Question

Many developers I have met suggest it’s best practice to go with simple loops and if conditions instead of one line list comprehension statements.

I have always found them very powerful as I can fit a lot of code in a single line and it saves a lot of variables from being created. Why is it still considered a bad practice?

(Is it slow?)

Answer 1

List comprehensions are used for creating lists, for example:

squares = [item ** 2 for item in some_list]

For loops are better for doing something with the elements of a list (or other objects):

for item in some_list:
    print(item)

Using a comprehension for its side effects, or a for-loop for creating a list, is generally frowned upon.

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Some of the other answers here advocate turning a comprehension into a loop once it becomes too long. I don’t think that’s good style: the append calls required for creating a list are still ugly. Instead, refactor into a function:

def polynomial(x):
    return x ** 4 + 7 * x ** 3 - 2 * x ** 2 + 3 * x - 4
result = [polynomial(x) for x in some_list]

Only if you’re concerned about speed – and you’ve done your profiling! – you should keep the long, unreadable list comprehension.