Simple Python variable scope

Question

It seems to me that functions can reference variables outside of their scope but cannot set them. Is this correct? Am I understanding this right?

I also included the globals usage. I know they are bad ju-ju and will avoid them; I know how to get around this, but just wanted to be clear.

My example program:

import foo

# beginning of functions

# this one works because I look at the variable but dont modify it
def do_something_working:

    if flag_to_do_something:
          print "I did it"

# this one does not work because I modify the var
def do_something_not_working:

    if flag_to_do_something:
          print "I did it"
          flag_to_do_something = 0

# this one works, but if I do this God kills a kitten
def do_something_using_globals_working_bad_ju_ju:

    global flag_to_do_something

    if flag_to_do_something:
         print "I did it"
         flag_to_do_something = 0


# end of functions

flag_to_do_something = 1

do_something_working()
do_something_not_working()
do_something_using_globals_working_bad_ju_ju()

Blue Peppers · Accepted Answer · 2010-07-23 21:30:10Z

5

Correct. Well mostly. When you flag_to_do_something = 0 you are not modifying the variable, you are creating a new variable. The flag_to_do_something that is created in the function will be a separate link to (in this case) the same object. However, if you had used a function or operator that modified the variable in place, then the code would have worked.

Example:

g = [1,2,3]
def a():
    g = [1,2]
a()
print g #outputs [1,2,3]

g = [1,2,3]
def b():
    g.remove(3)
b()
print g #outputs [1,2]

edited Jul 23, 2010 at 21:30

answered Jul 23, 2010 at 21:24

Blue Peppers

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2 Comments

William T Wild Over a year ago

Ahhh I see. So is there something like remove for non list variables ? Like flag_to_do_something.set(0) ?

Thomas Wouters Over a year ago

No, you can't mutate variables (which are just names), only objects. And immutable objects, like ints, can't be mutated. You can, however, assign to global variables by using the global declaration inside the function: global flag_to_do_something; flag_to_do_something = 0.

David Z · Accepted Answer · 2010-07-23 21:23:18Z

0

Yep, pretty much. Note that "global" variables in Python are actually module-level variables - their scope is that Python module (a.k.a. source file), not the entire program, as would be the case with a true global variable. So Python globals aren't quite as bad as globals in, say, C. But it's still preferable to avoid them.

answered Jul 23, 2010 at 21:23

David Z

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