根据我的理解，Python有一个单独的函数名称空间，所以如果我想在函数中使用全局变量，我可能应该使用global。
然而，我能够访问一个全局变量，即使没有全局:
>>> sub = ['0', '0', '0', '0']
>>> def getJoin():
... return '.'.join(sub)
...
>>> getJoin()
'0.0.0.0'
为什么会这样?
另请参阅第一次使用后重新分配局部变量时发生的UnboundLocalError，以了解试图分配给全局变量而不使用全局变量时发生的错误。有关如何使用全局变量的一般问题，请参阅在函数中使用全局变量。
当前回答
Global使变量为Global
def out():
global x
x = 1
print(x)
return
out()
print (x)
这使得'x'就像函数外的普通变量一样。如果你把全局变量拿出来，它就会报错，因为它不能在函数中打印变量。
def out():
# Taking out the global will give you an error since the variable x is no longer 'global' or in other words: accessible for other commands
x = 1
print(x)
return
out()
print (x)
其他回答
Global使变量为Global
def out():
global x
x = 1
print(x)
return
out()
print (x)
这使得'x'就像函数外的普通变量一样。如果你把全局变量拿出来，它就会报错，因为它不能在函数中打印变量。
def out():
# Taking out the global will give you an error since the variable x is no longer 'global' or in other words: accessible for other commands
x = 1
print(x)
return
out()
print (x)
访问名称和分配名称是不同的。在本例中，您只是访问一个名称。
如果在函数中赋值给某个变量，则假定该变量为局部变量，除非将其声明为全局变量。如果没有，就假定它是全局的。
>>> x = 1 # global
>>> def foo():
print x # accessing it, it is global
>>> foo()
1
>>> def foo():
x = 2 # local x
print x
>>> x # global x
1
>>> foo() # prints local x
2
虽然你可以在没有global关键字的情况下访问全局变量，但如果你想修改它们，你必须使用global关键字。例如:
foo = 1
def test():
foo = 2 # new local foo
def blub():
global foo
foo = 3 # changes the value of the global foo
在这里，你只是访问了list sub。
这在Python常见问题解答中有很好的解释
What are the rules for local and global variables in Python? In Python, variables that are only referenced inside a function are implicitly global. If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global. Though a bit surprising at first, a moment’s consideration explains this. On one hand, requiring global for assigned variables provides a bar against unintended side-effects. On the other hand, if global was required for all global references, you’d be using global all the time. You’d have to declare as global every reference to a built-in function or to a component of an imported module. This clutter would defeat the usefulness of the global declaration for identifying side-effects.
https://docs.python.org/3/faq/programming.html#what-are-the-rules-for-local-and-global-variables-in-python
在函数外部声明的任何变量都被假定为全局变量，只有在函数内部声明它们时(构造函数除外)，你必须指定变量为全局变量。
