Let's say I have a list of MyClass. I want to count the number of elements which have MyClass.SomeProperty set to True (assuming SomeProperty is a boolean which is always True or False). My concerns are:
I know I can do:
count = len([e for e in MyList if e.SomeProperty]) # For non booleans, something like e.SomeProperty == MyValue
But it seems inefficient.
e for e?Is it possible to do better?
You can use sum with a generator expression.
count = sum( e.SomeProperty for e in MyList )
Or for a general predicate p:
count = sum( p(e) for e in MyList )
This makes use of the fact True and False can be used as the integers 1 and 0, and the fact a generator is used will prevent a new list from being created.
If you insist on avoiding the for e in part, you can use map and attrgetter:
import opertor
count = sum(map(operator.attrgetter('SomeProperty'), MyList))
Or for a general predicate p:
count = sum(map(p, MyList))
However, this is less pythonic. I'd recommend the first approach.
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