Python：'global' 和 globals().update(var) 之间的区别
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Python: Difference between 'global' & globals().update(var)
提问by frank
What is the difference between initializing a variable as global varor calling globals().update(var).
将变量初始化为global var或调用globals().update(var).
Thanks
谢谢
回答by unutbu
When you say
当你说
global var
you are telling Python that var is the same var that was defined in a global context. You would use it in the following way:
你告诉 Python var 是在全局上下文中定义的同一个 var。您可以通过以下方式使用它：
var=0
def f():
global var
var=1
f()
print(var)
# 1 <---- the var outside the "def f" block is affected by calling f()
Without the global statement, the var inside the "def f" block would be a local variable, and setting its value would have no effect on the var outside the "def f" block.
如果没有 global 语句，“def f”块内的 var 将是一个局部变量，设置其值对“def f”块外的 var 没有影响。
var=0
def f():
var=1
f()
print(var)
# 0 <---- the var outside the "def f" block is unaffected
When you say globals.update(var) I am guessing you actually mean globals().update(var). Let's break it apart.
当您说 globals.update(var) 时，我猜您实际上是指 globals().update(var)。让我们把它分开。
globals() returns a dict object. The dict's keys are the names of objects, and the dict's values are the associated object's values.
globals() 返回一个 dict 对象。字典的键是对象的名称，字典的值是关联对象的值。
Every dict has a method called "update". So globals().update() is a call to this method. The update method expects at least one argument, and that argument is expected to be a dict. If you tell Python
每个字典都有一个称为“更新”的方法。所以 globals().update() 是对这个方法的调用。update 方法需要至少一个参数，并且该参数应该是一个 dict。如果你告诉 Python
globals().update(var)
then var had better be a dict, and you are telling Python to update the globals() dict with the contents of the var dict.
那么 var 最好是一个 dict，并且你告诉 Python 用 var dict 的内容更新 globals() dict。
For example:
例如：
#!/usr/bin/env python
# Here is the original globals() dict
print(globals())
# {'__builtins__': <module '__builtin__' (built-in)>, '__name__': '__main__', '__file__': '/home/unutbu/pybin/test.py', '__doc__': None}
var={'x':'Howdy'}
globals().update(var)
# Now the globals() dict contains both var and 'x'
print(globals())
# {'var': {'x': 'Howdy'}, 'x': 'Howdy', '__builtins__': <module '__builtin__' (built-in)>, '__name__': '__main__', '__file__': '/home/unutbu/pybin/test.py', '__doc__': None}
# Lo and behold, you've defined x without saying x='Howdy' !
print(x)
Howdy
