Variable scope in classes

[Robert Brewer]

Larry Bates wrote:
> I'm confused about variable scope in a class
> that is derived from a base class framework.
> 
> Can someone enlighten me on why bar.__init__
> method can't see self.__something variable
> in the foo.__init__?
> 
> Example:
> 
> 
> class foo:
>     def __init__(self):
>         print self.__something
> 
> 
> class bar(foo):
>     __something="This is a test"
> 
>     def __init__(self):
>         foo.__init__(self)
> 
> 
> x=bar()
> 
> returns the following traceback:
> 
> Traceback (most recent call last):
>   File
> "C:\Python22\Lib\site-packages\Pythonwin\pywin\framework\scrip
> tutils.py",
> line 301, in RunScript
>     exec codeObject in __main__.__dict__
>   File "F:\SYSCON\SMARTROUTE\classjunk.py", line 13, in ?
>     x=bar()
>   File "F:\SYSCON\SMARTROUTE\classjunk.py", line 10, in __init__
>     foo.__init__()
>   File "F:\SYSCON\SMARTROUTE\classjunk.py", line 3, in __init__
>     print self.__something
> AttributeError: bar instance has no attribute '_foo__something'
> 
> Is there some "other" way to do this without passing information
> through foo.__init__ argument list?

The quick and easy way would be to name your "__something" variable in
such a way that it doesn't start with two underscores. Try "_something"
with one underscore if you want to avoid the name mangling. See
http://www.python.org/doc/current/ref/atom-identifiers.html for a
discussion of mangling if you're not familiar with it.

The other way would be to form the mangled name yourself.

class foo:
    def __init__(self):
        attr = "_" + self.__class__.__name__ + "__something"
        print getattr(self, attr)


...or something like that.


Robert Brewer
MIS
Amor Ministries
fumanchu at amor.org