Global variable and local variable in Python classes

Question

x = "xtop"
y = "ytop"
def func():
    x = "xlocal"
    y = "ylocal"
    class C:
        print x  #xlocal  of course
        print y  #ytop  why? I guess output may be 'ylocal' or '1'
        y = 1
        print y  #1  of course
func()

Why x and y are different here？
If I replace class C with a function scope I will get UnboundLocalError: local variable 'y' referenced before assignment，What is the difference between a class and a function in this situation?

Asked By: Abirdcfly

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Source

Answer 1

The reason for this is because the scope of class C is actually different than the scope of def func – and the different defaulting behaviors of scopes that python has for resolving names.

Here is basically how python looks for a variable in a step-by-step guide:

Look in current scope
If current scope doesn’t have it → use nearest enclosing scope
If current scope has it, but not yet defined → use global scope
If current scope has it, and already defined → use it
Otherwise we blow up

(If you remove ytop you get the exception NameError: name 'y' is not defined)

So basically, when the interpreter looks at the following section of code it does this

class C:
    print(x) # need x, current scope no  x → default to nearest (xlocal)
    print(y) # need y, current scope yes y → default to global  (ytop)
             #         but not yet defined 
    y = 1
    print(y) # okay we have local now, switch from global to local scope

Consider the following scenarios and the different outputs we would get in each case

1) class C:
    print(x)
    print(y)

>>> xlocal
>>> ylocal

2) class C:
    y = 1
    print(x)
    print(y)  

>>> xlocal
>>> 1

3) class C:
    print(x)
    print(y)
    x = 1
 
>>> xtop
>>> ylocal

Answered By: AlanSTACK

Global variable and local variable in Python classes

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