When you declare variables inside a function definition, they have no relationship to other variables with the same name outside the function, i.e. variable names are local to the function . This is called the scope of the variable . The scope of all variables is the block in which they are defined, from the point where their names are defined.
use local variables
Example 7.3 Using local variables
#!/usr/bin/python
# Filename: func_local.py
def func(x):
print 'x is', x
x = 2
print 'Changed local x to', x
x = 50
func(x)
print 'x is still', x
(source file: code/func_local.py )
output
$ python func_local.py
x is 50
Changed local x to 2
x is still 50
how it works
In a function, the first time we use xa value , Python uses the value of the parameter declared by the function.
Next, we 2assign the value to x. xis a local variable of the function. So, when we change the value inside the function x, the one defined in the main block xis not affected.
In the last printstatement, we prove that the values in the main block xare indeed not affected.
use the global statement
If you want to assign a value to a variable defined outside a function, then you have to tell Python that the variable name is not local, but global . We use globalstatements to accomplish this functionality. Without globala statement, it is impossible to assign a value to a variable defined outside a function.
You can use the value of a variable defined outside the function (assuming there is no variable with the same name inside the function). However, I do not encourage you to do this, and you should try to avoid it, because it makes it unclear to the reader of the program where the variable is defined. A using globalstatement makes it clear that variables are defined outside of the block.
Example 7.4 Using the global statement
#!/usr/bin/python
# Filename: func_global.py
def func():
global x
print 'x is', x
x = 2
print 'Changed local x to', x
x = 50
func()
print 'Value of x is', x
（源文件：code/func_global.py）
输出
$ python func_global.py
x is 50
Changed global x to 2
Value of x is 2
它如何工作
global语句被用来声明x是全局的——因此，当我们在函数内把值赋给x的时候，这个变化也反映在我们在主块中使用x的值的时候。
你可以使用同一个global语句指定多个全局变量。例如global x, y, z。
http://woodpecker.org.cn/abyteofpython_cn/chinese/ch07s03.html
作者推荐：
http://www.tubemate.video