How to make lists contain only distinct element in Python

Question

I have a list in Python  how can I make it s values unique

User · Answer

The simplest way to remove duplicates whilst preserving order is to use collections OrderedDict  Python 2 7     from collections import OrderedDict d   OrderedDict   for x in mylist      d x    True print d iterkeys

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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 it s a little bit tricky here if x not in seen and not seen add x    In  1   0 not in  1 2 3  and not print  add   add Out 1   True   Why does it return True  print  and set add  returns nothing   In  3   type seen add 10   Out 3    lt type  NoneType  gt    and not None    True  but   In  2   1 not in  1 2 3  and not print  add   Out 2   False   Why does it print  add  in  1  but not in  2   See False and print  add    and doesn t 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          seen add x          yield x   else      for x in seq        x   idfun x        if x not in seen          seen add x          yield x   Without order  it s faster    def f seq     Not order preserving   return list set seq

User · Answer

From http   www peterbe com plog uniqifiers-benchmark   def f5 seq  idfun None           order preserving     if idfun is None          def idfun x   return x     seen          result          for item in seq          marker   idfun item            in old Python versions            if seen has key marker            but in new ones          if marker in seen  continue         seen marker    1         result append item      return result

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How about dictionary comprehensions    gt  gt  gt  mylist    3  2  1  3  4  4  4  5  5  3    gt  gt  gt   x 1 for x in mylist  keys    1  2  3  4  5    EDIT To  Danny s comment  my original suggestion does not keep the keys ordered  If you need the keys sorted  try    gt  gt  gt  from collections import OrderedDict   gt  gt  gt  OrderedDict   x 1  for x in mylist   keys    3  2  1  4  5    which keeps elements in the order by the first occurrence of the element  not extensively tested

User · Answer

The characteristics of sets in Python are that the data items in a set are unordered and duplicates are not allowed  If you try to add a data item to a set that already contains the data item  Python simply ignores it    gt  gt  gt  l     a    a    bb    b    c    c    10    10    8   8   10  10  6  10  11 2  11 2  11  11   gt  gt  gt  distinct l   set l   gt  gt  gt  print distinct l  set   a    10    c    b   6   bb   10  11  11 2   8

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Let me explain to you by an example   if you have Python list   gt  gt  gt  randomList     a   f    b    c    d    a    c    e    d    f    e     and you want to remove duplicates from it    gt  gt  gt  uniqueList        gt  gt  gt  for letter in randomList      if letter not in uniqueList          uniqueList append letter    gt  gt  gt  uniqueList   a    f    b    c    d    e     This is how you can remove duplicates from the list

User · Answer

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 won t 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

User · Answer

one-liner and preserve order  list OrderedDict fromkeys  2 1 1 3      although you ll need  from collections import OrderedDict

User · Answer

To preserve the order   l    1  1  2  2  3  result   list   map lambda x  not x in result and result append x   l  result    1  2  3

User · Answer

If all elements of the list may be used as dictionary keys  i e  they are all hashable  this is often faster  Python Programming FAQ  d      for x in mylist      d x    1 mylist   list d keys

[python] How to make lists contain only distinct element in Python?

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