[Tut] Python Function Call Inside List Comprehension

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Python Function Call Inside List Comprehension

Question: Is it possible to call a function inside a list comprehension statement?

Background: List comprehension is a compact way of creating lists. The simple formula is

Code:

[expression + context]

.

• Expression: What to do with each list element?
  
  
• Context: What elements to select? The context consists of an arbitrary number of
  Code:

  for
  and
  Code:

  if
  statements.
  

For example, the code

Code:

[x**2 for x in range(3)]

creates the list of square numbers

Code:

[0, 1, 4]

with the help of the expression

Code:

x**2

.

Related article: List Comprehension in Python — A Helpful Illustrated Guide

So, can you use a function with or without return value as an expression inside a list comprehension?

Answer: You can use any expression inside the list comprehension, including functions and methods. An expression can be an integer

Code:

42

, a numerical computation

Code:

2+2 (=4)

, or even a function call

Code:

np.sum(x)

on any iterable

Code:

x

. Any function without return value, returns

Code:

None

per default. That’s why you can even call functions with side effects within a list comprehension statement.

Here’s an example:

Code:

[print('hi') for _ in range(10)] ''' hi hi hi hi hi hi hi hi hi hi '''

You use the throw-away underscore

Code:

_

because you want to execute the same function ten times. If you want to print the first 10 numbers to the shell, the following code does the trick:

Code:

[print(i) for i in range(10)] ''' 0 1 2 3 4 5 6 7 8 9 '''

Let’s have a look at the content of the list you just created:

Code:

lst = [print(i) for i in range(10)] print(lst) # [None, None, None, None, None, None, None, None, None, None]

The list contains ten

Code:

None

values because the return value of the

Code:

print()

function is

Code:

None

. The side effect of executing the print function within the list comprehension statement is that the first ten values from 0 to 9 appear on your standard output.

Walrus Operator

Python 3.8 has introduced the walrus operator, also known as the assignment expression. This operator is useful if executing a certain function has side effects that you don’t want. For example, if you have a string creation method inside the list comprehension statement, conditioned by some filtering criterion in the if suffix. Without the walrus operator, Python would execute this same routine multiple times—even though this is highly redundant. You can avoid this redundancy by assigning it to a variable

Code:

s

once using the walrus operator and reusing this exact variable in the expression.

Code:

import random def get_random_string(): return f'sss {random.randrange(0, 100)}' # Goal: Print all random strings that contain 42 # WRONG lst = [get_random_string() for _ in range(1000) if '42' in get_random_string()] print(lst) # ['sss 74', 'sss 13', 'sss 76', 'sss 13', 'sss 92', 'sss 96', 'sss 27', 'sss 43', 'sss 80'] # CORRECT lst = [s for _ in range(1000) if '42' in (s := get_random_string())] print(lst) # ['sss 42', 'sss 42', 'sss 42', 'sss 42', 'sss 42', 'sss 42', 'sss 42', 'sss 42', 'sss 42', 'sss 42', 'sss 42', 'sss 42', 'sss 42']

With the walrus operator

Code:

s := get_random_string()

, you store the result of the function call in the variable

Code:

s

and retrieve it inside the expression part of the list comprehension. All of this happens inside the list comprehension statement.

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Related Video: List Comprehension



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