为什么 python 无法通过 function 参数名称更改访问全局变量值？

Question

in these two pieces of code, why the second one gives error about local variable assignment? two codes are similar just function parameters is different, in the second one it's able to read the global variable why not in the first one what changes with parameter name change about symbol table?

first one:

def a(z):
    z+=1
z=3
a(z)

second one:

def a(z):
    b += 1
b = 5
a(b)

Answer 1

You should concider def blocks as stand alone.

In the first snippet:

def a(z):
    z+=1

What is z ? It's the first parameter of a

In the second snippet:

def a(z):
    b += 1

What is b ? It is unknown. That's why this code fails.

You should also notice that in your first snippet, z inside the function is not the same than z=3 :

>>> def a(z):
...     z+=1
...
>>> z=3
>>> a(z)
>>>
>>> z
3

Answer 2

There aren't any global variables in use here.

In the first example, z is a parameter to the function, not a global. Note that when you increment z , it does not change in the calling scope, because the z inside the function is a copy of the z you passed in from outside the function.

In the second example, there is no b inside the function (the parameter is z ), which is why you get an error inside the function when you try to modify it.

To do what you're trying to do, you should make the parameter a mutable object that contains the value you're trying to mutate; that way you can modify the value inside the function and the caller will have access to the new value. You could define a class for this purpose, or you could simply make it a single-element list:

def a(z):
    z[0] += 1
b = [5]
a(b)  # b == [6]

Or, if possible, a better approach IMO is not to depend on mutability, and to just return the new value, requiring the caller to explicitly re-assign it within its scope:

def a(z)
    return z + 1
b = 5
b = a(b)  # b == 6

Answer 3

In the second code, the parameter is "z", and you tried to access that parameter with "b"

def a(z):
    b += 1