if statement on lenght of a list

[MRAB]

On 17/05/2011 19:02, Joe Leonardo wrote:
> Hey all,
>
> Totally baffled by this…maybe I need a nap. Writing a small function to
> reject input that is not a list of 19 fields.
>
> def breakLine(value):
>     if value.__class__() != [] and value.__len__() != 19:
>         print 'You must pass a list that contains 19 fields.'
>     else:
>         print 'YAY!'
>
> If I pass:
>
> breakLine([])
>
> I get:
>
> YAY!
>
> I expect:
>
> You must pass a list that contains 19 fields.
>
> If I print len(value) I get: 0
>
> What is going on here?
>
If value is [], then value.__class__() is [], so value.__class__() !=
[] is False.

What you meant was:

     if value.__class__() != [] or value.__len__() != 19:
         print 'You must pass a list that contains 19 fields.'
     else:
         print 'YAY!'

although the Pythonic way to write it would be:

     if not isinstance(value, list) or len(value) != 19:
         print 'You must pass a list that contains 19 fields.'
     else:
         print 'YAY!'

or, even better:

     if isinstance(value, list) and len(value) == 19:
         print 'YAY!'
     else:
         print 'You must pass a list that contains 19 fields.'