How can I use predicates in Python

Question

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?

sum(x.contains("f") for x in l); bool is a subclass of int, and a sequence of bool values can be summed directly. — chepner
– chepner, Commented Jul 22, 2020 at 20:19

chepner · Accepted Answer · 2020-07-22 20:23:42Z

2

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"))

answered Jul 22, 2020 at 20:23

chepner

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2 Comments

donkopotamus Over a year ago

Or sum(x.contains("f") for x in l)

chepner Over a year ago

Ugh, how did I forget to include that i my answer?

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