I was wondering how can I know if when a user inputs a value, that value already exists in a list.
For example;
lis = ['foo', 'boo', 'hoo'] user inputs:
'boo' Now my question is how can I tell the user this value already exists inside the list.
Use in operator:
>>> lis = ['foo', 'boo', 'hoo'] >>> 'boo' in lis True >>> 'zoo' in lis False You can also use lis.index which will return the index of the element.
>>> lis.index('boo') 1 If the element is not found, it will raise ValueError:
>>> lis.index('zoo') Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: 'zoo' is not in list UPDATE
As Nick T commented, if you don't care about order of items, you can use set:
>>> lis = {'foo', 'boo', 'hoo'} # set literal == set(['foo', 'boo', 'hoo']) >>> lis.add('foo') # duplicated item is not added. >>> lis {'boo', 'hoo', 'foo'} 
One more way you can do is use collections :-
import collections lis = ['foo', 'boo', 'hoo'] # Now if user inputs boo lis.append('boo') print [x for x, y in collections.Counter(lis).items() if y > 1] # Now it will print the duplicate value in output:- boo But the above one is not efficient. So for make it efficient use set as falsetru indicates in the answer:-
totalList= set() uniq = [] for x in lis: if x not in totalList: uniq.append(x) totalList.add(x) 
set, which, if you attempt to add a duplicate, it ignores it.int,dict,list, andstr.