Is there a simple way to delete a list element by value?

Question

I want to remove a value from a list if it exists in the list (which it may not).

a = [1, 2, 3, 4]
b = a.index(6)

del a[b]
print(a)

The abov

Answer 1

this is my answer, just use while and for

def remove_all(data, value):
    i = j = 0
    while j < len(data):
        if data[j] == value:
            j += 1
            continue
        data[i] = data[j]
        i += 1
        j += 1
    for x in range(j - i):
        data.pop()

Answer 2

Another possibility is to use a set instead of a list, if a set is applicable in your application.

IE if your data is not ordered, and does not have duplicates, then

my_set=set([3,4,2])
my_set.discard(1)

is error-free.

Often a list is just a handy container for items that are actually unordered. There are questions asking how to remove all occurences of an element from a list. If you don't want dupes in the first place, once again a set is handy.

my_set.add(3)

doesn't change my_set from above.

Answer 3

To remove an element's first occurrence in a list, simply use list.remove:

>>> a = ['a', 'b', 'c', 'd']
>>> a.remove('b')
>>> print(a)
['a', 'c', 'd']

Mind that it does not remove all occurrences of your element. Use a list comprehension for that.

>>> a = [10, 20, 30, 40, 20, 30, 40, 20, 70, 20]
>>> a = [x for x in a if x != 20]
>>> print(a)
[10, 30, 40, 30, 40, 70]

Answer 4

Usually Python will throw an Exception if you tell it to do something it can't so you'll have to do either:

if c in a:
    a.remove(c)

or:

try:
    a.remove(c)
except ValueError:
    pass

An Exception isn't necessarily a bad thing as long as it's one you're expecting and handle properly.

Answer 5

As stated by numerous other answers, list.remove() will work, but throw a ValueError if the item wasn't in the list. With python 3.4+, there's an interesting approach to handling this, using the suppress contextmanager:

from contextlib import suppress
with suppress(ValueError):
    a.remove('b')

Answer 6

In one line:

a.remove('b') if 'b' in a else None

sometimes it usefull.

Even easier:

if 'b' in a: a.remove('b')