return value of a function is determined by the last return statement executed. 函数的返回值由最后执行的return语句确定。Since the finally clause always executes, a return statement executed in the finally clause will always be the last one executed:由于finally子句始终执行，因此finally子句中执行的return语句将始终是最后执行的语句：

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

Additional information on exceptions can be found in section Exceptions, and information on using the raise statement to generate exceptions may be found in section The raise statement.有关异常的其他信息可以在异常部分中找到，有关使用raise语句生成异常的信息可以在raise语句部分中找到。

Changed in version 3.8:版本3.8中更改： Prior to Python 3.8, a continue statement was illegal in the finally clause due to a problem with the implementation.在Python 3.8之前，由于实现存在问题，finally子句中的continue语句是非法的。

8.5. The with statementwith语句

The with statement is used to wrap the execution of a block with methods defined by a context manager (see section With Statement Context Managers). with语句用于使用上下文管理器定义的方法包装块的执行（请参阅with语句上下文管理器一节）。This allows common try…except…finally usage patterns to be encapsulated for convenient reuse.这允许封装常见的try…except…finally使用模式，以便于重用。

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

The execution of the with statement with one “item” proceeds as follows:使用一个“item”执行with语句的过程如下：

1. The context expression (the expression given in the with_item) is evaluated to obtain a context manager.对上下文表达式（with_item中给出的表达式）进行求值以获得上下文管理器。

2. The context manager’s __enter__() is loaded for later use.将加载上下文管理器的__enter__()，以供以后使用。

3. The context manager’s __exit__() is loaded for later use.将加载上下文管理器的__exit__()以供以后使用。

4. The context manager’s __enter__() method is invoked.将调用上下文管理器的__enter__()方法。

5. If a target was included in the with statement, the return value from __enter__() is assigned to it.如果with语句中包含目标，则会为其分配来自__enter__()的返回值。

   Note

   The with statement guarantees that if the __enter__() method returns without an error, then __exit__() will always be called. with语句保证，如果返回的__enter__()方法没有错误，那么将始终调用__exit__()方法。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. 因此，如果在分配给目标列表的过程中发生错误，它将被视为与套件中发生的错误相同。See step 6 below.请参阅下面的步骤6。

6. The suite is executed.该套件已执行。

7. The context manager’s __exit__() method is invoked. 将调用上下文管理器的__exit__()方法。If an exception caused the suite to be exited, its type, value, and traceback are passed as arguments to __exit__(). 如果异常导致套件退出，则其类型、值和回溯将作为参数传递给__exit__()。Otherwise, three None arguments are supplied.否则，将提供三个None参数。

   If the suite was exited due to an exception, and the return value from the __exit__() method was false, the exception is reraised. 如果套件因异常而退出，并且__exit__()方法的返回值为false，则会重新引发异常。If the return value was true, the exception is suppressed, and execution continues with the statement following the with statement.如果返回值为true，则会抑制异常，并继续执行with语句后面的语句。

   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.如果套件因异常以外的任何原因退出，则忽略来自__exit__()的返回值，并在所采取的退出类型的正常位置继续执行。

The following code:以下代码：

with EXPRESSION as TARGET:
    SUITE

is semantically equivalent to:在语义上等同于：

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)

With more than one item, the context managers are processed as if multiple with statements were nested:对于多个项，上下文管理器的处理就像嵌套了多个with语句一样：

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

is semantically equivalent to:在语义上等同于：

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. 如果项目被括号包围，您还可以在多行中编写多项目上下文管理器。For example:例如：

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

Changed in version 3.1:版本3.1中更改： Support for multiple context expressions.支持多个上下文表达式。

Changed in version 3.10:版本3.10中更改： Support for using grouping parentheses to break the statement in multiple lines.支持使用分组括号将语句分隔为多行。

See also参阅

PEP 343 - The “with” statement“with”语句

The specification, background, and examples for the Python with statement.Python with语句的规范、背景和示例。

8.6. The match statementmatch语句

New in version 3.10.版本3.10中新增。

The match statement is used for pattern matching. match语句用于模式匹配。Syntax:语法：

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

Note

This section uses single quotes to denote soft keywords.本节使用单引号表示软关键字。

Pattern matching takes a pattern as input (following case) and a subject value (following match). 模式匹配将模式作为输入（跟着case）和主题值（跟着match）。The pattern (which may contain subpatterns) is matched against the subject value. 模式（可能包含子模式）与主题值匹配。The outcomes are:结果是：

• A match success or failure (also termed a pattern success or failure).匹配的成功或失败（也称为模式成功或失败）。

• Possible binding of matched values to a name. 匹配值可能绑定到名称。The prerequisites for this are further discussed below.下面将进一步讨论这方面的先决条件。

The match and case keywords are soft keywords.match关键字和case关键字是软关键字。

See also

• PEP 634 – Structural Pattern Matching: Specification结构模式匹配：规范

• PEP 636 – Structural Pattern Matching: Tutorial结构填充图案匹配：教程

8.6.1. Overview概述

Here’s an overview of the logical flow of a match statement:下面是match语句逻辑流的概述：

1. The subject expression subject_expr is evaluated and a resulting subject value obtained. 对主题表达式subject_expr进行求值，并获得结果subject值。If the subject expression contains a comma, a tuple is constructed using the standard rules.如果主题表达式包含逗号，则使用标准规则构造元组。

2. Each pattern in a case_block is attempted to match with the subject value. case_block中的每个图案都试图与主题值匹配。The specific rules for success or failure are described below. 成功或失败的具体规则如下所述。The match attempt can also bind some or all of the standalone names within the pattern. 匹配尝试还可以绑定模式中的部分或所有独立名称。The precise pattern binding rules vary per pattern type and are specified below. 精确的模式绑定规则因模式类型而异，如下所述。Name bindings made during a successful pattern match outlive the executed block and can be used after the match statement.在成功的模式匹配期间进行的名称绑定比执行的块更有效，并且可以在match语句之后使用。

   Note

   During failed pattern matches, some subpatterns may succeed. 在失败的模式匹配期间，某些子模式可能会成功。Do not rely on bindings being made for a failed match. 不要依赖为失败的匹配所做的绑定。Conversely, do not rely on variables remaining unchanged after a failed match. 相反，不要依赖匹配失败后保持不变的变量。The exact behavior is dependent on implementation and may vary. 具体行为取决于实现，可能会有所不同。This is an intentional decision made to allow different implementations to add optimizations.这是一个有意的决定，允许不同的实现添加优化。

3. If the pattern succeeds, the corresponding guard (if present) is evaluated. 如果模式成功，将评估相应的保护（如果存在）。In this case all name bindings are guaranteed to have happened.在这种情况下，保证所有名称绑定都已发生。

   • If the guard evaluates as true or is missing, the block inside case_block is executed.如果guard的计算结果为true或缺失，则执行case_block中的块。

   • Otherwise, the next case_block is attempted as described above.否则，如上所述尝试下一个case_block。

   • If there are no further case blocks, the match statement is completed.如果没有其他case块，则完成match语句。

Note

Users should generally never rely on a pattern being evaluated. 用户通常不应该依赖于正在评估的模式。Depending on implementation, the interpreter may cache values or use other optimizations which skip repeated evaluations.根据实现的不同，解释器可以缓存值或使用跳过重复计算的其他优化。

A sample match statement:匹配语句示例：

>>> 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. 在这种情况下，if标志是一个护卫。Read more about that in the next section.请在下一节中阅读更多相关内容。

8.6.2. Guards护卫

guard ::=  "if" named_expression

A guard (which is part of the case) must succeed for code inside the case block to execute. 护卫guard（属于case的一部分）必须成功，才能执行case块内的代码。It takes the form: if followed by an expression.其形式为：if后跟表达式。

The logical flow of a case block with a guard follows:具有护卫的case块的逻辑流如下所示：

1. Check that the pattern in the case block succeeded. 检查case块中的模式是否成功。If the pattern failed, the guard is not evaluated and the next case block is checked.如果模式失败，则不评估护卫，并检查下一个case块。

2. If the pattern succeeded, evaluate the guard.如果模式成功，请评估护卫。

   • If the guard condition evaluates as true, the case block is selected.如果guard条件的评估结果为真，则选择case块。

   • If the guard condition evaluates as false, the case block is not selected.如果guard条件评估为false，则不会选择case块。

   • If the guard raises an exception during evaluation, the exception bubbles up.如果guard在评估期间引发异常，则会出现异常。

Guards are allowed to have side effects as they are expressions. 警卫被允许有副作用，因为他们是表达。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. 守卫评估必须从第一个案例块开始到最后一个案例块，一次一个，跳过模式不完全成功的案例块。(I.e., guard evaluation must happen in order.) （即，必须按顺序进行防护评估。）Guard evaluation must stop once a case block is selected.选择案例块后，必须停止防护评估。

8.6.3. Irrefutable Case Blocks无可辩驳的case块

An irrefutable case block is a match-all case block. 无可辩驳的案例块是匹配所有案例块。A match statement may have at most one irrefutable case block, and it must be last.match语句最多只能有一个无可辩驳的case块，并且必须是最后一个。

A case block is considered irrefutable if it has no guard and its pattern is irrefutable. 如果一个case块没有护卫装置，并且其模式是无可辩驳的，那么它就被认为是无可辩驳的。A pattern is considered irrefutable if we can prove from its syntax alone that it will always succeed. 如果我们仅从其语法就可以证明它总是成功的，那么模式被认为是无可辩驳的。Only the following patterns are irrefutable:只有以下模式是无可辩驳的：

• AS Patterns模式 whose left-hand side is irrefutable他的左手边是无可辩驳的

• OR Patterns模式 containing at least one irrefutable pattern至少包含一种无可辩驳的模式

• Capture Patterns捕获模式

• Wildcard Patterns通配符模式

• parenthesized irrefutable patterns带圆括号的无可辩驳模式

8.6.4. Patterns模式

Note

This section uses grammar notations beyond standard EBNF:本节使用标准EBNF以外的语法符号：

• the notation SEP.RULE+ is shorthand for RULE (SEP RULE)*符号SEP.RULE+是RULE (SEP RULE)*的简写形式

• the notation !RULE is shorthand for a negative lookahead assertion符号!RULE是消极前瞻断言的简写形式

The top-level syntax for patterns is:patterns的顶级语法是：

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). 下面的描述将包括“用简单的术语”描述一个模式的作用，以便于演示（雷蒙德·赫廷格的一篇文档激发了大部分描述的灵感）。Note that these descriptions are purely for illustration purposes and may not reflect the underlying implementation. 请注意，这些描述仅用于说明目的，可能不反映底层实现。Furthermore, they do not cover all valid forms.此外，它们并不涵盖所有有效的表格。

8.6.4.1. OR Patterns模式

An OR pattern is two or more patterns separated by vertical bars |. OR模式是由竖条|分隔的两个或多个模式。Syntax:语法：

or_pattern ::=  "|".closed_pattern+

Only the final subpattern may be irrefutable, and each subpattern must bind the same set of names to avoid ambiguity.只有最后的子模式可能是无可辩驳的，每个子模式必须绑定相同的名称集以避免歧义。

An OR pattern matches each of its subpatterns in turn to the subject value, until one succeeds. OR模式将其每个子模式依次与主题值匹配，直到其中一个成功。The OR pattern is then considered successful. 然后，OR模式被认为是成功的。Otherwise, if none of the subpatterns succeed, the OR pattern fails.否则，如果没有任何子模式成功，则OR模式将失败。

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.简单地说，P1 | P2 | ...将尝试匹配P1，如果失败，将尝试匹配P2，如果成功，将立即成功，否则将失败。

8.6.4.2. AS Patterns模式

An AS pattern matches an OR pattern on the left of the as keyword against a subject. AS模式将as关键字左侧的OR模式与主题匹配。Syntax:语法：

as_pattern ::=  or_pattern "as" capture_pattern

If the OR pattern fails, the AS pattern fails. 如果OR模式失败，则AS模式失败。Otherwise, the AS pattern binds the subject to the name on the right of the as keyword and succeeds. 否则，AS模式将主题绑定到AS关键字右侧的名称，并成功。capture_pattern cannot be a a _.不能是_。

In simple terms P as NAME will match with P, and on success it will set NAME = <subject>.简单地说，P as NAME将与P匹配，如果成功，它将设置NAME=<subject>。

8.6.4.3. Literal Patterns文字模式

A literal pattern corresponds to most literals in Python. 文字模式对应于Python中的大多数文字。Syntax:语法：

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. 规则strings和口令NUMBER在标准Python语法中定义。Triple-quoted strings are supported. 支持三引号字符串。Raw strings and byte strings are supported. 支持原始字符串和字节字符串。Formatted string literals are not supported.不支持格式化的字符串文字。

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. signed_number '+' NUMBER和signed_number '-' NUMBER的形式用于表示复数；它们需要在左边有一个实数，在右边有一个虚数。E.g. 3 + 4j.例如：3 + 4j。

In simple terms, LITERAL will succeed only if <subject> == LITERAL. 简单地说，只有当<subject> == LITERAL时，LITERAL才会成功。For the singletons None, True and False, the is operator is used.对于单例None，True和False，使用is运算符。

8.6.4.4. Capture Patterns捕获模式

A capture pattern binds the subject value to a name. 捕获模式将主题值绑定到名称。Syntax:语法：

capture_pattern ::=  !'_' NAME

A single underscore _ is not a capture pattern (this is what !'_' expresses). 单下划线_不是捕获模式（这是!'_'所表达的）。It is instead treated as a wildcard_pattern.相反，它被视为wildcard_pattern。

In a given pattern, a given name can only be bound once. 在给定的模式中，给定的名称只能绑定一次。E.g. case x, x: ... is invalid while case [x] | x: ... is allowed.例如，当允许使用case [x] | x: ...时，case x, x: ...是无效的。

Capture patterns always succeed. 捕获模式总是成功的。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.绑定遵循PEP 572中赋值表达式运算符建立的范围规则；除非有适用的global或nonlocal语句，否则名称将成为最近包含函数作用域中的局部变量。

In simple terms NAME will always succeed and it will set NAME = <subject>.简单来说，NAME总是成功的，它将设置NAME = <subject>。

8.6.4.5. Wildcard Patterns通配符模式

A wildcard pattern always succeeds (matches anything) and binds no name. 通配符模式总是成功（匹配任何内容），并且不绑定任何名称。Syntax:语法：

wildcard_pattern ::=  '_'

_ is a soft keyword within any pattern, but only within patterns. _是任何模式中的软关键字，但仅在模式中。It is an identifier, as usual, even within match subject expressions, guards, and case blocks.与往常一样，它是一个标识符，即使在match主题表达式、guard和case块中也是如此。

In simple terms, _ will always succeed.简单地说，_将永远成功。

8.6.4.6. Value Patterns值模式

A value pattern represents a named value in Python. 值模式表示Python中的命名值。Syntax:语法：

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. 使用标准Python名称解析规则查找模式中的虚线名称。The pattern succeeds if the value found compares equal to the subject value (using the == equality operator).如果找到的值与主题值比较相等（使用==相等运算符），则模式成功。

In simple terms NAME1.NAME2 will succeed only if <subject> == NAME1.NAME2简单地说，只有当<subject> == NAME1.NAME2时，NAME1.NAME2才会成功

Note

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. 如果同一个值在同一个match语句中多次出现，解释器可能会缓存找到的第一个值并重用它，而不是重复相同的查找。This cache is strictly tied to a given execution of a given match statement.此缓存严格绑定到给定匹配语句的给定执行。

8.6.4.7. Group Patterns组模式

A group pattern allows users to add parentheses around patterns to emphasize the intended grouping. 组模式允许用户在模式周围添加括号，以强调预期的分组。Otherwise, it has no additional syntax. 否则，它没有其他语法。Syntax:语法：

group_pattern ::=  "(" pattern ")"

In simple terms (P) has the same effect as P.简单地说，(P)与P具有相同的效果。

8.6.4.8. Sequence Patterns序列模式

A sequence pattern contains several subpatterns to be matched against sequence elements. 序列模式包含多个子模式，这些子模式与序列元素相匹配。The syntax is similar to the unpacking of a list or tuple.语法类似于列表或元组的解包。

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 [...] ).如果序列模式使用括号或方括号，则没有区别（即(...) vs [...]）。

Note

A single pattern enclosed in parentheses without a trailing comma (e.g. (3 | 4)) is a group pattern. 用括号括起来但不带尾随逗号的单个模式（例如(3 | 4)）是一种组模式。While a single pattern enclosed in square brackets (e.g. [3 | 4]) is still a sequence pattern.而方括号内的单一模式（如[3 | 4]）仍然是序列模式。

At most one star subpattern may be in a sequence pattern. 序列模式中最多可以有一个星形子模式。The star subpattern may occur in any position. 星形子模式可能出现在任何位置。If no star subpattern is present, the sequence pattern is a fixed-length sequence pattern; otherwise it is a variable-length sequence pattern.如果不存在星形子模式，则序列模式是固定长度的序列模式；否则，它是一个可变长度的序列模式。

The following is the logical flow for matching a sequence pattern against a subject value:以下是将序列模式与主题值匹配的逻辑流程：

1. If the subject value is not a sequence 2, the sequence pattern fails.如果主题值不是序列2，则序列模式失败。

2. If the subject value is an instance of str, bytes or bytearray the sequence pattern fails.如果主题值是str、bytes或bytearray的实例，则序列模式将失败。

3. The subsequent steps depend on whether the sequence pattern is fixed or variable-length.后续步骤取决于序列模式是固定长度还是可变长度。

   If the sequence pattern is fixed-length:如果序列模式为固定长度：

   1. If the length of the subject sequence is not equal to the number of subpatterns, the sequence pattern fails如果主题序列的长度不等于子模式的数量，则序列模式失败

   2. Subpatterns in the sequence pattern are matched to their corresponding items in the subject sequence from left to right. 序列模式中的子模式与主题序列中的相应项目从左到右匹配。Matching stops as soon as a subpattern fails. 一旦子模式失败，匹配就会停止。If all subpatterns succeed in matching their corresponding item, the sequence pattern succeeds.如果所有子模式都成功匹配其对应的项，则序列模式成功。

   Otherwise, if the sequence pattern is variable-length:否则，如果序列模式为可变长度：

   1. If the length of the subject sequence is less than the number of non-star subpatterns, the sequence pattern fails.如果主题序列的长度小于非星形子模式的数量，则序列模式失败。

   2. The leading non-star subpatterns are matched to their corresponding items as for fixed-length sequences.对于固定长度序列，前导的非星形子模式与其对应的项相匹配。

   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.如果上一步成功，则星形子模式匹配由剩余主题项组成的列表，不包括与星形子模式之后的非星形子模式相对应的剩余项。

   4. Remaining non-star subpatterns are matched to their corresponding subject items, as for a fixed-length sequence.剩余的非星形子模式与其对应的主题项相匹配，就像固定长度的序列一样。

   Note

   The length of the subject sequence is obtained via len() (i.e. via the __len__() protocol). 主题序列的长度是通过len()获得的（即通过__len__()协议）。This length may be cached by the interpreter in a similar manner as value patterns.解释器可以用与值模式类似的方式缓存该长度。

In simple terms [P1, P2, P3, … , P<N>] matches only if all the following happens:简单地说，[P1, P2, P3, ... , P<N>]仅在以下所有情况下匹配：

• check <subject> is a sequence检查<subject>是一个序列

• len(subject) == <N>

• P1 matches 匹配<subject>[0] (note that this match can also bind names)（请注意，此匹配也可以绑定名称）

• P2 matches 匹配<subject>[1] (note that this match can also bind names)（请注意，此匹配也可以绑定名称）

• … and so on for the corresponding pattern/element.对于相应的模式/元素，依此类推。

8.6.4.9. Mapping Patterns映射模式

A mapping pattern contains one or more key-value patterns. 映射模式包含一个或多个键值模式。The syntax is similar to the construction of a dictionary. 语法类似于词典的构造。Syntax:语法：

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. 映射模式中最多可以有一个双星模式。The double star pattern must be the last subpattern in the mapping pattern.双星模式必须是映射模式中的最后一个子模式。

Duplicate keys in mapping patterns are disallowed. 映射模式中不允许存在重复键。Duplicate literal keys will raise a SyntaxError. 重复的文本键将引发SyntaxError。Two keys that otherwise have the same value will raise a ValueError at runtime.否则具有相同值的两个键将在运行时引发ValueError。

The following is the logical flow for matching a mapping pattern against a subject value:以下是将映射模式与主题值匹配的逻辑流程：

1. If the subject value is not a mapping 3,the mapping pattern fails.如果主题值不是映射3，则映射模式失败。

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.如果映射模式中给定的每个键都存在于主题映射中，并且每个键的模式与主题映射的相应项匹配，则映射模式成功。

3. If duplicate keys are detected in the mapping pattern, the pattern is considered invalid. 如果在映射模式中检测到重复的密钥，则该模式被视为无效。A SyntaxError is raised for duplicate literal values; or a ValueError for named keys of the same value.对于重复的文字值，会引发SyntaxError；或具有相同值的命名键的ValueError。

Note

Key-value pairs are matched using the two-argument form of the mapping subject’s get() method. 使用映射主题的get()方法的两个参数形式匹配键值对。Matched key-value pairs must already be present in the mapping, and not created on-the-fly via __missing__() or __getitem__().匹配的键值对必须已经存在于映射中，并且不能通过__missing__()或__getitem__()动态创建。

In simple terms {KEY1: P1, KEY2: P2, ... } matches only if all the following happens:简单来说{KEY1: P1, KEY2: P2, ... }仅当发生以下所有情况时匹配：

• check <subject> is a mapping检查<subject>是否为映射

• KEY1 in <subject>

• P1 matches 比赛<subject>[KEY1]

• … and so on for the corresponding KEY/pattern pair.对于相应的键/模式对，依此类推。

8.6.4.10. Class Patterns类模式

A class pattern represents a class and its positional and keyword arguments (if any). 类模式表示类及其位置参数和关键字参数（如果有）。Syntax:语法：

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.类模式中不应重复相同的关键字。

The following is the logical flow for matching a class pattern against a subject value:以下是将类模式与主题值进行匹配的逻辑流程：

1. If name_or_attr is not an instance of the builtin type , raise TypeError.如果name_or_attr不是内置type的实例，将引发TypeError。

2. If the subject value is not an instance of name_or_attr (tested via isinstance()), the class pattern fails.如果subject值不是name_or_attr的实例（通过isinstance()测试），则类模式将失败。

3. If no pattern arguments are present, the pattern succeeds. 如果不存在模式参数，则模式成功。Otherwise, the subsequent steps depend on whether keyword or positional argument patterns are present.否则，后续步骤取决于是否存在关键字或位置参数模式。

   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.对于许多内置类型（如下所述），可以接受与整个主题匹配的单个位置子模式；对于这些类型，关键字模式也适用于其他类型。

   If only keyword patterns are present, they are processed as follows, one by one:如果只存在关键字模式，则按如下方式逐一处理：

   I. The keyword is looked up as an attribute on the subject.关键字作为主题的属性进行查找。

   • If this raises an exception other than AttributeError, the exception bubbles up.如果这引发了AttributeError以外的异常，则会出现异常。

   • If this raises AttributeError, the class pattern has failed.如果这引发AttributeError，则类模式已失败。

   • Else, the subpattern associated with the keyword pattern is matched against the subject’s attribute value. 否则，与关键字模式关联的子模式将与主题的属性值匹配。If this fails, the class pattern fails; if this succeeds, the match proceeds to the next keyword.如果失败，则类模式失败；如果成功，匹配将继续到下一个关键字。

   II. If all keyword patterns succeed, the class pattern succeeds.如果所有关键字模式都成功，则类模式成功。

   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:如果存在任何位置模式，则在匹配之前，将使用类名或属性上的__match_args__属性将其转换为关键字模式：

   I. The equivalent of getattr(cls, "__match_args__", ()) is called.调用getattr(cls, "__match_args__", ())的等效项。

   • If this raises an exception, the exception bubbles up.如果这引发异常，则会出现异常。

   • If the returned value is not a tuple, the conversion fails and TypeError is raised.如果返回的值不是元组，则转换失败并引发TypeError。

   • If there are more positional patterns than len(cls.__match_args__), TypeError is raised.如果位置模式多于len(cls.__match_args__)，则会引发TypeError。

   • Otherwise, positional pattern i is converted to a keyword pattern using __match_args__[i] as the keyword. 否则，位置模式i将使用__match_args__[i]作为关键字转换为关键字模式。__match_args__[i] must be a string; if not TypeError is raised.__match_args__[i]必须是字符串；如果不是，则引发TypeError。

   • If there are duplicate keywords, TypeError is raised.如果存在重复的关键字，则会引发TypeError。

   See also参阅

   Customizing positional arguments in class pattern matching自定义类模式匹配中的位置参数

   II. Once all positional patterns have been converted to keyword patterns,一旦所有位置模式都转换为关键字模式，

   the match proceeds as if there were only keyword patterns.匹配过程就像只有关键字模式一样进行。

   For the following built-in types the handling of positional subpatterns is different:对于以下内置类型，位置子模式的处理是不同的：

   • 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. 这些类接受单个位置参数，其中的模式与整个对象而不是属性匹配。For example int(0|1) matches the value 0, but not the values 0.0 or False.例如，int(0|1)与值0匹配，但与值0.0或False不匹配。

In simple terms CLS(P1, attr=P2) matches only if the following happens:简单来说，CLS(P1, attr=P2)仅在以下情况下匹配：

• isinstance(<subject>, CLS)

• convert P1 to a keyword pattern using CLS.__match_args__使用CLS.__match_args__将P1转换为关键字模式

• For each keyword argument attr=P2:对于每个关键字参数attr=P2：

  • hasattr(<subject>, "attr")

  • P2 matches 比赛<subject>.attr

• … and so on for the corresponding keyword argument/pattern pair.对于相应的关键字参数/模式对，依此类推。

See also

• PEP 634 – Structural Pattern Matching: Specification结构模式匹配：规范

• PEP 636 – Structural Pattern Matching: Tutorial结构填充图案匹配：教程

8.7. Function definitions函数定义

A function definition defines a user-defined function object (see section The standard type hierarchy):函数定义定义用户定义的函数对象（请参见“标准类型层次结构”一节）：

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

A function definition is an executable statement. 函数定义是一条可执行语句。Its execution binds the function name in the current local namespace to a function object (a wrapper around the executable code for the function). 它的执行将当前本地名称空间中的函数名绑定到函数对象（围绕函数的可执行代码的包装器）。This function object contains a reference to the current global namespace as the global namespace to be used when the function is called.此函数对象包含对当前全局命名空间的引用，作为调用函数时要使用的全局命名空间。

The function definition does not execute the function body; this gets executed only when the function is called. 函数定义不执行函数体；这仅在调用函数时执行。4

A function definition may be wrapped by one or more decorator expressions. 函数定义可以由一个或多个装饰符表达式包装。Decorator expressions are evaluated when the function is defined, in the scope that contains the function definition. 定义函数时，将在包含函数定义的范围内计算装饰器表达式。The result must be a callable, which is invoked with the function object as the only argument. 结果必须是一个可调用的，它是以函数对象作为唯一参数调用的。The returned value is bound to the function name instead of the function object. 返回的值绑定到函数名，而不是函数对象。Multiple decorators are applied in nested fashion. 多个装饰器以嵌套方式应用。For example, the following code例如，以下代码

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

is roughly equivalent to大致相当于

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

except that the original function is not temporarily bound to the name func.除了原始函数没有临时绑定到名称func之外。

Changed in version 3.9:版本3.9中更改： Functions may be decorated with any valid assignment_expression. 函数可以用任何有效的assignment_expression修饰。Previously, the grammar was much more restrictive; see PEP 614 for details.以前，语法的限制性要大得多；详见PEP 614。

When one or more parameters have the form parameter = expression, the function is said to have “default parameter values.” 当一个或多个参数的形式为parameter=expression时，该函数被称为具有“默认参数值”For a parameter with a default value, the corresponding argument may be omitted from a call, in which case the parameter’s default value is substituted. 对于具有默认值的参数，可以从调用中省略相应的参数，在这种情况下，将替换参数的默认值。If a parameter has a default value, all following parameters up until the “*” must also have a default value — this is a syntactic restriction that is not expressed by the grammar.如果一个参数有一个默认值，那么直到“*”之前的所有后续参数也必须有一个默认值-这是语法中没有表示的语法限制。

Default parameter values are evaluated from left to right when the function definition is executed.执行函数定义时，将从左到右计算默认参数值。 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. 这意味着在定义函数时，表达式只计算一次，并且每次调用都使用相同的“预计算”值。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. 了解默认参数值是可变对象（如列表或字典）时，这一点尤其重要：如果函数修改对象（例如，通过向列表中添加项目），则默认参数值实际上会被修改。This is generally not what was intended. 这通常不是预期的结果。A way around this is to use None as the default, and explicitly test for it in the body of the function, e.g.:解决此问题的一种方法是使用None作为默认值，并在函数体中显式测试它，例如：

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 Calls. 函数调用语义将在调用一节中进行更详细的描述。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. 函数调用总是从位置参数、关键字参数或默认值为参数列表中提到的所有参数赋值。If the form “*identifier” is present, it is initialized to a tuple receiving any excess positional parameters, defaulting to the empty tuple. 如果存在格式“*identifier”，则将其初始化为接收任何多余位置参数的元组，默认为空元组。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. 如果存在表格“**identifier”，则会将其初始化为新的有序映射，并接收任何多余的关键字参数，默认为相同类型的新空映射。Parameters after “*” or “*identifier” are keyword-only parameters and may only be passed by keyword arguments. “*”或“*identifier”后的参数是仅限关键字的参数，只能通过关键字参数传递。Parameters before “/” are positional-only parameters and may only be passed by positional arguments.“/”之前的参数是仅位置参数，只能由位置参数传递。

Changed in version 3.8:版本3.8中更改： The / function parameter syntax may be used to indicate positional-only parameters. /函数参数语法可用于指示仅位置参数。See PEP 570 for details.详见PEP 570。

Parameters may have an annotation of the form “: expression” following the parameter name. 参数名称后面可能有一个格式为“: expression”的注释。Any parameter may have an annotation, even those of the form *identifier or **identifier. 任何参数都可以有注释，甚至是“*identifier”或“**identifier”形式的注释。Functions may have “return” annotation of the form “-> expression” after the parameter list. 函数可以在参数列表后有“-> expression”形式的“return”注释。These annotations can be any valid Python expression. 这些注释可以是任何有效的Python表达式。The presence of annotations does not change the semantics of a function. 注释的存在不会改变函数的语义。The annotation values are available as values of a dictionary keyed by the parameters’ names in the __annotations__ attribute of the function object. 注释值可以作为字典的值使用，字典由函数对象的__annotations__属性中的参数名称进行键控。If the annotations import from __future__ is used, annotations are preserved as strings at runtime which enables postponed evaluation. 如果使用从__future__导入的annotations，则注释将在运行时保留为字符串，从而启用延迟的计算。Otherwise, they are evaluated when the function definition is executed. 否则，将在执行函数定义时对其进行计算。In this case annotations may be evaluated in a different order than they appear in the source code.在这种情况下，注释的求值顺序可能与它们在源代码中出现的顺序不同。

It is also possible to create anonymous functions (functions not bound to a name), for immediate use in expressions. 还可以创建匿名函数（未绑定到名称的函数），以便在表达式中立即使用。This uses lambda expressions, described in section Lambdas. 这使用lambda表达式，如Lambdas一节所述。Note that the lambda expression is merely a shorthand for a simplified function definition; a function defined in a “def” statement can be passed around or assigned to another name just like a function defined by a lambda expression. 请注意，lambda表达式只是简化函数定义的简写；“def”语句中定义的函数可以传递或分配给其他名称，就像lambda表达式定义的函数一样。The “def” form is actually more powerful since it allows the execution of multiple statements and annotations.“def”表单实际上更强大，因为它允许执行多个语句和注释。

Programmer’s note: Functions are first-class objects. 函数是一级对象。A “def” statement executed inside a function definition defines a local function that can be returned or passed around. 在函数定义中执行的“def”语句定义了可以返回或传递的本地函数。Free variables used in the nested function can access the local variables of the function containing the def. 嵌套函数中使用的自由变量可以访问包含def的函数的局部变量。See section Naming and binding for details.有关详细信息，请参阅命名和绑定部分。

See also参阅

PEP 3107 - Function Annotations函数注解

The original specification for function annotations.函数注释的原始规范。

PEP 484 - Type Hints键入提示

Definition of a standard meaning for annotations: type hints.注释的标准含义定义：键入提示。

PEP 526 - Syntax for Variable Annotations变量注释的语法

Ability to type hint variable declarations, including class variables and instance variables能够键入提示变量声明，包括类变量和实例变量

PEP 563 - Postponed Evaluation of Annotations延期评估注释

Support for forward references within annotations by preserving annotations in a string form at runtime instead of eager evaluation.通过在运行时以字符串形式保留注释，而不是急于求值，支持在注释内转发引用。

8.8. Class definitions类定义

A class definition defines a class object (see section The standard type hierarchy):类定义定义了类对象（请参见“标准类型层次结构”一节）：

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

A class definition is an executable statement. 类定义是一个可执行语句。The inheritance list usually gives a list of base classes (see Metaclasses for more advanced uses), so each item in the list should evaluate to a class object which allows subclassing. 继承列表通常给出基类的列表（有关更高级的用途，请参阅元类），因此列表中的每个项都应计算为允许子类化的类对象。Classes without an inheritance list inherit, by default, from the base class object; hence,默认情况下，没有继承列表的类从基类object继承；因此

class Foo:
    pass

is equivalent to相当于

class Foo(object):
    pass

The class’s suite is then executed in a new execution frame (see Naming and binding), using a newly created local namespace and the original global namespace. 然后，使用新创建的本地名称空间和原始全局名称空间，在新的执行框架中执行类的套件（请参见命名和绑定）。(Usually, the suite contains mostly function definitions.) （通常，套件主要包含函数定义。）When the class’s suite finishes execution, its execution frame is discarded but its local namespace is saved. 当类的套件完成执行时，将丢弃其执行框架，但会保存其本地命名空间。5 A class object is then created using the inheritance list for the base classes and the saved local namespace for the attribute dictionary. 然后使用基类的继承列表和属性字典保存的本地名称空间创建类对象。The class name is bound to this class object in the original local namespace.类名绑定到原始本地命名空间中的该类对象。

The order in which attributes are defined in the class body is preserved in the new class’s __dict__. 在类主体中定义属性的顺序保留在新类的__dict__中。Note that this is reliable only right after the class is created and only for classes that were defined using the definition syntax.请注意，只有在创建类之后，并且仅对于使用定义语法定义的类，这才是可靠的。

Class creation can be customized heavily using metaclasses.可以大量使用元类自定义类创建。

Classes can also be decorated: just like when decorating functions,类也可以被修饰：就像修饰函数一样，

@f1(arg)
@f2
class Foo: pass

is roughly equivalent to大致相当于

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

The evaluation rules for the decorator expressions are the same as for function decorators. decorator表达式的计算规则与函数decorator的计算规则相同。The result is then bound to the class name.然后将结果绑定到类名。

Changed in version 3.9:版本3.9中更改： Classes may be decorated with any valid assignment_expression. 类可以用任何有效的assignment_expression修饰。Previously, the grammar was much more restrictive; see PEP 614 for details.以前，语法的限制性要大得多；详见PEP 614。

Programmer’s note: Variables defined in the class definition are class attributes; they are shared by instances. 类定义中定义的变量为类属性；它们由实例共享。Instance attributes can be set in a method with self.name = value. 可以在self.name=value的方法中设置实例属性。Both class and instance attributes are accessible through the notation “self.name”, and an instance attribute hides a class attribute with the same name when accessed in this way. 类和实例属性都可以通过标记“self.name”访问，并且实例属性在以这种方式访问时隐藏具有相同名称的类属性。Class attributes can be used as defaults for instance attributes, but using mutable values there can lead to unexpected results. 类属性可以用作实例属性的默认值，但在其中使用可变值可能会导致意外结果。Descriptors描述符 can be used to create instance variables with different implementation details.可以用于创建具有不同实现细节的实例变量。

See also参阅

PEP 3115 - Metaclasses in Python 3000

The proposal that changed the declaration of metaclasses to the current syntax, and the semantics for how classes with metaclasses are constructed.将元类声明更改为当前语法的建议，以及如何构造具有元类的类的语义。

PEP 3129 - Class Decorators

The proposal that added class decorators. 添加类装饰器的建议。Function and method decorators were introduced in PEP 318.PEP 318中介绍了装饰器的功能和方法。

8.9. Coroutines协同程序

New in version 3.5.版本3.5中新增。

8.9.1. Coroutine function definition协同例程函数定义

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

Execution of Python coroutines can be suspended and resumed at many points (see coroutine). Python协同路由的执行可以在多个点暂停和恢复（请参见协同例程）。await expressions, async for and async with can only be used in the body of a coroutine function.await表达式、async for和async with只能在协同例程函数的主体中使用。

Functions defined with async def syntax are always coroutine functions, even if they do not contain await or async keywords.使用async def语法定义的函数始终是协程函数，即使它们不包含await或async关键字。

It is a SyntaxError to use a yield from expression inside the body of a coroutine function.在协同例程函数体中使用yield from表达式是一种SyntaxError。

An example of a coroutine function:协同例程函数的一个示例：

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

Changed in version 3.7:版本3.7中更改： await and async are now keywords; previously they were only treated as such inside the body of a coroutine function.await和async现在是关键字；以前，它们仅在协同例程函数的主体内被视为这样处理。

8.9.2. The async for statementasync for语句

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.异步iterable提供了一个__aiter__方法，该方法直接返回异步迭代器，该迭代器可以在其__anext__方法中调用异步代码。

The async for statement allows convenient iteration over asynchronous iterables.async for语句允许在异步iterables上进行方便的迭代。

The following code:以下代码：

async for TARGET in ITER:
    SUITE
else:
    SUITE2

Is semantically equivalent to:在语义上等同于：

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.有关详细信息，请参阅__aiter__()和__anext__()。

It is a SyntaxError to use an async for statement outside the body of a coroutine function.在协同例程函数体外部使用async for语句是一种SyntaxError。

8.9.3. The async with statementasync with语句

async_with_stmt ::=  "async" with_stmt

An asynchronous context manager is a context manager that is able to suspend execution in its enter and exit methods.异步上下文管理器是一种上下文管理器，它能够在其enter和exit方法中暂停执行。

The following code:以下代码：

async with EXPRESSION as TARGET:
    SUITE

is semantically equivalent to:在语义上等同于：

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.有关详细信息，请参阅__aenter__()和__aexit__()。

It is a SyntaxError to use an async with statement outside the body of a coroutine function.在协同例程函数体外部使用async with语句是一种SyntaxError。

See also

PEP 492 - Coroutines with async and await syntax使用async和await语法的协同路由

The proposal that made coroutines a proper standalone concept in Python, and added supporting syntax.该提案使协同路由在Python中成为一个适当的独立概念，并添加了支持语法。

Footnotes

1

The exception is propagated to the invocation stack unless there is a finally clause which happens to raise another exception. 除非有一个finally子句碰巧引发另一个异常，否则该异常会传播到调用堆栈。That new exception causes the old one to be lost.新的异常会导致旧的异常丢失。

2

In pattern matching, a sequence is defined as one of the following:在模式匹配中，序列定义为以下之一：

• a class that inherits from collections.abc.Sequence从collections.abc.Sequence继承的类

• a Python class that has been registered as collections.abc.Sequence已注册为collections.abc.Sequence的Python类

• a builtin class that has its (CPython) Py_TPFLAGS_SEQUENCE bit set设置了（CPython）Py_TPFLAGS_SEQUENCE位的内置类

• a class that inherits from any of the above从上述任何一个继承的类

The following standard library classes are sequences:以下标准库类是序列：

• array.array

• collections.deque

• list

• memoryview

• range

• tuple

Note

Subject values of type str, bytes, and bytearray do not match sequence patterns.str、bytes和bytearray类型的主题值与序列模式不匹配。

3

In pattern matching, a mapping is defined as one of the following:在模式匹配中，映射定义为以下之一：

• a class that inherits from collections.abc.Mapping从collections.abc.Mapping继承的类

• a Python class that has been registered as collections.abc.Mapping已注册为collections.abc.Mapping的Python类

• a builtin class that has its (CPython) Py_TPFLAGS_MAPPING bit set设置了（CPython）Py_TPFLAGS_MAPPING位的内置类

• a class that inherits from any of the above从上述任何一个继承的类

The standard library classes dict and types.MappingProxyType are mappings.标准库类dict和types.MappingProxyType类型是映射。

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.作为函数体中第一条语句出现的字符串文字将转换为函数的__doc__属性，从而转换为函数的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.作为类主体中的第一条语句出现的字符串文字将转换为命名空间的__doc__项，从而转换为类的docstring。