how to safely extract dict values

[David Zaret]

thanks for the many responses.

i have zero control over the dict.  in practice, i'm using turbogears 
which is "automatically" populating the result dict with zero-to-many 
choices from a generated list of HTML checkboxes.  the user can select 
none, one, or many, and submit.  cherrypy packs the **kwargs arg full of 
the selection, as i described in my op.

using a try/exception case to switch between append and extend will not 
work.  the scalar, coming back as a string, can certainly be iterated 
and i'll end up with a bunch of characters in my list.

i like what's posted below - that's more compact - and at least closer 
to what i was looking for.

thanks again, great newsgroup.

---- dz




Bruno Desthuilliers wrote:
> David Zaret wrote:
>> i have a dict with a particular key - the values for this key will be
>> None, one valid scalar, or a list:
>>
>>     {mykey, None}
>>     {mykey, "foo"}
>>     {mykey, ["bar", "baz"]}
>>
>> let's ignore the None case - in the case of the one or many values, i
>> want to suck the values into a list.  here's one way to do this:
>>
>>             if mydict.has_key(mykey):
>>                 vals=[]
>>                 _v = mydict[mykey]
>>                 if isinstance(_v, types.ListType):
>>                     vals.extend(_v)
>>                 else:
>>                     vals.append(_v)
>>                 #   now we can safely iterate through acts
>>         for val in vals:
>>             .....
>>
>>
>> my way is ugly.  what's a better way?
> 
> If you have control over the dict's creation and 'filling', you may want
> to handle the case there - eventually using a custom dict-like object
> that stores everything (but None) in lists. The implementation code for
> this object will not be less ugly than the one above, but at least
> ugliness will be hidden away !-)
> 
> Else, you can shorten the code a bit (NB : ignoring the 'None' case...):
> 
>   v = d[k]
>   v = ([v], v)[isinstance(v, list)]
> 
> And this can be hidden away in a function:
> 
> def get_as_list(d, k):
>   v = d[k]
>   return ([v], v)[isinstance(v, list)]
> 
> vals = get_as_list(mydict, mykey)
> 
> 
> I don't like using isinstance() tests too much, but it depends on the
> context...
> 
> HTH
>