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-   -   Tkinter WEIRDNESS or Python WEIRDNESS? (http://www.velocityreviews.com/forums/t342552-tkinter-weirdness-or-python-weirdness.html)

steve 03-12-2005 05:06 AM

Tkinter WEIRDNESS or Python WEIRDNESS?
 
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:

----------------------------------------
import string
import types
from Tkinter import *

# globals
lookingAtAbilities = 1
currentAbility = 2

(snip: some class and function definitions)

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()

def drawMainButtons():
back = Button(win, text="prev", command=prevThing)
back.grid(row=20, column=0)
next = Button(win, text="next", command=nextThing)
next.grid(row=20, column=1)

# START OF EXECUTION #

masterWin = Tk()
win = Frame(masterWin)
win.grid()

drawMainButtons()
masterWin.mainloop()
------------------------------------------

This has me flumoxed

thanks if you can work it out,

Steve

Brian van den Broek 03-12-2005 06:34 AM

Re: Tkinter WEIRDNESS or Python WEIRDNESS?
 
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

Vincent Wehren 03-12-2005 06:50 AM

Re: Tkinter WEIRDNESS or Python WEIRDNESS?
 
steve wrote:
> 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"
>


If you're assigning an object to a name, it will be assigned to that
name in local namespace. So, in your case, you are doing augmented
assignment on a local name that does not yet exist (hence the error
message). You need to tell the interpreter that what you want is assign
to that name in global namespace. You can do this by using the 'global'
statement, as in:

>>> my_global = 1
>>> def some_func():

.... global my_global
.... my_global += 1
>> some_func()
>> my_global

2


--

Vincent Wehren

steve 03-12-2005 11:46 PM

Re: Tkinter WEIRDNESS or Python WEIRDNESS?
 
Brian van den Broek <bvande@po-box.mcgill.ca> wrote in message news:<mailman.312.1110609301.1799.python-list@python.org>...
> 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:
> > 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.


Thanks to both of you, this has fixed the problem. I wouldnt have
guessed that in a million years.

But it leads me to ask what is the use of something being in the
gloabl namespace if it cant be accessed globally?

I also find the continual need for objects to reference their own
variables by prefixing 'self' annoying. It seems that Python is the
reverse of my assuptions. whats the rationale for that?

> 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


Brian van den Broek 03-13-2005 12:34 AM

Re: Tkinter WEIRDNESS or Python WEIRDNESS?
 
steve said unto the world upon 2005-03-12 18:46:
> Brian van den Broek <bvande@po-box.mcgill.ca> wrote in message news:<mailman.312.1110609301.1799.python-list@python.org>...
>
>>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:
>>>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.

>
>
> Thanks to both of you, this has fixed the problem. I wouldnt have
> guessed that in a million years.


Hi Steve,

you're welcome. I'm just glad I've finally understood enough to offer
helpful answers :-) (Though note the self-correction at the end of
this post.)

> But it leads me to ask what is the use of something being in the
> gloabl namespace if it cant be accessed globally?


I think that your question shows there is some residual confusion.
Things in the global namespace can be accessed just fine. The issue
with your code was that, while you had a global name currentAbility in
the global namespace sitting around just waiting to be accessed, your
first use of that name in your function was to assign it.

Assignments are by default to names in the local namespace. If you'd
put the global line in, you'd have told the interpreter to make the
assignment in the global scope. Without this, your function told the
interpreter to create a new name in the local namespace and make it
refer to the object to the right of the '=' which, in your case, was
not well-defined.

There is a certain logic to making assignments be assignments within
the active namepace by default. There may be a deeper reason, but I
don't know it. (To anyone more expert: if there is, I'd love to learn it).

> I also find the continual need for objects to reference their own
> variables by prefixing 'self' annoying. It seems that Python is the
> reverse of my assuptions. whats the rationale for that?


The self prefix is just a conventional way of indicating a reference
to the current class instance. I hated it at first too. Python is my
first language since some BASIC long ago and the selfs were one reason
I steered clear of OOP at first. But, a few evenings of programming
with classes (a recent thing for me) and the aversion went away. So:
give it a bit of time; you might get so you aren't bothered by them.

I should also note there was some residual confusion in what I wrote, too:

>>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.


This won't work. Witness:

>>> a = 42
>>> def my_mistake():

.... temp = a
.... a = a * 2
....
>>> my_mistake()

Traceback (most recent call last):
File "<interactive input>", line 1, in ?
File "<interactive input>", line 2, in my_mistake
UnboundLocalError: local variable 'a' referenced before assignment
>>>


Sorry 'bout that :-[

Best,

Brian vdB



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