CODEWITHSUNDEEP
pythonpredicatedeclarative
Michael Ray Lovett
Michael Ray Lovett

Reputation: 7060

How can I use predicates in Python

I want to use python in a more declarative way, using predicates as much as possible to simplify things, but I'm having trouble seeing how to do that "simply" in python.

For example, when counting items in a list I want to be able to pass in a predicate, like this:

mylist.count(lambda x: x.contains("f"))

to count items in the list that contain and "f".

I can see how to do this with itertools:

sum(map(lambda x: 1 if x.contains("f") else 0, l))

but that's worse than a for loop. Am I missing something or does Python just not allow for this kind of expression?

Upvotes: 0

Views: 2831

Answers (1)

chepner
chepner

Reputation: 532053

The "simplest" (or at least, most decomposable) way would use operator.methodcaller along with sum.

from operator import methodcaller

p = methodcaller("contains", "f")
sum(map(p, l))

This works because contains returns an instance of bool, which is a subclass of int.

You can also use a generator expression to create a sequence of 1s from a filtered iterable.

sum(1 for x in l if x.contains("f"))

Upvotes: 2

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