Why don't my global variables work?

Gordon McMillan gmcm at hypernet.com
Thu Jun 17 18:56:25 CEST 1999


Nathan J Froyd writes:

> Can somebody explain this point to me? I have the following program

> Could somebody look at this, perhaps running the program and explain
> to me what fundamental fact I am missing here? Because it looks like
> my program should run without errors. Thanks!
/snip/
> # I HAVE DEFINED `count' HERE, PYTHON!
> count = 0
/snip/
> def topic_read(regmatch, evilcount):
>   # if you comment this line, the program will not work
>   count = evilcount
>   print "\t" * count, "<topic name=\"%s\">" % regmatch.group(1)
>   count = count + 1

Alas, the FAQ is down, so I can't point you there.

Changing the binding of a global var requires a "global" statment.

def topic_read(...):
   global count
   count = count + 1

Without that, Python (at the time the "def" is scanned) decides you 
are creating a local var named count. Then you go and reference said 
local var before you've given it a value. Boom. NameError. 
Non-intuitive, perhaps, but think of the strange bugs you'd have if 
locals didn't shadow globals of the same name.

- Gordon




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