Tkinter WEIRDNESS or Python WEIRDNESS?

[Brian van den Broek]

steve said unto the world upon 2005-03-12 00:06:
> In a nutshell, my problem is that I am getting this runtime error, and
> I have no clue why.  Please bear in mind its my first python program:
> 
> "localvariable 'currentAbility' referenced before asignment"
> 
> in this function which is a callback for a button:
> 
> def nextThing():
>     if lookingAtAbilities == 1:
>         currentAbility = currentAbility + 1
>         if currentAbility not in ability:
>             currentAbility = 0
>         clearScreen()
>         ability[currentAbility].draw(currentAbility)
>         drawMainButtons()
>     else:
>         currentCreature = currentCreature + 1
>         if currentCreature not in creature:
>             currentCreature = 0
>         clearScreen()
>         creature[currentCreature].draw(currentCreature)
>         drawMainButtons()
> 
> now the thing is, both lookingAtAbilities  and  currentAbility are
> global variables.  In fact, they are both assigned at the same part of
> the program, right at the start.  yet lookingAtAbilities  is
> recognized as a global by this function, and currentAbility is not,
> generating this runtime error.
> 
> here is the structure of the code:

<SNIP>

> This has me flumoxed
> 
> thanks if you can work it out,
> 
> Steve

Hi Steve,

While both lookingAtAbilities and currentAbility are names in the 
global namespace, nextThing() doesn't know that ;-).

The first mention of if lookingAtAbilities in nextThing() is:
.    if lookingAtAbilities == 1:
This is a reference to the name, so the interpreter fetches 
lookingAtAbilities value from the first namespace in which it finds 
such a name. (The global namespace.)

By contrast, the first mention of currentAbility is:
.        currentAbility = currentAbility + 1
So, the interpreter `creates' (I am a bit fuzzy on the terminology and 
not an accomplished Pythoner) a name in the function local namespace 
and then looks to see what value to point the name at. When it sees it 
should have currentAbility (a name without a value as yet) point to
currentAbility + 1, it barfs.

One way to fix it, is to put `global currentAbility' at the top of 
your function definition. (Many frown on globals as it messes with the 
namespaces, and gives bad mojo.)

Another would be to pass currentAbility in to your function as an 
argument.

Yet another (and probably the best) would be to wrap your code into a 
class and then reference self.currentAbility.

Still another (but one not so good for clarity or beauty) would be to 
put a line like:
temp = currentAbility # temp a name to reference global scope name
                       # before assignment to follow.

Perhaps more expert voices will chime in and say better things. :-)

HTH,

Brian vdB