7.
简单语句
¶
简单语句由单逻辑行构成。由分号分隔的单行可能出现几条简单语句。简单语句语法：
simple_stmt ::= expression_stmt
| assert_stmt
| assignment_stmt
| augmented_assignment_stmt
| annotated_assignment_stmt
| pass_stmt
| del_stmt
| return_stmt
| yield_stmt
| raise_stmt
| break_stmt
| continue_stmt
| import_stmt
| future_stmt
| global_stmt
| nonlocal_stmt
| type_stmt
7.1.
表达式语句
¶
Expression statements are used (mostly interactively) to compute and write a value, or (usually) to call a procedure (a function that returns no meaningful result; in Python, procedures return the value
None
). Other uses of expression statements are allowed and occasionally useful. The syntax for an expression statement is:
expression_stmt ::= starred_expression
An expression statement evaluates the expression list (which may be a single expression).
在交互模式下，若值不是
None
, it is converted to a string using the built-in
repr()
function and the resulting string is written to standard output on a line by itself (except if the result is
None
, so that procedure calls do not cause any output.)
7.2.
赋值语句
¶
Assignment statements are used to (re)bind names to values and to modify attributes or items of mutable objects:
assignment_stmt ::= (target_list "=")+ (starred_expression | yield_expression)
target_list ::= target ("," target)* [","]
target ::= identifier
| "(" [target_list] ")"
| "[" [target_list] "]"
| attributeref
| subscription
| slicing
| "*" target
(见章节
基元
for the syntax definitions for
attributeref
,
subscription
，和
slicing
)。
An assignment statement evaluates the expression list (remember that this can be a single expression or a comma-separated list, the latter yielding a tuple) and assigns the single resulting object to each of the target lists, from left to right.
Assignment is defined recursively depending on the form of the target (list). When a target is part of a mutable object (an attribute reference, subscription or slicing), the mutable object must ultimately perform the assignment and decide about its validity, and may raise an exception if the assignment is unacceptable. The rules observed by various types and the exceptions raised are given with the definition of the object types (see section
标准类型层次结构
).
Assignment of an object to a target list, optionally enclosed in parentheses or square brackets, is recursively defined as follows.
Assignment of an object to a single target is recursively defined as follows.
-
若目标是标识符 (名称)：
-
If the name does not occur in a
global
or
nonlocal
statement in the current code block: the name is bound to the object in the current local namespace.
-
Otherwise: the name is bound to the object in the global namespace or the outer namespace determined by
nonlocal
，分别。
The name is rebound if it was already bound. This may cause the reference count for the object previously bound to the name to reach zero, causing the object to be deallocated and its destructor (if it has one) to be called.
-
If the target is an attribute reference: The primary expression in the reference is evaluated. It should yield an object with assignable attributes; if this is not the case,
TypeError
is raised. That object is then asked to assign the assigned object to the given attribute; if it cannot perform the assignment, it raises an exception (usually but not necessarily
AttributeError
).
Note: If the object is a class instance and the attribute reference occurs on both sides of the assignment operator, the right-hand side expression,
a.x
can access either an instance attribute or (if no instance attribute exists) a class attribute. The left-hand side target
a.x
is always set as an instance attribute, creating it if necessary. Thus, the two occurrences of
a.x
do not necessarily refer to the same attribute: if the right-hand side expression refers to a class attribute, the left-hand side creates a new instance attribute as the target of the assignment:
class Cls:
x = 3 # class variable
inst = Cls()
inst.x = inst.x + 1 # writes inst.x as 4 leaving Cls.x as 3
This description does not necessarily apply to descriptor attributes, such as properties created with
property()
.
If the target is a subscription: The primary expression in the reference is evaluated. It should yield either a mutable sequence object (such as a list) or a mapping object (such as a dictionary). Next, the subscript expression is evaluated.
If the primary is a mutable sequence object (such as a list), the subscript must yield an integer. If it is negative, the sequence’s length is added to it. The resulting value must be a nonnegative integer less than the sequence’s length, and the sequence is asked to assign the assigned object to its item with that index. If the index is out of range,
IndexError
is raised (assignment to a subscripted sequence cannot add new items to a list).
If the primary is a mapping object (such as a dictionary), the subscript must have a type compatible with the mapping’s key type, and the mapping is then asked to create a key/value pair which maps the subscript to the assigned object. This can either replace an existing key/value pair with the same key value, or insert a new key/value pair (if no key with the same value existed).
For user-defined objects, the
__setitem__()
method is called with appropriate arguments.
If the target is a slicing: The primary expression in the reference is evaluated. It should yield a mutable sequence object (such as a list). The assigned object should be a sequence object of the same type. Next, the lower and upper bound expressions are evaluated, insofar they are present; defaults are zero and the sequence’s length. The bounds should evaluate to integers. If either bound is negative, the sequence’s length is added to it. The resulting bounds are clipped to lie between zero and the sequence’s length, inclusive. Finally, the sequence object is asked to replace the slice with the items of the assigned sequence. The length of the slice may be different from the length of the assigned sequence, thus changing the length of the target sequence, if the target sequence allows it.
CPython 实现细节：
In the current implementation, the syntax for targets is taken to be the same as for expressions, and invalid syntax is rejected during the code generation phase, causing less detailed error messages.
Although the definition of assignment implies that overlaps between the left-hand side and the right-hand side are ‘simultaneous’ (for example
a, b =
b, a
swaps two variables), overlaps
在
the collection of assigned-to variables occur left-to-right, sometimes resulting in confusion. For instance, the following program prints
[0, 2]
:
x = [0, 1]
i = 0
i, x[i] = 1, 2 # i is updated, then x[i] is updated
print(x)
另请参阅
-
PEP 3132
- 扩展可迭代解包
-
规范对于
*target
特征。
7.2.1.
自变量赋值语句
¶
Augmented assignment is the combination, in a single statement, of a binary operation and an assignment statement:
augmented_assignment_stmt ::= augtarget augop (expression_list | yield_expression)
augtarget ::= identifier | attributeref | subscription | slicing
augop ::= "+=" | "-=" | "*=" | "@=" | "/=" | "//=" | "%=" | "**="
| ">>=" | "<<=" | "&=" | "^=" | "|="
(见章节
基元
for the syntax definitions of the last three symbols.)
An augmented assignment evaluates the target (which, unlike normal assignment statements, cannot be an unpacking) and the expression list, performs the binary operation specific to the type of assignment on the two operands, and assigns the result to the original target. The target is only evaluated once.
An augmented assignment statement like
x += 1
can be rewritten as
x = x +
1
to achieve a similar, but not exactly equal effect. In the augmented version,
x
is only evaluated once. Also, when possible, the actual operation is performed
in-place
, meaning that rather than creating a new object and assigning that to the target, the old object is modified instead.
Unlike normal assignments, augmented assignments evaluate the left-hand side
before
evaluating the right-hand side. For example,
a[i] += f(x)
first looks-up
a[i]
, then it evaluates
f(x)
and performs the addition, and lastly, it writes the result back to
a[i]
.
With the exception of assigning to tuples and multiple targets in a single statement, the assignment done by augmented assignment statements is handled the same way as normal assignments. Similarly, with the exception of the possible
in-place
behavior, the binary operation performed by augmented assignment is the same as the normal binary operations.
For targets which are attribute references, the same
关于类和实例属性的告诫
applies as for regular assignments.
7.2.2.
注解赋值语句
¶
Annotation
assignment is the combination, in a single statement, of a variable or attribute annotation and an optional assignment statement:
annotated_assignment_stmt ::= augtarget ":" expression
["=" (starred_expression | yield_expression)]
The difference from normal
赋值语句
is that only a single target is allowed.
The assignment target is considered “simple” if it consists of a single name that is not enclosed in parentheses. For simple assignment targets, if in class or module scope, the annotations are evaluated and stored in a special class or module attribute
__annotations__
that is a dictionary mapping from variable names (mangled if private) to evaluated annotations. This attribute is writable and is automatically created at the start of class or module body execution, if annotations are found statically.
If the assignment target is not simple (an attribute, subscript node, or parenthesized name), the annotation is evaluated if in class or module scope, but not stored.
If a name is annotated in a function scope, then this name is local for that scope. Annotations are never evaluated and stored in function scopes.
If the right hand side is present, an annotated assignment performs the actual assignment before evaluating annotations (where applicable). If the right hand side is not present for an expression target, then the interpreter evaluates the target except for the last
__setitem__()
or
__setattr__()
调用。
另请参阅
-
PEP 526
- 变量注解句法
-
The proposal that added syntax for annotating the types of variables (including class variables and instance variables), instead of expressing them through comments.
-
PEP 484
- 类型提示
-
The proposal that added the
typing
module to provide a standard syntax for type annotations that can be used in static analysis tools and IDEs.
3.8 版改变：
Now annotated assignments allow the same expressions in the right hand side as regular assignments. Previously, some expressions (like un-parenthesized tuple expressions) caused a syntax error.
7.3.
The
assert
语句
¶
assert 语句是将调试断言插入程序的方便方式：
assert_stmt ::= "assert" expression ["," expression]
简单形式
assert expression
，相当于
if __debug__:
if not expression: raise AssertionError
扩展形式
assert expression1, expression2
，相当于
if __debug__:
if not expression1: raise AssertionError(expression2)
这些等价假定
__debug__
and
AssertionError
引用具有这些名称的内置变量。在当前实现中，内置变量
__debug__
is
True
在正常情况下，
False
当请求优化时 (命令行选项
-O
). The current code generator emits no code for an
assert
statement when optimization is requested at compile time. Note that it is unnecessary to include the source code for the expression that failed in the error message; it will be displayed as part of the stack trace.
赋值
__debug__
是非法的。内置变量值是确定的，当解释器启动时。
7.4.
The
pass
语句
¶
pass_stmt ::= "pass"
pass
is a null operation — when it is executed, nothing happens. It is useful as a placeholder when a statement is required syntactically, but no code needs to be executed, for example:
def f(arg): pass # a function that does nothing (yet)
class C: pass # a class with no methods (yet)
7.5.
The
del
语句
¶
del_stmt ::= "del" target_list
Deletion is recursively defined very similar to the way assignment is defined. Rather than spelling it out in full details, here are some hints.
Deletion of a target list recursively deletes each target, from left to right.
Deletion of a name removes the binding of that name from the local or global namespace, depending on whether the name occurs in a
global
statement in the same code block. If the name is unbound, a
NameError
exception will be raised.
Deletion of attribute references, subscriptions and slicings is passed to the primary object involved; deletion of a slicing is in general equivalent to assignment of an empty slice of the right type (but even this is determined by the sliced object).
3.2 版改变：
Previously it was illegal to delete a name from the local namespace if it occurs as a free variable in a nested block.
7.7.
The
yield
语句
¶
yield_stmt ::= yield_expression
A
yield
语句语义上相当于
yield 表达式
。
yield
statement can be used to omit the parentheses that would otherwise be required in the equivalent yield expression statement. For example, the yield statements
yield <expr>
yield from <expr>
are equivalent to the yield expression statements
(yield <expr>)
(yield from <expr>)
Yield expressions and statements are only used when defining a
generator
function, and are only used in the body of the generator function. Using
yield
in a function definition is sufficient to cause that definition to create a generator function instead of a normal function.
对于完整细节的
yield
语义，参考
yield 表达式
章节。
7.8.
The
raise
语句
¶
raise_stmt ::= "raise" [expression ["from" expression]]
If no expressions are present,
raise
re-raises the exception that is currently being handled, which is also known as the
active exception
. If there isn’t currently an active exception, a
RuntimeError
exception is raised indicating that this is an error.
否则，
raise
evaluates the first expression as the exception object. It must be either a subclass or an instance of
BaseException
. If it is a class, the exception instance will be obtained when needed by instantiating the class with no arguments.
The
type
of the exception is the exception instance’s class, the
值
is the instance itself.
A traceback object is normally created automatically when an exception is raised and attached to it as the
__traceback__
attribute. You can create an exception and set your own traceback in one step using the
with_traceback()
exception method (which returns the same exception instance, with its traceback set to its argument), like so:
raise Exception("foo occurred").with_traceback(tracebackobj)
The
from
clause is used for exception chaining: if given, the second
表达式
must be another exception class or instance. If the second expression is an exception instance, it will be attached to the raised exception as the
__cause__
attribute (which is writable). If the expression is an exception class, the class will be instantiated and the resulting exception instance will be attached to the raised exception as the
__cause__
attribute. If the raised exception is not handled, both exceptions will be printed:
>>> try:
... print(1 / 0)
... except Exception as exc:
... raise RuntimeError("Something bad happened") from exc
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
print(1 / 0)
~~^~~
ZeroDivisionError: division by zero
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
raise RuntimeError("Something bad happened") from exc
RuntimeError: Something bad happened
A similar mechanism works implicitly if a new exception is raised when an exception is already being handled. An exception may be handled when an
except
or
finally
子句，或
with
statement, is used. The previous exception is then attached as the new exception’s
__context__
属性：
>>> try:
... print(1 / 0)
... except:
... raise RuntimeError("Something bad happened")
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
print(1 / 0)
~~^~~
ZeroDivisionError: division by zero
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
raise RuntimeError("Something bad happened")
RuntimeError: Something bad happened
Exception chaining can be explicitly suppressed by specifying
None
在
from
子句：
>>> try:
... print(1 / 0)
... except:
... raise RuntimeError("Something bad happened") from None
...
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
RuntimeError: Something bad happened
可以找到有关异常的额外信息在章节
异常
, and information about handling exceptions is in section
try 语句
.
3.3 版改变：
None
现在准许作为
Y
in
raise X from Y
.
添加
__suppress_context__
attribute to suppress automatic display of the exception context.
3.11 版改变：
If the traceback of the active exception is modified in an
except
clause, a subsequent
raise
statement re-raises the exception with the modified traceback. Previously, the exception was re-raised with the traceback it had when it was caught.
7.9.
The
break
语句
¶
break_stmt ::= "break"
break
may only occur syntactically nested in a
for
or
while
loop, but not nested in a function or class definition within that loop.
It terminates the nearest enclosing loop, skipping the optional
else
clause if the loop has one.
若
for
loop is terminated by
break
, the loop control target keeps its current value.
当
break
传递控制脱离
try
语句带有
finally
子句，
finally
clause is executed before really leaving the loop.
7.10.
The
continue
语句
¶
continue_stmt ::= "continue"
continue
may only occur syntactically nested in a
for
or
while
loop, but not nested in a function or class definition within that loop. It continues with the next cycle of the nearest enclosing loop.
当
continue
传递控制脱离
try
语句带有
finally
子句，
finally
clause is executed before really starting the next loop cycle.
7.11.
The
import
语句
¶
import_stmt ::= "import" module ["as" identifier] ("," module ["as" identifier])*
| "from" relative_module "import" identifier ["as" identifier]
("," identifier ["as" identifier])*
| "from" relative_module "import" "(" identifier ["as" identifier]
("," identifier ["as" identifier])* [","] ")"
| "from" relative_module "import" "*"
module ::= (identifier ".")* identifier
relative_module ::= "."* module | "."+
The basic import statement (no
from
clause) is executed in two steps:
-
find a module, loading and initializing it if necessary
-
define a name or names in the local namespace for the scope where the
import
statement occurs.
When the statement contains multiple clauses (separated by commas) the two steps are carried out separately for each clause, just as though the clauses had been separated out into individual import statements.
The details of the first step, finding and loading modules, are described in greater detail in the section on the
导入系统
, which also describes the various types of packages and modules that can be imported, as well as all the hooks that can be used to customize the import system. Note that failures in this step may indicate either that the module could not be located,
or
that an error occurred while initializing the module, which includes execution of the module’s code.
If the requested module is retrieved successfully, it will be made available in the local namespace in one of three ways:
-
If the module name is followed by
as
, then the name following
as
is bound directly to the imported module.
-
If no other name is specified, and the module being imported is a top level module, the module’s name is bound in the local namespace as a reference to the imported module
-
If the module being imported is
not
a top level module, then the name of the top level package that contains the module is bound in the local namespace as a reference to the top level package. The imported module must be accessed using its full qualified name rather than directly
The
from
form uses a slightly more complex process:
-
find the module specified in the
from
clause, loading and initializing it if necessary;
-
for each of the identifiers specified in the
import
子句：
-
check if the imported module has an attribute by that name
-
if not, attempt to import a submodule with that name and then check the imported module again for that attribute
-
if the attribute is not found,
ImportError
被引发。
-
otherwise, a reference to that value is stored in the local namespace, using the name in the
as
clause if it is present, otherwise using the attribute name
范例：
import foo # foo imported and bound locally
import foo.bar.baz # foo, foo.bar, and foo.bar.baz imported, foo bound locally
import foo.bar.baz as fbb # foo, foo.bar, and foo.bar.baz imported, foo.bar.baz bound as fbb
from foo.bar import baz # foo, foo.bar, and foo.bar.baz imported, foo.bar.baz bound as baz
from foo import attr # foo imported and foo.attr bound as attr
If the list of identifiers is replaced by a star (
'*'
), all public names defined in the module are bound in the local namespace for the scope where the
import
statement occurs.
The
public names
defined by a module are determined by checking the module’s namespace for a variable named
__all__
; if defined, it must be a sequence of strings which are names defined or imported by that module. The names given in
__all__
are all considered public and are required to exist. If
__all__
is not defined, the set of public names includes all names found in the module’s namespace which do not begin with an underscore character (
'_'
).
__all__
should contain the entire public API. It is intended to avoid accidentally exporting items that are not part of the API (such as library modules which were imported and used within the module).
The wild card form of import —
from module import *
— is only allowed at the module level. Attempting to use it in class or function definitions will raise a
SyntaxError
.
When specifying what module to import you do not have to specify the absolute name of the module. When a module or package is contained within another package it is possible to make a relative import within the same top package without having to mention the package name. By using leading dots in the specified module or package after
from
you can specify how high to traverse up the current package hierarchy without specifying exact names. One leading dot means the current package where the module making the import exists. Two dots means up one package level. Three dots is up two levels, etc. So if you execute
from . import mod
from a module in the
pkg
package then you will end up importing
pkg.mod
. If you execute
from ..subpkg2
import
mod
from within
pkg.subpkg1
you will import
pkg.subpkg2.mod
. The specification for relative imports is contained in the
包相对导入
章节。
importlib.import_module()
is provided to support applications that determine dynamically the modules to be loaded.
引发
审计事件
import
采用自变量
module
,
filename
,
sys.path
,
sys.meta_path
,
sys.path_hooks
.
7.11.1.
未来语句
¶
A
未来语句
is a directive to the compiler that a particular module should be compiled using syntax or semantics that will be available in a specified future release of Python where the feature becomes standard.
The future statement is intended to ease migration to future versions of Python that introduce incompatible changes to the language. It allows use of the new features on a per-module basis before the release in which the feature becomes standard.
future_stmt ::= "from" "__future__" "import" feature ["as" identifier]
("," feature ["as" identifier])*
| "from" "__future__" "import" "(" feature ["as" identifier]
("," feature ["as" identifier])* [","] ")"
feature ::= identifier
A future statement must appear near the top of the module. The only lines that can appear before a future statement are:
The only feature that requires using the future statement is
annotations
(见
PEP 563
).
All historical features enabled by the future statement are still recognized by Python 3. The list includes
absolute_import
,
division
,
generators
,
generator_stop
,
unicode_literals
,
print_function
,
nested_scopes
and
with_statement
. They are all redundant because they are always enabled, and only kept for backwards compatibility.
A future statement is recognized and treated specially at compile time: Changes to the semantics of core constructs are often implemented by generating different code. It may even be the case that a new feature introduces new incompatible syntax (such as a new reserved word), in which case the compiler may need to parse the module differently. Such decisions cannot be pushed off until runtime.
For any given release, the compiler knows which feature names have been defined, and raises a compile-time error if a future statement contains a feature not known to it.
The direct runtime semantics are the same as for any import statement: there is a standard module
__future__
, described later, and it will be imported in the usual way at the time the future statement is executed.
The interesting runtime semantics depend on the specific feature enabled by the future statement.
Note that there is nothing special about the statement:
import __future__ [as name]
That is not a future statement; it’s an ordinary import statement with no special semantics or syntax restrictions.
Code compiled by calls to the built-in functions
exec()
and
compile()
that occur in a module
M
containing a future statement will, by default, use the new syntax or semantics associated with the future statement. This can be controlled by optional arguments to
compile()
— see the documentation of that function for details.
A future statement typed at an interactive interpreter prompt will take effect for the rest of the interpreter session. If an interpreter is started with the
-i
option, is passed a script name to execute, and the script includes a future statement, it will be in effect in the interactive session started after the script is executed.
另请参阅
-
PEP 236
- Back to the __future__
-
The original proposal for the __future__ mechanism.
7.12.
The
global
语句
¶
global_stmt ::= "global" identifier ("," identifier)*
The
global
statement causes the listed identifiers to be interpreted as globals. It would be impossible to assign to a global variable without
global
, although free variables may refer to globals without being declared global.
The
global
statement applies to the entire scope of a function or class body. A
SyntaxError
is raised if a variable is used or assigned to prior to its global declaration in the scope.
程序员注意：
global
is a directive to the parser. It applies only to code parsed at the same time as the
global
statement. In particular, a
global
statement contained in a string or code object supplied to the built-in
exec()
function does not affect the code block
包含
the function call, and code contained in such a string is unaffected by
global
statements in the code containing the function call. The same applies to the
eval()
and
compile()
函数。
7.13.
The
nonlocal
语句
¶
nonlocal_stmt ::= "nonlocal" identifier ("," identifier)*
When the definition of a function or class is nested (enclosed) within the definitions of other functions, its nonlocal scopes are the local scopes of the enclosing functions. The
nonlocal
statement causes the listed identifiers to refer to names previously bound in nonlocal scopes. It allows encapsulated code to rebind such nonlocal identifiers. If a name is bound in more than one nonlocal scope, the nearest binding is used. If a name is not bound in any nonlocal scope, or if there is no nonlocal scope, a
SyntaxError
被引发。
The
nonlocal
statement applies to the entire scope of a function or class body. A
SyntaxError
is raised if a variable is used or assigned to prior to its nonlocal declaration in the scope.
程序员注意：
nonlocal
is a directive to the parser and applies only to code parsed along with it. See the note for the
global
语句。
7.14.
The
type
语句
¶
type_stmt ::= 'type' identifier [type_params] "=" expression
The
type
statement declares a type alias, which is an instance of
typing.TypeAliasType
.
For example, the following statement creates a type alias:
type Point = tuple[float, float]
This code is roughly equivalent to:
annotation-def VALUE_OF_Point():
return tuple[float, float]
Point = typing.TypeAliasType("Point", VALUE_OF_Point())
annotation-def
indicates an
annotation scope
, which behaves mostly like a function, but with several small differences.
The value of the type alias is evaluated in the annotation scope. It is not evaluated when the type alias is created, but only when the value is accessed through the type alias’s
__value__
attribute (see
Lazy evaluation
). This allows the type alias to refer to names that are not yet defined.
Type aliases may be made generic by adding a
type parameter list
after the name. See
Generic type aliases
for more.
type
是
soft keyword
.
3.12 版添加。
另请参阅
-
PEP 695
- Type Parameter Syntax
-
引入
type
statement and syntax for generic classes and functions.
