Python Count Elements in a List of Objects with Matching Attributes

Question:

I am trying to find a simple and fast way of counting the number of Objects in a list that match a criteria.
e.g.

class Person:
    def __init__(self, Name, Age, Gender):
        self.Name = Name
        self.Age = Age
        self.Gender = Gender

# List of People
PeopleList = [Person("Joan", 15, "F"), 
              Person("Henry", 18, "M"), 
              Person("Marg", 21, "F")]

Now what’s the simplest function for counting the number of objects in this list that match an argument based on their attributes?
E.g., returning 2 for Person.Gender == “F” or Person.Age < 20.

Asked By: FacesOfMu

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Source

Answers:

class Person:
    def __init__(self, Name, Age, Gender):
        self.Name = Name
        self.Age = Age
        self.Gender = Gender


>>> PeopleList = [Person("Joan", 15, "F"), 
              Person("Henry", 18, "M"), 
              Person("Marg", 21, "F")]
>>> sum(p.Gender == "F" for p in PeopleList)
2
>>> sum(p.Age < 20 for p in PeopleList)
2
Answered By: jamylak

Personally I think that defining a function is more simple over multiple uses:

def count(seq, pred):
    return sum(1 for v in seq if pred(v))

print(count(PeopleList, lambda p: p.Gender == "F"))
print(count(PeopleList, lambda p: p.Age < 20))

Particularly if you want to reuse a query.

Answered By: kampu

I prefer this:

def count(iterable):
    return sum(1 for _ in iterable)

Then you can use it like this:

femaleCount = count(p for p in PeopleList if p.Gender == "F")

which is cheap (doesn’t create useless lists etc) and perfectly readable (I’d say better than both sum(1 for … if …) and sum(p.Gender == "F" for …)).

Answered By: Alfe

I found that using a list comprehension and getting its length was faster than using sum().

According to my tests…

len([p for p in PeopleList if p.Gender == 'F'])

…runs 1.59 times as fast as…

sum(p.Gender == "F" for p in PeopleList)
Answered By: Webucator

I know this is an old question but these days one stdlib way to do this would be

from collections import Counter

c = Counter(getattr(person, 'gender') for person in PeopleList)
# c now is a map of attribute values to counts -- eg: c['F']
Answered By: lonetwin