8. 复合语句¶
复合语句是包含其它语句(语句组)的语句;它们会以某种方式影响或控制所包含其它语句的执行。 通常,复合语句会跨越多行,虽然在某些简单形式下整个复合语句也可能包含于一行之内。
if
, while
和 for
语句用来实现传统的控制流程构造。 try
语句为一组语句指定异常处理和/和清理代码,而 with
语句允许在一个代码块周围执行初始化和终结化代码。 函数和类定义在语法上也属于复合语句。
一条复合语句由一个或多个‘子句’组成。 一个子句则包含一个句头和一个‘句体’。 特定复合语句的子句头都处于相同的缩进层级。 每个子句头以一个作为唯一标识的关键字开始并以一个冒号结束。 子句体是由一个子句控制的一组语句。 子句体可以是在子句头的冒号之后与其同处一行的一条或由分号分隔的多条简单语句,或者也可以是在其之后缩进的一行或多行语句。 只有后一种形式的子句体才能包含嵌套的复合语句;以下形式是不合法的,这主要是因为无法分清某个后续的 else
子句应该属于哪个 if
子句:
if test1: if test2: print(x)
还要注意的是在这种情形下分号的绑定比冒号更紧密,因此在以下示例中,所有 print()
调用或者都不执行,或者都执行:
if x < y < z: print(x); print(y); print(z)
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 INDENTstatement
+ DEDENT statement ::=stmt_list
NEWLINE |compound_stmt
stmt_list ::=simple_stmt
(";"simple_stmt
)* [";"]
请注意语句总是以 NEWLINE
结束,之后可能跟随一个 DEDENT
。 还要注意可选的后续子句总是以一个不能作为语句开头的关键字作为开头,因此不会产生歧义(‘悬空的 else
’问题在 Python 中是通过要求嵌套的 if
语句必须缩进来解决的)。
为了保证清晰,以下各节中语法规则采用将每个子句都放在单独行中的格式。
8.1. if
语句¶
if
语句用于条件性执行:
if_stmt ::= "if"assignment_expression
":"suite
("elif"assignment_expression
":"suite
)* ["else" ":"suite
]
它通过对表达式逐个求值直至找到一个真值(请参阅 布尔运算 了解真值与假值的定义)在子句体中选择唯一匹配的一个;然后执行该子句体(而且 if
语句的其他部分不会被执行或求值)。 如果所有表达式均为假值,则如果 else
子句体存在就会被执行。
8.2. while
语句¶
while
语句用于在表达式保持为真的情况下重复地执行:
while_stmt ::= "while"assignment_expression
":"suite
["else" ":"suite
]
这将重复地检验表达式,并且如果其值为真就执行第一个子句体;如果表达式值为假(这可能在第一次检验时就发生)则如果 else
子句体存在就会被执行并终止循环。
第一个子句体中的 break
语句在执行时将终止循环且不执行 else
子句体。 第一个子句体中的 continue
语句在执行时将跳过子句体中的剩余部分并返回检验表达式。
8.3. for
语句¶
for
语句用于对序列(例如字符串、元组或列表)或其他可迭代对象中的元素进行迭代:
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 赋值语句), 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.
第一个子句体中的 break
语句在执行时将终止循环且不执行 else
子句体。 第一个子句体中的 continue
语句在执行时将跳过子句体中的剩余部分并转往下一项继续执行,或者在没有下一项时转往 else
子句执行。
for 循环会对目标列表中的变量进行赋值。 这将覆盖之前对这些变量的所有赋值,包括在 for 循环体中的赋值:
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
目标列表中的名称在循环结束时不会被删除,但如果序列为空,则它们根本不会被循环所赋值。 提示:内置函数 range()
会返回一个可迭代的整数序列,适用于模拟 Pascal 中的 for i := a to b do
这种效果;例如 list(range(3))
会返回列表 [0, 1, 2]
。
在 3.11 版更改: Starred elements are now allowed in the expression list.
8.4. try
语句¶
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
有关异常的更多信息可以在 异常 一节找到,有关使用 raise
语句生成异常的信息可以在 raise 语句 一节找到。
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
被转写为
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.
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 标准类型层级结构) identifying
the point in the program where the exception occurred. The details about the
exception accessed via sys.exc_info()
are restored to their previous values
when leaving an exception handler:
>>> print(sys.exc_info())
(None, None, None)
>>> try:
... raise TypeError
... except:
... print(sys.exc_info())
... try:
... raise ValueError
... except:
... print(sys.exc_info())
... print(sys.exc_info())
...
(<class 'TypeError'>, TypeError(), <traceback object at 0x10efad080>)
(<class 'ValueError'>, ValueError(), <traceback object at 0x10efad040>)
(<class 'TypeError'>, TypeError(), <traceback object at 0x10efad080>)
>>> print(sys.exc_info())
(None, None, None)
8.4.2. except*
clause¶
The except*
clause(s) are used for handling
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 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.
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¶
如果控制流离开 try
子句体时没有引发异常,并且没有执行 return
, continue
或 break
语句,可选的 else
子句将被执行。 else
语句中的异常不会由之前的 except
子句处理。
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'
在 3.8 版更改: Prior to Python 3.8, a continue
statement was illegal in the
finally
clause due to a problem with the implementation.
8.5. with
语句¶
with
语句用于包装带有使用上下文管理器 (参见 with 语句上下文管理器 一节) 定义的方法的代码块的执行。 这允许对普通的 try
…except
…finally
使用模式进行封装以方便地重用。
with_stmt ::= "with" ( "("with_stmt_contents
","? ")" |with_stmt_contents
) ":"suite
with_stmt_contents ::=with_item
(","with_item
)* with_item ::=expression
["as"target
]
带有一个“项目”的 with
语句的执行过程如下:
The context expression (the expression given in the
with_item
) is evaluated to obtain a context manager.载入上下文管理器的
__enter__()
以便后续使用。载入上下文管理器的
__exit__()
以便后续使用。发起调用上下文管理器的
__enter__()
方法。如果
with
语句中包含一个目标,来自__enter__()
的返回值将被赋值给它。备注
with
语句会保证如果__enter__()
方法返回时未发生错误,则__exit__()
将总是被调用。 因此,如果在对目标列表赋值期间发生错误,则会将其视为在语句体内部发生的错误。 参见下面的第 6 步。执行语句体。
发起调用上下文管理器的
__exit__()
方法。 如果语句体的退出是由异常导致的,则其类型、值和回溯信息将被作为参数传递给__exit__()
。 否则的话,将提供三个None
参数。如果语句体的退出是由异常导致的,并且来自
__exit__()
方法的返回值为假,则该异常会被重新引发。 如果返回值为真,则该异常会被抑制,并会继续执行with
语句之后的语句。如果语句体由于异常以外的任何原因退出,则来自
__exit__()
的返回值会被忽略,并会在该类退出正常的发生位置继续执行。
以下代码:
with EXPRESSION as TARGET:
SUITE
在语义上等价于:
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)
如果有多个项目,则会视作存在多个 with
语句嵌套来处理多个上下文管理器:
with A() as a, B() as b:
SUITE
在语义上等价于:
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. For example:
with (
A() as a,
B() as b,
):
SUITE
在 3.1 版更改: 支持多个上下文表达式。
在 3.10 版更改: Support for using grouping parentheses to break the statement in multiple lines.
8.6. The match
statement¶
3.10 新版功能.
The match statement is used for pattern matching. Syntax:
match_stmt ::= 'match'subject_expr
":" NEWLINE INDENTcase_block
+ DEDENT subject_expr ::=star_named_expression
","star_named_expressions
? |named_expression
case_block ::= 'case'patterns
[guard
] ":"block
备注
This section uses single quotes to denote soft keywords.
Pattern matching takes a pattern as input (following case
) and a subject
value (following match
). The pattern (which may contain subpatterns) is
matched against the subject value. The outcomes are:
A match success or failure (also termed a pattern success or failure).
Possible binding of matched values to a name. The prerequisites for this are further discussed below.
The match
and case
keywords are soft keywords.
参见
8.6.1. 概述¶
Here’s an overview of the logical flow of a match statement:
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.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.备注
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.
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
insidecase_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.
备注
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:
Check that the pattern in the
case
block succeeded. If the pattern failed, theguard
is not evaluated and the nextcase
block is checked.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
parenthesized irrefutable patterns
8.6.4. Patterns¶
备注
This section uses grammar notations beyond standard EBNF:
the notation
SEP.RULE+
is shorthand forRULE (SEP RULE)*
the notation
!RULE
is shorthand for a negative lookahead assertion
The top-level syntax for patterns
is:
patterns ::=open_sequence_pattern
|pattern
pattern ::=as_pattern
|or_pattern
closed_pattern ::= |literal_pattern
|capture_pattern
|wildcard_pattern
|value_pattern
|group_pattern
|sequence_pattern
|mapping_pattern
|class_pattern
The descriptions below will include a description “in simple terms” of what a pattern does for illustration purposes (credits to Raymond Hettinger for a document that inspired most of the descriptions). Note that these descriptions are purely for illustration purposes and may not reflect the underlying implementation. Furthermore, they do not cover all valid forms.
8.6.4.1. OR Patterns¶
An OR pattern is two or more patterns separated by vertical
bars |
. Syntax:
or_pattern ::= "|".closed_pattern
+
Only the final subpattern may be irrefutable, and each subpattern must bind the same set of names to avoid ambiguity.
An OR pattern matches each of its subpatterns in turn to the subject value, until one succeeds. The OR pattern is then considered successful. Otherwise, if none of the subpatterns succeed, the OR pattern fails.
In simple terms, P1 | P2 | ...
will try to match P1
, if it fails it will try to
match P2
, succeeding immediately if any succeeds, failing otherwise.
8.6.4.2. AS Patterns¶
An AS pattern matches an OR pattern on the left of the as
keyword against a subject. Syntax:
as_pattern ::=or_pattern
"as"capture_pattern
If the OR pattern fails, the AS pattern fails. Otherwise, the AS pattern binds
the subject to the name on the right of the as keyword and succeeds.
capture_pattern
cannot be a a _
.
In simple terms P as NAME
will match with P
, and on success it will
set NAME = <subject>
.
8.6.4.3. Literal Patterns¶
A literal pattern corresponds to most literals in Python. Syntax:
literal_pattern ::=signed_number
|signed_number
"+" NUMBER |signed_number
"-" NUMBER |strings
| "None" | "True" | "False" |signed_number
: NUMBER | "-" NUMBER
The rule strings
and the token NUMBER
are defined in the
standard Python grammar. Triple-quoted strings are
supported. Raw strings and byte strings are supported. 格式字符串字面值 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
备注
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 [...]
).
备注
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:
If the subject value is not a sequence 2, the sequence pattern fails.
If the subject value is an instance of
str
,bytes
orbytearray
the sequence pattern fails.The subsequent steps depend on whether the sequence pattern is fixed or variable-length.
If the sequence pattern is fixed-length:
If the length of the subject sequence is not equal to the number of subpatterns, the sequence pattern fails
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:
If the length of the subject sequence is less than the number of non-star subpatterns, the sequence pattern fails.
The leading non-star subpatterns are matched to their corresponding items as for fixed-length sequences.
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.
Remaining non-star subpatterns are matched to their corresponding subject items, as for a fixed-length sequence.
备注
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 sequencelen(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:
If the subject value is not a mapping 3,the mapping pattern fails.
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.
If duplicate keys are detected in the mapping pattern, the pattern is considered invalid. A
SyntaxError
is raised for duplicate literal values; or aValueError
for named keys of the same value.
备注
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 mappingKEY1 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:
If
name_or_attr
is not an instance of the builtintype
, raiseTypeError
.If the subject value is not an instance of
name_or_attr
(tested viaisinstance()
), the class pattern fails.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 classname_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 notTypeError
is raised.If there are duplicate keywords,
TypeError
is raised.
- 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:
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 value0
, but not the value0.0
.
In simple terms CLS(P1, attr=P2)
matches only if the following happens:
isinstance(<subject>, CLS)
convert
P1
to a keyword pattern usingCLS.__match_args__
- For each keyword argument
attr=P2
: hasattr(<subject>, "attr")
P2
matches<subject>.attr
- For each keyword argument
… and so on for the corresponding keyword argument/pattern pair.
8.7. 函数定义¶
函数定义就是对用户自定义函数的定义(参见 标准类型层级结构 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
函数定义是一条可执行语句。 它执行时会在当前局部命名空间中将函数名称绑定到一个函数对象(函数可执行代码的包装器)。 这个函数对象包含对当前全局命名空间的引用,作为函数被调用时所使用的全局命名空间。
函数定义并不会执行函数体;只有当函数被调用时才会执行此操作。 4
一个函数定义可以被一个或多个 decorator 表达式所包装。 当函数被定义时将在包含该函数定义的作用域中对装饰器表达式求值。 求值结果必须是一个可调用对象,它会以该函数对象作为唯一参数被发起调用。 其返回值将被绑定到函数名称而非函数对象。 多个装饰器会以嵌套方式被应用。 例如以下代码
@f1(arg)
@f2
def func(): pass
大致等价于
def func(): pass
func = f1(arg)(f2(func))
不同之处在于原始函数并不会被临时绑定到名称 func
。
在 3.9 版更改: Functions may be decorated with any valid
assignment_expression
. Previously, the grammar was
much more restrictive; see PEP 614 for details.
当一个或多个 形参 具有 形参 =
表达式 这样的形式时,该函数就被称为具有“默认形参值”。 对于一个具有默认值的形参,其对应的 argument 可以在调用中被省略,在此情况下会用形参的默认值来替代。 如果一个形参具有默认值,后续所有在 “*
” 之前的形参也必须具有默认值 — 这个句法限制并未在语法中明确表达。
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
函数调用的语义在 调用 一节中有更详细的描述。 函数调用总是会给形参列表中列出的所有形参赋值,或是用位置参数,或是用关键字参数,或是用默认值。 如果存在 “*identifier
” 这样的形式,它会被初始化为一个元组来接收任何额外的位置参数,默认为一个空元组。 如果存在 “**identifier
” 这样的形式,它会被初始化为一个新的有序映射来接收任何额外的关键字参数,默认为一个相同类型的空映射。 在 “*
” 或 “*identifier
” 之后的形参都是仅限关键字形参因而只能通过关键字参数传入。 在 “/
” 之前的形参都是仅限位置形参因而只能通过位置参数传入。
在 3.8 版更改: 可以使用 /
函数形参语法来标示仅限位置形参。 请参阅 PEP 570 了解详情。
形参可以带有 标注,其形式为在形参名称后加上 “: expression
”。 任何形参都可以带有标注,甚至 *identifier
或 **identifier
这样的形参也可以。 函数可以带有“返回”标注,其形式为在形参列表后加上 “-> expression
”。 这些标注可以是任何有效的 Python 表达式。 标注的存在不会改变函数的语义。 标注值可以作为函数对象的 __annotations__
属性中以对应形参名称为键的字典值被访问。 如果使用了 annotations
import from __future__
的方式,则标注会在运行时保存为字符串以启用延迟求值特性。 否则,它们会在执行函数定义时被求值。 在这种情况下,标注的求值顺序可能与它们在源代码中出现的顺序不同。
创建匿名函数(未绑定到一个名称的函数)以便立即在表达式中使用也是可能的。 这需要使用 lambda 表达式,具体描述见 lambda 表达式 一节。 请注意 lambda 只是简单函数定义的一种简化写法;在 “def
” 语句中定义的函数也可以像用 lambda 表达式定义的函数一样被传递或赋值给其他名称。 “def
” 形式实际上更为强大,因为它允许执行多条语句和使用标注。
程序员注意事项: 函数属于一类对象。 在一个函数内部执行的 “def
” 语句会定义一个局部函数并可被返回或传递。 在嵌套函数中使用的自由变量可以访问包含该 def 语句的函数的局部变量。 详情参见 命名与绑定 一节。
8.8. 类定义¶
类定义就是对类对象的定义 (参见 标准类型层级结构 一节):
classdef ::= [decorators
] "class"classname
[inheritance
] ":"suite
inheritance ::= "(" [argument_list
] ")" classname ::=identifier
类定义是一条可执行语句。 其中继承列表通常给出基类的列表 (进阶用法请参见 元类),列表中的每一项都应当被求值为一个允许子类的类对象。 没有继承列表的类默认继承自基类 object
;因此,:
class Foo:
pass
等价于
class Foo(object):
pass
随后类体将在一个新的执行帧 (参见 命名与绑定) 中被执行,使用新创建的局部命名空间和原有的全局命名空间。 (通常,类体主要包含函数定义。) 当类体结束执行时,其执行帧将被丢弃而其局部命名空间会被保存。 5 一个类对象随后会被创建,其基类使用给定的继承列表,属性字典使用保存的局部命名空间。 类名称将在原有的全局命名空间中绑定到该类对象。
在类体内定义的属性的顺序保存在新类的 __dict__
中。 请注意此顺序的可靠性只限于类刚被创建时,并且只适用于使用定义语法所定义的类。
类的创建可使用 元类 进行重度定制。
类也可以被装饰:就像装饰函数一样,:
@f1(arg)
@f2
class Foo: pass
大致等价于
class Foo: pass
Foo = f1(arg)(f2(Foo))
装饰器表达式的求值规则与函数装饰器相同。 结果随后会被绑定到类名称。
在 3.9 版更改: Classes may be decorated with any valid
assignment_expression
. Previously, the grammar was
much more restrictive; see PEP 614 for details.
程序员注意事项: 在类定义内定义的变量是类属性;它们将被类实例所共享。 实例属性可通过 self.name = value
在方法中设定。 类和实例属性均可通过 “self.name
” 表示法来访问,当通过此方式访问时实例属性会隐藏同名的类属性。 类属性可被用作实例属性的默认值,但在此场景下使用可变值可能导致未预期的结果。 可以使用 描述器 来创建具有不同实现细节的实例变量。
8.9. 协程¶
3.5 新版功能.
8.9.1. 协程函数定义¶
async_funcdef ::= [decorators
] "async" "def"funcname
"(" [parameter_list
] ")" ["->"expression
] ":"suite
Execution of Python coroutines can be suspended and resumed at many points
(see coroutine). await
expressions, async for
and
async with
can only be used in the body of a coroutine function.
使用 async def
语法定义的函数总是为协程函数,即使它们不包含 await
或 async
关键字。
在协程函数体中使用 yield from
表达式将引发 SyntaxError
。
协程函数的例子:
async def func(param1, param2):
do_stuff()
await some_coroutine()
在 3.7 版更改: 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
语句¶
async_for_stmt ::= "async" for_stmt
asynchronous iterable 提供了 __aiter__
方法,该方法会直接返回 asynchronous iterator,它可以在其 __anext__
方法中调用异步代码。
async for
语句允许方便地对异步可迭代对象进行迭代。
以下代码:
async for TARGET in ITER:
SUITE
else:
SUITE2
在语义上等价于:
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.
在协程函数体之外使用 async for
语句将引发 SyntaxError
。
8.9.3. async with
语句¶
async_with_stmt ::= "async" with_stmt
asynchronous context manager 是一种 context manager,能够在其 enter 和 exit 方法中暂停执行。
以下代码:
async with EXPRESSION as TARGET:
SUITE
在语义上等价于:
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.
在协程函数体之外使用 async with
语句将引发 SyntaxError
。
参见
- PEP 492 - 使用 async 和 await 语法实现协程
将协程作为 Python 中的一个正式的单独概念,并增加相应的支持语法。
备注
- 1
异常会被传播给发起调用栈,除非存在一个
finally
子句正好引发了另一个异常。 新引发的异常将导致旧异常的丢失。- 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 seta class that inherits from any of the above
The following standard library classes are sequences:
备注
Subject values of type
str
,bytes
, andbytearray
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 seta class that inherits from any of the above
The standard library classes
dict
andtypes.MappingProxyType
are mappings.- 4
作为函数体的第一条语句出现的字符串字面值会被转换为函数的
__doc__
属性,也就是该函数的 docstring。- 5
作为类体的第一条语句出现的字符串字面值会被转换为命名空间的
__doc__
条目,也就是该类的 docstring。