Global variable and local variable in Python classes


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
  1. Why x and y are different here?

  2. 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



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:

>>> xlocal
>>> ylocal

2) class C:
    y = 1

>>> xlocal
>>> 1

3) class C:
    x = 1
>>> xtop
>>> ylocal
Answered By: AlanSTACK
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