I was working on a work task last week, and needed to filter out one instance of a class from a list of class instances. No matter how you do this speed doesn’t matter too much if you’re doing this operation once or a few times.

However, I this operation needs to be done about 100K times each time the script runs - so speed definitely does matter in this case.

First, I naively started off with using filter(). When that lead to waiting more than ten minutes, I read that list comprehensions are faster.

Second, I tried list comprehensions. Also waited more then ten minutes and gave up again.

Last, I thought perhaps it would work to make a dictionary where the keys are the things I need to filter on and the values the class instances. This was the answer! Super fast.

Now some examples of what I’m talking about.

Make a class called Stuff.

class Stuff:
  def __init__(self, letter):
    self.letter = letter
    super(Stuff, self).__init__()
  def __repr__(self):
        return "<{} ({})>".format(self.__class__.__name__, self.letter)

x = Stuff('s')
#> <Stuff (s)>
#> 's'

Make a list of instances of the class Stuff

import string

lst = []
for x in string.ascii_lowercase:
#> [<Stuff (a)>, <Stuff (b)>, <Stuff (c)>, <Stuff (d)>, <Stuff (e)>, <Stuff (f)>, <Stuff (g)>, <Stuff (h)>, <Stuff (i)>, <Stuff (j)>, <Stuff (k)>, <Stuff (l)>, <Stuff (m)>, <Stuff (n)>, <Stuff (o)>, <Stuff (p)>, <Stuff (q)>, <Stuff (r)>, <Stuff (s)>, <Stuff (t)>, <Stuff (u)>, <Stuff (v)>, <Stuff (w)>, <Stuff (x)>, <Stuff (y)>, <Stuff (z)>]
#> 26

List comprehension: This is how I did the list comprehension method. Filter the list lst where some attibute matched some value.

[x for x in lst if x.letter == 'f']
#> [<Stuff (f)>]

Filter: This is how I did the filter method. Filter the list lst where some attibute matched some value.

list(filter(lambda x: x.letter == 'f', lst))
#> [<Stuff (f)>]

And here’s the dictionary approach. Here, I first make a dictionary via dict(zip()) where the keys are some attribute of each instance. You can lookup by key.

A major difference/drawback of this approach is that it only works if there’s only one match per key because dictionaries don’t allow duplicate keys.

lst_map = dict(zip([w.letter for w in lst], lst))
#> <Stuff (f)>

And better yet, use .get() so you don’t run into a KeyError when the key doesn’t exist

#> <Stuff (f)>
#> (returns None)

What about the timings:

from timeit import timeit
n = 100000
time_list_comp = timeit("[x for x in lst if x.letter == 'f']", number=n, globals=globals())
time_filter = timeit("list(filter(lambda x: x.letter == 'f', lst))", number=n, globals=globals())
time_dict = timeit("lst_map['f']", number=n, globals=globals())
time_dict_get = timeit("lst_map.get('f')", number=n, globals=globals())

round(time_list_comp, 3)
#> 0.088
round(time_filter, 3)
#> 0.134
round(time_dict, 3)
#> 0.002
round(time_dict_get, 3)
#> 0.003

For bracketed lookup, the list comprehension is 39 times slower, and the filter is 59 times slower.

For the get() lookup, the list comprehension is 26 times slower, and the filter is 39 times slower.