[Tutor] global

[Jeff Shannon]

Kirk Bailey wrote:

> Got a script that wants to bark. Early in the program I define some 
> defaults for certain variables to define state (they are used as 
> boolian flags, 1/0 stuff). Later, a function referrs o one- and barks. 
> Says it does not know the variable- undefined. Hmmm, sure it is... 
> Apparently this function is not aware the variable is already defined. 
> How do I force a variable to be global so it will automatically be 
> available to the insiode structure of a defined function without 
> having to formally pass it as an arguement? 


Presuming that this is within the same module, nothing needs to be done. 
 However, rebinding a name that [supposedly] points to a global object 
can result in a problem like what you mention --

 >>> spam = 5
 >>> def foo(value):
...     print "Had %d spam," % spam
...     spam = value
...     print "Now have %d spam" % spam
...    
 >>> foo(6)
Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
  File "<interactive input>", line 2, in foo
UnboundLocalError: local variable 'spam' referenced before assignment
 >>> spam
5
 >>>

Since there is an assignment to spam within the function foo(), spam 
becomes a local variable.  Because of certain optimizations used for 
local variables, this means that if spam isn't found in the locals() 
[psuedo-]dict, it won't get looked up in the globals() dict.  As a 
result, attempting to access spam before assigning to it causes an error 
-- Python knows that it should be a local, but can't find it there. 
 This can be resolved by placing the statement 'global spam' at the top 
of foo().  That tells Python that this *isn't* a local variable, and 
that it should always be looked for in the globals() dict.

If this isn't what's happening to you, then post the relevant code (the 
module-level definitions and the function that's using them) and the 
full traceback of the error.

Jeff Shannon
Technician/Programmer
Credit International