8. 复合语句 ¶

复合语句包含 (分组) 其它语句；它们以某种方式影响 (或控制) 其它语句的执行。一般而言，复合语句跨多行，尽管简单化身可以将整个复合语句包含在一行中。

The if , while and for statements implement traditional control flow constructs. 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. Function and class definitions are also syntactically compound statements.

A compound statement consists of one or more ‘clauses.’ A clause consists of a header and a ‘suite.’ 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. A suite is a group of statements controlled by a clause. 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. 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:

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:

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

汇总：

compound_stmt ::=  if_stmt
                   | while_stmt
                   | for_stmt
                   | try_stmt
                   | with_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 . 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).

The formatting of the grammar rules in the following sections places each clause on a separate line for clarity.

8.1. The `if` 语句 ¶

The 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 布尔运算 for the definition of true and false); then that suite is executed (and no other part of the if statement is executed or evaluated). If all expressions are false, the suite of the else clause, if present, is executed.

8.2. The `while` 语句 ¶

The while 语句用于重复执行只要表达式为 True：

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

这将反复测试表达式，若为 True，执行第一套件；若表达式为 False (可能是首次测试)，执行套件 else 子句，若它存在，执行并循环终止。

A break 语句在第一套件中执行将终止循环而不执行 else 子句套件。 continue 语句在第一套件中执行将跳过套件的其余部分并回到测试表达式。

8.3. The `for` 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_stmt ::=  "for" target_list "in" expression_list ":" suite
              ["else" ":" suite]

The expression list is evaluated once; it should yield an iterable object. An iterator is created for the result of the expression_list . The suite is then executed once for each item provided by the iterator, in the order returned by the iterator. Each item in turn is assigned to the target list using the standard rules for assignments (see 赋值语句 ), and then the suite is executed. 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.

A break 语句在第一套件中执行将终止循环而不执行 else 子句套件。 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.

The for-loop makes assignments to the variables in the target list. This overwrites all previous assignments to those variables including those made in the suite of the for-loop:

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 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] .

注意

There is a subtlety when the sequence is being modified by the loop (this can only occur for mutable sequences, e.g. lists). An internal counter is used to keep track of which item is used next, and this is incremented on each iteration. When this counter has reached the length of the sequence the loop terminates. This means that if the suite deletes the current (or a previous) item from the sequence, the next item will be skipped (since it gets the index of the current item which has already been treated). Likewise, if the suite inserts an item in the sequence before the current item, the current item will be treated again the next time through the loop. This can lead to nasty bugs that can be avoided by making a temporary copy using a slice of the whole sequence, e.g.,

for x in a[:]:
    if x < 0: a.remove(x)

8.4. The `try` 语句 ¶

The 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. 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 it is the class or a base class of the exception object, or a tuple containing an item that is the class or a 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

was translated to

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

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

Before an except clause’s suite is executed, details about the exception are stored in the sys module and can be accessed via sys.exc_info() . sys.exc_info() returns a 3-tuple consisting of the exception class, the exception instance and a traceback object (see section 标准类型层次结构 ) identifying the point in the program where the exception occurred. sys.exc_info() values are restored to their previous values (before the call) when returning from a function that handled an exception.

可选 else 子句会被执行，若控制流离开 try suite, no exception was raised, and no return , continue ，或 break statement was executed. Exceptions in the else clause are not handled by the preceding except clauses.

若 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 子句执行 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 子句。

当 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'

可以找到有关异常的额外信息在章节异常 , and information on using the raise statement to generate exceptions may be found in section raise 语句 .

3.8 版改变： Prior to Python 3.8, a continue statement was illegal in the finally clause due to a problem with the implementation.

8.5. The `with` 语句 ¶

The with statement is used to wrap the execution of a block with methods defined by a context manager (see section with 语句上下文管理器 )。这允许常见 try … except … finally usage patterns to be encapsulated for convenient reuse.

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

The execution of the with statement with one “item” proceeds as follows:

The context expression (the expression given in the with_item ) is evaluated to obtain a context manager.
The context manager’s __enter__() is loaded for later use.
The context manager’s __exit__() is loaded for later use.
The context manager’s __enter__() method is invoked.
If a target was included in the with statement, the return value from __enter__() is assigned to it.

注意

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 6 below.
The suite is executed.
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 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.

以下代码：

with EXPRESSION as TARGET:
    SUITE

语义上相当于：

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:

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

语义上相当于：

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

3.1 版改变：支持多上下文表达式。

另请参阅

PEP 343 - with 语句

PEP 3107 - 函数注解
PEP 484 - 类型提示
PEP 526 - 变量注解句法
PEP 563 - 注解延期评估

PEP 3115 - Python 3000 的元类
PEP 3129 - 类装饰器

PEP 492 具有 async 和 await 句法的协程

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8. 复合语句 ¶

8.1. The `if` 语句 ¶

8.2. The `while` 语句 ¶

8.3. The `for` 语句 ¶

8.4. The `try` 语句 ¶

8.5. The `with` 语句 ¶

8.6. 函数定义 ¶

8.7. 类定义 ¶

8.8. 协程 ¶

8.8.1. 协程函数定义 ¶

8.8.2. The `async for` 语句 ¶

8.8.3. The `async with` 语句 ¶

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8. 复合语句 ¶

8.1. The if 语句 ¶

8.2. The while 语句 ¶

8.3. The for 语句 ¶

8.4. The try 语句 ¶

8.5. The with 语句 ¶

8.6. 函数定义 ¶

8.7. 类定义 ¶

8.8. 协程 ¶

8.8.1. 协程函数定义 ¶

8.8.2. The async for 语句 ¶

8.8.3. The async with 语句 ¶

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8.1. The `if` 语句 ¶

8.2. The `while` 语句 ¶

8.3. The `for` 语句 ¶

8.4. The `try` 语句 ¶

8.5. The `with` 语句 ¶

8.8.2. The `async for` 语句 ¶

8.8.3. The `async with` 语句 ¶