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[Python-Dev] Improve error message "UnboundLocalError: local variable referenced before assignment"

R. David Murray rdmurray at bitdance.com
Wed Oct 31 21:28:06 CET 2012 
On Wed, 31 Oct 2012 21:57:28 +0200, anatoly techtonik <techtonik at gmail.com> wrote:
> Here is the code:
> 
> ---[cut]-----------------------------
> 
> DEBUG = []
> FONT_NAMES = []
> 
> def names():
>   if len(DEBUG):
>     print(len(DEBUG))
>   if len(FONT_NAMES):
>     print(len(FONT_NAMES))
>   if len(FONT_NAMES)==0:
>     FONT_NAMES = "query()"
> 
> names()
> ---[cut]-----------------------------
> 
> Here is the output:
> 
> Traceback (most recent call last):
>   File "globalocal.py", line 13, in <module>
>     names()
>   File "globalocal.py", line 8, in names
>     if len(FONT_NAMES):
> UnboundLocalError: local variable 'FONT_NAMES' referenced before assignment
> 
> 
> As you may see there is inconsistency between handling of line 6 -
> "if len(DEBUG):" and line 8 - "if len(FONT_NAMES):". This is very magical
> and hard to troubleshoot. I wonder if this message can be improved with a
> pointer to the concept on when global variables become local?

There is no inconsistency here.  Only FONT_NAMES is assigned a value
in the local scope.  "local variable referenced before assignment" *is*
a pointer to the concept of when global variables become local...perhaps
there is a better wording, do you have a suggestion?

--David
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