Is there a short contains function for lists?

Question

Given a list xs and a value item, how can I check whether xs contains item (i.e., if any of the elements of xs is equal to item)? Is there something like xs.contains(item)?

_{For performance considerations, see Fastest way to check if a value exists in a list.}

Asked By: Joan Venge

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Source

Answer 1

Use:

if my_item in some_list:
    ...

Also, inverse operation:

if my_item not in some_list:
    ...

It works fine for lists, tuples, sets and dicts (check keys).

Note that this is an O(n) operation in lists and tuples, but an O(1) operation in sets and dicts.

Answered By: defuz

Answer 2

The list method index will return -1 if the item is not present, and will return the index of the item in the list if it is present. Alternatively in an if statement you can do the following:

if myItem in list:
    #do things

You can also check if an element is not in a list with the following if statement:

if myItem not in list:
    #do things

Answered By: Mr. Squig

Answer 3

In addition to what other have said, you may also be interested to know that what in does is to call the list.__contains__ method, that you can define on any class you write and can get extremely handy to use python at his full extent.

A dumb use may be:

>>> class ContainsEverything:
    def __init__(self):
        return None
    def __contains__(self, *elem, **k):
        return True


>>> a = ContainsEverything()
>>> 3 in a
True
>>> a in a
True
>>> False in a
True
>>> False not in a
False
>>>

Answered By: Ant

Answer 4

I came up with this one liner recently for getting True if a list contains any number of occurrences of an item, or False if it contains no occurrences or nothing at all. Using next(...) gives this a default return value (False) and means it should run significantly faster than running the whole list comprehension.

list_does_contain = next((True for item in list_to_test if item == test_item), False)

Answered By: Dustin Raimondi

Answer 5

There is also the list method:

[2, 51, 6, 8, 3].__contains__(8)
# Out[33]: True
[2, 51, 6, 3].__contains__(8)
# Out[33]: False

Answered By: Andreas

Answer 6

There is one another method that uses index. But I am not sure if this has any fault or not.

list = [5,4,3,1]
try:
    list.index(2)
    #code for when item is expected to be in the list
    print("present")
except:
    #code for when item is not expected to be in the list
    print("not present")

Output:

not present

Answered By: rsonx

Is there a short contains function for lists?

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