Find object by its member inside a List in python

Question

lets assume the following simple Object:

class Mock:
    def __init__(self, name, age):
        self.name = name
        self.age = age

then I

Answer 1

List comprehensions are almost always the faster way to do these things (2x as fast here), though as mentioned earlier indexing is even faster if you're concerned about speed.

~$ python -mtimeit -s"from mock import myList" "filter(lambda x: x.age==21, myList)"
1000000 loops, best of 3: 1.34 usec per loop
~$ python -mtimeit -s"from mock import myList" "[x for x in myList if x.age==21]"
1000000 loops, best of 3: 0.63 usec per loop

For file mock.py in current directory:

class Mock:
    def __init__(self, name, age):
        self.name = name
        self.age = age

myList = [Mock('Tom', 20), Mock('Dick', 21), Mock('Harry', 21), Mock('John', 22)]

Answer 2

You could try a filter():

filter(lambda x: x.age == 30, myList)

This would return a list with only those objects satisfying the lambda expression.

Answer 3

List comprehensions can pick these up:

new_list = [x for x in myList if x.age == 30]

Answer 4

You might want to pre-index them -

from collections import defaultdict

class Mock(object):
    age_index = defaultdict(list)

    def __init__(self, name, age):
        self.name = name
        self.age = age
        Mock.age_index[age].append(self)

    @classmethod
    def find_by_age(cls, age):
        return Mock.age_index[age]

Edit: a picture is worth a thousand words:

X axis is number of Mocks in myList, Y axis is runtime in seconds.

• red dots are @dcrooney's filter() method
• blue dots are @marshall.ward's list comprehension
• green dots hiding behind the X axis are my index ;-)