How to Filter a List in Python

Posted on: Wed 16 May 2018

li is some list, and I want to filter a list -- comparable to np.where(...) for a pure python list

In [1]:

li = ['a', 'b', 'c', 'a', 'b', 'a']
print(li)

['a', 'b', 'c', 'a', 'b', 'a']

using `filter` and `lambda`¶

In [8]:

t_m1 = %timeit -o list(filter(lambda x:x=='a', li))

1.38 µs ± 19.2 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [11]:

filtered_list = list(filter(lambda x:x=='a', li))
print(filtered_list)

['a', 'a', 'a']

In [12]:

t_m2 = %timeit -o [x for x in li if x == 'a']

595 ns ± 10.5 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [13]:

list_comp = [x for x in li if x == 'a']
print(list_comp)

['a', 'a', 'a']

In [15]:

t_m1.average

Out[15]:

1.3784109998565066e-06

In [16]:

t_m2.average

Out[16]:

5.946756401442273e-07

In [18]:

t_m1.average / t_m2.average

Out[18]:

2.3179207399889443