Remove list element without mutation
Assume you have a list
>>> m = ['a','b','c']
I'd like to make a new list n that has everything except for a given item in m (for example the item 'a'). However, when I use
>>> m.remove('a') >>> m m = ['b', 'c']
the original list is mutated (the value 'a' is removed from the original list). Is there a way to get a new list sans-'a' without mutating the original? So I mean that m should still be [ 'a', 'b', 'c' ], and I will get a new list, which has to be [ 'b', 'c' ].
I assume you mean that you want to create a new list without a given element, instead of changing the original list. One way is to use a list comprehension:
m = ['a', 'b', 'c'] n = [x for x in m if x != 'a']
n is now a copy of m, but without the 'a' element.
Another way would of course be to copy the list first
m = ['a', 'b', 'c'] n = m[:] n.remove('a')
If removing a value by index, it is even simpler
n = m[:index] + m[index+1:]
You can create a new list without the offending element with a list-comprehension. This will preserve the value of the original list.
l = ['a','b','c'] [s for s in l if s!='a']
Another approach to list comprehension is numpy:
>>> import numpy >>> a = [1, 2, 3, 4] >>> list(numpy.remove(a,a.index(3))) [1, 2, 4]
There is a simple way to do that using built-in function :filter .
Here is ax example:
a = [1, 2, 3, 4] b = filter(lambda x: x != 3,a)