Variable interpolation question

[Bengt Richter]

On Mon, 17 Nov 2003 20:47:51 +0300, anton muhin <antonmuhin.REMOVE.ME.FOR.REAL.MAIL at rambler.ru> wrote:

>Andrew Fabbro wrote:
>
>> This is probably a beginner's question, but I'm stuck...please be kind
>> to an ex-perler ;)
>> 
>> How do I do something like this:
>> 
>> for attr in dir(some_obj):
>>   if ( some_obj.attr == 0 ):
>>     print "Missing data: %s field %s" % ( some_obj.name,
>> some_obj.attr)
>> 
>> Of course, this gives 
>> "AttributeError: foo instance has no attribute 'attr'"
>> 
>> I really don't want to use exec/eval, as that slows things down
>> dramatically.
>> 
>> Help?
>> 
>> Thanks.
>> 
>> -Drew
>
>You are probably looking for hasattr/getattr functions:
>
>for attr in dir(some_obj):
>     if hasattr(some_obj, attr) and getattr(some_obj, attr) == 0:
>          print 'blah...'
>
>of course, it could be shorter:
>
>....
>     if getattr(some_obj, attr, 0) == 0:
>         print 'blah'
>
Or why use an "== 0" test when just hasattr tests for presence/absence without using up
a possible value of the attribute for flag purposes?

BTW, The OP might want to note that in general 

    for attr in dir(some_object):
        if getattr(some_obj, attr, 0) == 0:
            ...

is not the same as (untested)

    objdict = vars(some_obj)     # vars raises exception if there's no some_obj.__dict__
    for attr in objdict:
        if objdict[attr] == 0:
            ...

or, safer, (untested)

    objdict = getattr(some_object, '__dict__', {})
    for attr in objdict:
        if objdict[attr] == 0:
            ...

or (untested)

    for attr in getattr(some_object, '__dict__', {}): ...
        if some_obj.__dict__[attr] == 0: ...
            ...

I.e., dir chases down all the inherited stuff, and getattr invokes all the magic
associated with attribute access, such as properties, whereas just using
some_obj.__dict__ bypasses that.

Regards,
Bengt Richter