On 30-06-2010 20:56, Gary Herron wrote: > On 06/30/2010 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. >> Now I'm often what to do something if I've more than 1 element in the result. >> So I test: >> >> 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 > > > Because the natural interpretation of len only makes sense for concepts such > as a container or > collection. The value None is no such thing. Assigning a meaning to > len(None) begs the question > of meanings for len(True), len(False), len(3.14), len(sys), ... This is a > slippery slope, best > avoided. > > But there are solutions: > > 1. Have your functions return [] or () or whatever. If they are to return > a list, and the list > may be empty, [] is correct. > thanks guys, I think that will be the best idea.
cheers, Stef > 2. If you insist on a function returning a list sometimes and other values > at other times (such > as None), then be prepared to write your code which uses the result with test > to determine which > type was returned. Fortunately that's not hard, as your one example shows. > > 3. Create a test function Empty(Result) which does what you want returning a > boolean and write > your tests as: > if Empty(Result): ... > > Gary Herron > > > > >
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