Hwy doesn't len(None) return zero ?
in [Python]
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From: Ian Kelly on 30 Jun 2010 15:35 On Wed, Jun 30, 2010 at 12:39 PM, Stef Mientki <stef.mientki(a)gmail.com> wrote: > So I wonder why len is not allowed on None > and if there are objections to extend the len function . For the same reason that (None + 42) doesn't return 42, and that (None.upper()) doesn't return NONE.
From: Emile van Sebille on 30 Jun 2010 16:18 On 6/30/2010 11:39 AM Stef Mientki said... > hello, > > I've lot of functions that returns their result in some kind of tuple / list / array, > and if there is no result, these functions return None. > Now I'm often what to do something if I've more than 1 element in the result. > So I test: which works fine if beforehand you do Result = presumedFuncCall() or [] particularly if you want to test len subsequently. Emile > > if len ( Result )> 1 : > > But to prevent exceptions, i've to write ( I often forget) > if Result and ( len ( Result )> 1 ) : > > So I wonder why len is not allowed on None > and if there are objections to extend the len function . > > thanks, > Stef Mientki > >
From: Steven D'Aprano on 30 Jun 2010 18:23
Please pardon me for breaking threading, but Stef's original post has not come through to me. On 6/30/10 11:39 AM, Stef Mientki wrote: > hello, > > I've lot of functions that returns their result in some kind of tuple / > list / array, > and if there is no result, these functions return None. Well there's your problem right there. If you have a function that returns a list of X, and there are no X to return, you should return an empty list, not None. >>> filter(lambda n: n%2 == 0, [1, 3, 5, 7]) # return even numbers [] There are good use-cases for functions that sometimes return X and sometimes return None, but they're rare. -- Steven |