A and B but not C in list

[Arnaud Delobelle]

Terry Reedy <tjreedy at udel.edu> writes:

> On 1/23/2011 4:05 PM, CM wrote:
>> In Python, is there a recommended way to write conditionals of the
>> form:
>>
>> "if A and B but not C or D in my list, do something."  ?
>>
>> I may also have variations on this, like "if A but not B, C, or D".
>>
>> Do I have to just write out all the if and elifs with all possible
>> conditions, or is there a handier and more code-maintainable way to
>> deal with this?
>
> The straightforward code
>
> if a in L and b in L and c not in L and d not in L
>
> scans the list 4 times. One scan be be done generically as follows:
>
> def req_pro(iterable, required = set(), prohibited = set()):
>     for item in iterable:
>         if item in prohibited:
>             return False
>         elif item in required:
>            required.remove(item)
>     else:
>         return not required # should now be empty
>
> if req_pro(my_list, {A,B}, {C,D}): ...
> # untested, of course..

But what's better? Four fast list scans or one slow one?

-- 
Arnaud