Move all zeroes to the beginning of a list in Python

Question

I have a list like below:

a = [4, 5, 0, 0, 6, 7, 0, 1, 0, 5]

and I want to push all zeroes to the beginning of that list. The result must be like below.

a = [0, 0, 0, 0, 4, 5, 6, 7, 1, 5]

How to do it in Python 2?

Answer 1

You could sort the list:

a.sort(key=lambda v: v != 0)

The key function tells Python to sort values by wether or not they are 0. False is sorted before True, and values are then sorted based on their original relative position.

For 0, False is returned, sorting all those values first. For the rest True is returned, leaving sort to put them last but leave their relative positions untouched.

Demo:

>>> a = [4, 5, 0, 0, 6, 7, 0, 1, 0, 5]
>>> a.sort(key=lambda v: v != 0)
>>> a
[0, 0, 0, 0, 4, 5, 6, 7, 1, 5]