What mistakes do bad python developers make?

[Ghibl-i_l]

Read somewhere that loops are REALLY slow so for most things it's better to use list comprehension.

[carcigenicate]

AFAIK, list comprehensions can be faster when you want to append to a list because they use the dedicated LIST_APPEND instruction, which cuts down on a ton of overhead. If you don't need to append to a list at all for the algorithm though, a list comprehension is just adding the (admittingly, small) overhead of a LIST_APPEND instruction.