python的作用域、globals()-全局变量 和 locals()-局部变量

在python中，函数会创建一个自己的作用域，也称为为命名空间。当我们在函数内部访问某个变量时，函数会优先在自己的命名空间中寻找。

我们自己定义的全局变量均在python内建的globals()函数中，以字典的形式保存。而locals()函数返回的是函数内部本地作用域中的变量名称字典。

查看全局变量和局部变量

a = 1

def b():
    e = 2
    print(locals())

class C:
    def __init__(self):
        pass

print(globals())
b()

输出结果

{'__name__': '__main__', '__doc__': None, '__package__': None, '__loader__': <_frozen_importlib_external.SourceFileLoader object at 0x00000000005055C0>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>, '__file__': 'D:/qqq.py', '__cached__': None, 'a': 1, 'b': <function b at 0x00000000004BC1E0>, 'C': <class '__main__.C'>}
{'e': 2}

由此可以看出，函数都有各自独立的命名空间。

通过全局变量，可以知道内置属性__file__指的是当前运行的文件名称，__name__指的是__main__。

变量解析规则

在python的作用域规则里面，创建变量时一定会在当前作用域里创建同样的变量，但访问或修改变量时，会在当前作用域中查找该变量，如果没找到匹配的变量，就会依次向上在闭合作用域中进行查找，所以在函数中直接访问全局变量也是可以的。

但是变量在调用之前必须被声明，否则报错。

var = "this is a global variable"

def test():
    print("在test函数内调用var：" + var)

test()
print(a)

输出结果

在test函数内调用var：this is a global variable
Traceback (most recent call last):
  File "D:/yibo/_git/study-notes/qqq.py", line 9, in <module>
    print(a)
NameError: name 'a' is not defined

需要注意的是我们在调用print()函数时，并没有定义这个函数，为什么没有报错？

这是因为，如果在全局变量中没有找到函数的变量时，还会去“__builtins__”对应的作用域去查找，我们常用的Python内建函数均放在这个作用域中。

print(globals()["__builtins__"].__dict__)

输出结果

{'__name__': 'builtins', '__doc__': "Built-in functions, exceptions, and other objects.\n\nNoteworthy: None is the `nil' object; Ellipsis represents `...' in slices.", '__package__': '', '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': ModuleSpec(name='builtins', loader=<class '_frozen_importlib.BuiltinImporter'>), '__build_class__': <built-in function __build_class__>, '__import__': <built-in function __import__>, 'abs': <built-in function abs>, 'all': <built-in function all>, 'any': <built-in function any>, 'ascii': <built-in function ascii>, 'bin': <built-in function bin>, 'breakpoint': <built-in function breakpoint>, 'callable': <built-in function callable>, 'chr': <built-in function chr>, 'compile': <built-in function compile>, 'delattr': <built-in function delattr>, 'dir': <built-in function dir>, 'divmod': <built-in function divmod>, 'eval': <built-in function eval>, 'exec': <built-in function exec>, 'format': <built-in function format>, 'getattr': <built-in function getattr>, 'globals': <built-in function globals>, 'hasattr': <built-in function hasattr>, 'hash': <built-in function hash>, 'hex': <built-in function hex>, 'id': <built-in function id>, 'input': <built-in function input>, 'isinstance': <built-in function isinstance>, 'issubclass': <built-in function issubclass>, 'iter': <built-in function iter>, 'len': <built-in function len>, 'locals': <built-in function locals>, 'max': <built-in function max>, 'min': <built-in function min>, 'next': <built-in function next>, 'oct': <built-in function oct>, 'ord': <built-in function ord>, 'pow': <built-in function pow>, 'print': <built-in function print>, 'repr': <built-in function repr>, 'round': <built-in function round>, 'setattr': <built-in function setattr>, 'sorted': <built-in function sorted>, 'sum': <built-in function sum>, 'vars': <built-in function vars>, 'None': None, 'Ellipsis': Ellipsis, 'NotImplemented': NotImplemented, 'False': False, 'True': True, 'bool': <class 'bool'>, 'memoryview': <class 'memoryview'>, 'bytearray': <class 'bytearray'>, 'bytes': <class 'bytes'>, 'classmethod': <class 'classmethod'>, 'complex': <class 'complex'>, 'dict': <class 'dict'>, 'enumerate': <class 'enumerate'>, 'filter': <class 'filter'>, 'float': <class 'float'>, 'frozenset': <class 'frozenset'>, 'property': <class 'property'>, 'int': <class 'int'>, 'list': <class 'list'>, 'map': <class 'map'>, 'object': <class 'object'>, 'range': <class 'range'>, 'reversed': <class 'reversed'>, 'set': <class 'set'>, 'slice': <class 'slice'>, 'staticmethod': <class 'staticmethod'>, 'str': <class 'str'>, 'super': <class 'super'>, 'tuple': <class 'tuple'>, 'type': <class 'type'>, 'zip': <class 'zip'>, '__debug__': True, 'BaseException': <class 'BaseException'>, 'Exception': <class 'Exception'>, 'TypeError': <class 'TypeError'>, 'StopAsyncIteration': <class 'StopAsyncIteration'>, 'StopIteration': <class 'StopIteration'>, 'GeneratorExit': <class 'GeneratorExit'>, 'SystemExit': <class 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'UnicodeTranslateError': <class 'UnicodeTranslateError'>, 'AssertionError': <class 'AssertionError'>, 'ArithmeticError': <class 'ArithmeticError'>, 'FloatingPointError': <class 'FloatingPointError'>, 'OverflowError': <class 'OverflowError'>, 'ZeroDivisionError': <class 'ZeroDivisionError'>, 'SystemError': <class 'SystemError'>, 'ReferenceError': <class 'ReferenceError'>, 'MemoryError': <class 'MemoryError'>, 'BufferError': <class 'BufferError'>, 'Warning': <class 'Warning'>, 'UserWarning': <class 'UserWarning'>, 'DeprecationWarning': <class 'DeprecationWarning'>, 'PendingDeprecationWarning': <class 'PendingDeprecationWarning'>, 'SyntaxWarning': <class 'SyntaxWarning'>, 'RuntimeWarning': <class 'RuntimeWarning'>, 'FutureWarning': <class 'FutureWarning'>, 'ImportWarning': <class 'ImportWarning'>, 'UnicodeWarning': <class 'UnicodeWarning'>, 'BytesWarning': <class 'BytesWarning'>, 'ResourceWarning': <class 'ResourceWarning'>, 'ConnectionError': <class 'ConnectionError'>, 'BlockingIOError': 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All Rights Reserved.

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变量生存周期

变量在一个个的命名空间中是有自己的生存周期的。全局变量的生存周期是在整个程序执行期间有效，而局部变量的生存周期只在当前作用域中有效，一旦这个作用域不存在了，比如函数执行退出了，变量的生存周期就结束了，生产周期结束的变量不能被访问，也就是说在函数外部不能使用函数内部定义的局部变量。

var = "this is a global variable"

def test():
    var = "this is a Local variable"
    a = 1
    print("在函数内的var：" + var)

test()
print("在函数外的var：" + var)
print(a)

输出结果

在函数内的var：this is a Local variable
在函数外的var：this is a global variable
Traceback (most recent call last):
  File "D:/qqq.py", line 12, in <module>
    print(a)
NameError: name 'a' is not defined