# How to make lists contain only distinct element in Python?

## How to make lists contain only distinct element in Python?

The simplest is to convert to a set then back to a list:

``````my_list = list(set(my_list))
``````

One disadvantage with this is that it wont preserve the order. You may also want to consider if a set would be a better data structure to use in the first place, instead of a list.

Modified versions of http://www.peterbe.com/plog/uniqifiers-benchmark

To preserve the order:

``````def f(seq): # Order preserving
Modified version of Dave Kirby solution
seen = set()
return [x for x in seq if x not in seen and not seen.add(x)]
``````

OK, now how does it work, because its a little bit tricky here `if x not in seen and not seen.add(x)`:

``````In : 0 not in [1,2,3] and not print(add)
Out: True
``````

Why does it return True? print (and set.add) returns nothing:

``````In : type(seen.add(10))
Out: <type NoneType>
``````

and `not None == True`, but:

``````In : 1 not in [1,2,3] and not print(add)
Out: False
``````

Why does it print add in  but not in ? See `False and print(add)`, and doesnt check the second argument, because it already knows the answer, and returns true only if both arguments are True.

More generic version, more readable, generator based, adds the ability to transform values with a function:

``````def f(seq, idfun=None): # Order preserving
return list(_f(seq, idfun))

def _f(seq, idfun=None):
Originally proposed by Andrew Dalke
seen = set()
if idfun is None:
for x in seq:
if x not in seen:
yield x
else:
for x in seq:
x = idfun(x)
if x not in seen:
yield x
``````

Without order (its faster):

``````def f(seq): # Not order preserving
return list(set(seq))
``````

#### How to make lists contain only distinct element in Python?

one-liner and preserve order

``````list(OrderedDict.fromkeys([2,1,1,3]))
``````

although youll need

``````from collections import OrderedDict
``````