Python basic operation 1st: List comprehension notation

#Python basic operation


Review the basic operations of Python only for the parts that you are uneasy about. As the first step, we describe list comprehension notation.

Why do we need list comprehensions?

・ Speeding up processing ・ Simplification of structure Mainly because these benefits may be obtained. Here, the description method and evaluation will be performed.

Description method 1

List the variables that store the processing contents and their processing in parentheses []

[Variable processing] Example) data = [i for i in range (10)]

Description method 2

List the variables that store the processing contents and their processing and execution conditions in parentheses [].

[Variable processing conditions] Example) data = [i for i in range (10) if i%2 == 0]

Description method 3

When processing is required when the condition is not satisfied, such as if ~ else ..., the conditions are arranged in between.

[Variable condition processing] Example) data = [i if i%2 == 0 else 0 for i in range (10)]


@Yuya Shimizu

List comprehension
#Description method 1:[Variable processing]
data = [i for i in range(10)]

#Description method 2:[Variable processing conditions]
data = [x for x in range(10) if x%2 == 0]

#Description method 3:[Variable condition processing]
data = [y if y%2 == 0 else "xxx" for y in range(10)]

Is it really faster?

Certainly, I think that the structure can be simplified with one line. But is it really faster? In the following, the verification is performed using the time module. Since it was difficult to understand the difference when the number of repetitions was small, It was verified under 5 conditions of repetition number 1000, 2000, 3000, 4000, 5000.


@Yuya Shimizu

List comprehension
iteration = 1000 #Repeat condition{1000,2000,3000,4000,5000}
def func1():
    #[Variable processing]
    data = []
    for i in range(iteration):

    #[Variable processing conditions]
    data = []
    for x in range(iteration):
        if x%2 == 0:

    #[Variable condition processing]
    data = []
    for y in range(iteration):
        if y%2 == 0:

def func2():
    #[Variable processing]
    data = [i for i in range(iteration)]

    #[Variable processing conditions]
    data = [x for x in range(iteration) if x%2 == 0]

    #[Variable condition processing]
    data = [y if y%2 == 0 else "xxx" for y in range(iteration)]

######Execution time measurement
import time

start1 = time.time()
process_time1 = time.time() - start1

start2 = time.time()
process_time2 = time.time() - start2

print("List comprehension(iteration={})----None:{}Seconds, Yes:{}Seconds".format(iteration, process_time1, process_time2))

inspection result

Number of repetitions[Times] Execution time without list comprehension[s] Execution time with list comprehension[s]
1000 1.139344 1.121223
2000 1.026536 0.578534
3000 0.856933 0.884612
4000 0.766533 0.579598
5000 0.667517 0.605027

Speeding up is certainly seen at 2000 and 4000 times, but it is not always possible. It can be seen that the speed may be increased. In fact, if you look closely, the result is that when the number of trials is 3000, it is rather slower to use the list comprehension notation.


According to the verification results, it is not always possible to increase the speed, but if there is a possibility and the delay is slight, I wondered if it should be utilized. Besides, I think that the simplification of the structure is convenient for debugging. If there are situations where it can be used, I would definitely like to use list comprehension notation.


Introduction to algorithms starting with Python: Standards and computational complexity learned with traditional algorithms Written by Toshikatsu Masui, Shoeisha

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