Reverse invert a dictionary mapping

Question

Given a dictionary like so   my map     a   1   b   2    How can one invert this map to get   inv map    1   a   2   b

User · Answer

I am aware that this question already has many good answers, but I wanted to share this very neat solution that also takes care of duplicate values:

def dict_reverser(d):
    seen = set()
    return {v: k for k, v in d.items() if v not in seen or seen.add(v)}

This relies on the fact that set.add always returns None in Python.

User · Answer

This expands upon the answer by Robert  applying to when the values in the dict aren t unique   class ReversibleDict dict        def reversed self                       Return a reversed dict  with common values in the original dict         grouped into a list in the returned dict           Example           gt  gt  gt  d   ReversibleDict   a   3   c   2   b   2   e   3   d   1   f   2            gt  gt  gt  d reversed            1    d    2    c    b    f    3    a    e                         revdict              for k  v in self iteritems                revdict setdefault v      append k          return revdict   The implementation is limited in that you cannot use reversed twice and get the original back  It is not symmetric as such  It is tested with Python 2 6  Here is a use case of how I am using to print the resultant dict   If you d rather use a set than a list  and there could exist unordered applications for which this makes sense  instead of setdefault v      append k   use setdefault v  set    add k

User · Answer

Try this   inv map   dict zip my map values    my map keys        Note that the Python docs on dictionary views explicitly guarantee that  keys   and  values   have their elements in the same order  which allows the approach above to work    Alternatively   inv map   dict  my map k   k  for k in my map    or using python 3 0 s dict comprehensions  inv map    my map k    k for k in my map

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In addition to the other functions suggested above  if you like lambdas   invert   lambda mydict   v k for k  v in mydict items      Or  you could do it this way too   invert   lambda mydict  dict  zip mydict values    mydict keys

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Assuming that the values in the dict are unique   dict  v  k  for k  v in my map iteritems

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I found that this version is more than 10  faster than the accepted version of a dictionary with 10000 keys  d    i  str i  for i in range 10000    new d   dict zip d values    d keys

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If the values in my map aren t unique  inv map      for k  v in my map iteritems        inv map v    inv map get v         k

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Try this   inv map   dict zip my map values    my map keys        Note that the Python docs on dictionary views explicitly guarantee that  keys   and  values   have their elements in the same order  which allows the approach above to work    Alternatively   inv map   dict  my map k   k  for k in my map    or using python 3 0 s dict comprehensions  inv map    my map k    k for k in my map

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To do this while preserving the type of your mapping  assuming that it is a dict or a dict subclass    def inverse mapping f       return f   class   map reversed  f items

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Assuming that the values in the dict are unique   dict  v  k  for k  v in my map iteritems

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If values aren t unique AND may be a hash  one dimension    for k  v in myDict items        if len v   gt  1          for item in v              invDict item    invDict get item                  invDict item  append k      else          invDict v    invDict get v              invDict v  append k    And with a recursion if you need to dig deeper then just one dimension   def digList lst       temp          for item in lst          if type item  is list              temp append digList item           else              temp append item      return set temp   for k  v in myDict items        if type v  is list          items   digList v          for item in items              invDict item    invDict get item                  invDict item  append k      else          invDict v    invDict get v              invDict v  append k

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This handles non-unique values and retains much of the look of the unique case    inv map    v  k for k in my map if my map k     v  for v in my map itervalues      For Python 3 x  replace itervalues with values

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A case where the dictionary values is a set  Like  some dict     quot 1 quot    quot a quot   quot b quot   quot c quot             quot 2 quot    quot d quot   quot e quot   quot f quot             quot 3 quot    quot g quot   quot h quot   quot i quot     The inverse would like  some dict    vi  k  for k  v in some dict items   for vi in v   The output is like this    c    1     b    1     a    1     f    2     d    2     e    2     g    3     h    3     i    3

User · Answer

In addition to the other functions suggested above  if you like lambdas   invert   lambda mydict   v k for k  v in mydict items      Or  you could do it this way too   invert   lambda mydict  dict  zip mydict values    mydict keys

User · Answer

If the values in my map aren t unique  inv map      for k  v in my map iteritems        inv map v    inv map get v         k

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Function is symmetric for values of type list  Tuples are coverted to lists when performing reverse dict reverse dict dictionary    def reverse dict dictionary       reverse dict          for key  value in dictionary iteritems            if not isinstance value   list  tuple                value    value          for val in value              reverse dict val    reverse dict get val                  reverse dict val  append key      for key  value in reverse dict iteritems            if len value     1              reverse dict key    value 0      return reverse dict

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Try this   inv map   dict zip my map values    my map keys        Note that the Python docs on dictionary views explicitly guarantee that  keys   and  values   have their elements in the same order  which allows the approach above to work    Alternatively   inv map   dict  my map k   k  for k in my map    or using python 3 0 s dict comprehensions  inv map    my map k    k for k in my map

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Another  more functional  way   my map      a   1   b  2   dict map reversed  my map items

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Combination of list and dictionary comprehension  Can handle duplicate keys   v  i for i in d keys   if d i     v   for k v in d items

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A lambda solution for current python 3 x versions   d1   dict alice  apples   bob  bananas   d2   dict map lambda key   d1 key   key   d1 keys     print d2    Result     apples    alice    bananas    bob     This solution does not check for duplicates   Some remarks    The lambda construct can access d1 from the outer scope  so we only pass in the current key  It returns a tuple  The dict   constructor accepts a list of tuples  It also accepts the result of a map  so we can skip the conversion to a list  This solution has no explicit for loop  It also avoids using a list comprehension for those who are bad at math  -

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def reverse dictionary input dict         out            for v in input dict values                for value in v                if value not in out                    out value lower                for i in input dict            for j in out                if j in map  lambda x   x lower   input dict i                      out j  append i lower                      out j  sort         return out   this code do like this   r   reverse dictionary   Accurate     exact    precise     exact     precise     astute     Smart    clever     smart     clever    bright    talented      print r     precise     accurate    exact     clever     astute    smart     talented     smart     bright     smart     exact     accurate     smart     astute

User · Answer

I am aware that this question already has many good answers, but I wanted to share this very neat solution that also takes care of duplicate values:

def dict_reverser(d):
    seen = set()
    return {v: k for k, v in d.items() if v not in seen or seen.add(v)}

This relies on the fact that set.add always returns None in Python.

User · Answer

Python 3   inv map    v  k for k  v in my map items     Python 2  inv map    v  k for k  v in my map iteritems

User · Answer

If the values in my map aren t unique  inv map      for k  v in my map iteritems        inv map v    inv map get v         k

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To do this while preserving the type of your mapping  assuming that it is a dict or a dict subclass    def inverse mapping f       return f   class   map reversed  f items

User · Answer

A lambda solution for current python 3 x versions   d1   dict alice  apples   bob  bananas   d2   dict map lambda key   d1 key   key   d1 keys     print d2    Result     apples    alice    bananas    bob     This solution does not check for duplicates   Some remarks    The lambda construct can access d1 from the outer scope  so we only pass in the current key  It returns a tuple  The dict   constructor accepts a list of tuples  It also accepts the result of a map  so we can skip the conversion to a list  This solution has no explicit for loop  It also avoids using a list comprehension for those who are bad at math  -

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For instance  you have the following dictionary   dict     a    fire    b    ice    c    fire    d    water     And you wanna get it in such an inverted form   inverted dict     fire     a    c     ice     b     water     d      First Solution  For inverting key-value pairs in your dictionary use a for-loop approach     Use this code to invert dictionaries that have non-unique values  inverted dict   dict   for key  value in dict items        inverted dict setdefault value  list    append key    Second Solution  Use a dictionary comprehension approach for inversion     Use this code to invert dictionaries that have unique values  inverted dict    value  key for key  value in dict items      Third Solution  Use reverting the inversion approach  relies on second solution      Use this code to invert dictionaries that have lists of values  dict    value  key for key in inverted dict for value in my map key

User · Answer

Here is another way to do it  my map     a   1   b   2   inv map     for key in my map keys         val   my map key      inv map val    key

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Python 3   inv map    v  k for k  v in my map items     Python 2  inv map    v  k for k  v in my map iteritems

User · Answer

I found that this version is more than 10  faster than the accepted version of a dictionary with 10000 keys  d    i  str i  for i in range 10000    new d   dict zip d values    d keys

User · Answer

Try this for python 2 7 3 x  inv map     for i in my map      inv map my map i   i     print inv map

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We can also reverse a dictionary with duplicate keys using defaultdict   from collections import Counter  defaultdict  def invert dict d       d inv   defaultdict list      for k  v in d items            d inv v  append k      return d inv  text    aaa bbb ccc ddd aaa bbb ccc aaa   c   Counter text split      Counter   aaa   3   bbb   2   ccc   2   ddd   1   dict invert dict c      1    ddd    2    bbb    ccc    3    aaa        See here      This technique is simpler and faster than an equivalent technique using dict setdefault

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I think the best way to do this is to define a class  Here is an implementation of a  symmetric dictionary    class SymDict      def   init   self           self aToB              self bToA           def assocAB self  a  b             Stores and returns a tuple  a b  of overwritten bindings         currB   None         if a in self aToB  currB   self bToA a          currA   None         if b in self bToA  currA   self aToB b           self aToB a    b         self bToA b    a         return  currA  currB       def lookupA self  a           if a in self aToB              return self aToB a          return None      def lookupB self  b           if b in self bToA              return self bToA b          return None   Deletion and iteration methods are easy enough to implement if they re needed   This implementation is way more efficient than inverting an entire dictionary  which seems to be the most popular solution on this page   Not to mention  you can add or remove values from your SymDict as much as you want  and your inverse-dictionary will always stay valid -- this isn t true if you simply reverse the entire dictionary once

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Try this for python 2 7 3 x  inv map     for i in my map      inv map my map i   i     print inv map

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Fast functional solution for non-bijective maps  values not unique    from itertools import imap  groupby  def fst s       return s 0   def snd s       return s 1   def inverseDict d               input d  a - gt  b     output   b - gt  set a              return           v   set imap fst  kv iter           for  v  kv iter  in groupby              sorted d iteritems                       key snd               key snd                   In theory this should be faster than adding to the set  or appending to the list  one by one like in the imperative solution   Unfortunately the values have to be sortable  the sorting is required by groupby

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Not something completely different  just a bit rewritten recipe from Cookbook  It s futhermore optimized by retaining setdefault method  instead of each time getting it through the instance   def inverse mapping               A function to inverse mapping  collecting keys with simillar values     in list  Careful to retain original type and to be fast       gt  gt  d   dict a 1  b 2  c 1  d 3  e 2  f 1  g 5  h 2       gt  gt  inverse d       1    f    c    a    2    h    b    e    3    d    5    g                res          setdef   res setdefault     for key  value in mapping items            setdef value      append key      return res if mapping   class    dict else mapping   class   res    Designed to be run under CPython 3 x  for 2 x replace mapping items   with mapping iteritems    On my machine runs a bit faster  than other examples here

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Since dictionaries require one unique key within the dictionary unlike values  we have to append the reversed values into a list of sort to be included within the new specific keys    def r maping dictionary       List z        Map         for z  x in dictionary iteritems     iterate through the keys and values         Map setdefault x List z  append z   Setdefault is the same as dict key  default  The method returns the key value available in the dictionary and if given key is not available then it will return provided default value  Afterward  we will append into the default list our new values for the specific key      return Map

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If the values aren t unique  and you re a little hardcore   inv map   dict       v   k for  k  xx  in filter lambda  key  value   value    v  my map items           for v in set my map values        Especially for a large dict  note that this solution is far less efficient than the answer Python reverse   invert a mapping because it loops over items   multiple times

User · Answer

For instance  you have the following dictionary   dict     a    fire    b    ice    c    fire    d    water     And you wanna get it in such an inverted form   inverted dict     fire     a    c     ice     b     water     d      First Solution  For inverting key-value pairs in your dictionary use a for-loop approach     Use this code to invert dictionaries that have non-unique values  inverted dict   dict   for key  value in dict items        inverted dict setdefault value  list    append key    Second Solution  Use a dictionary comprehension approach for inversion     Use this code to invert dictionaries that have unique values  inverted dict    value  key for key  value in dict items      Third Solution  Use reverting the inversion approach  relies on second solution      Use this code to invert dictionaries that have lists of values  dict    value  key for key in inverted dict for value in my map key

User · Answer

Combination of list and dictionary comprehension  Can handle duplicate keys   v  i for i in d keys   if d i     v   for k v in d items

User · Answer

If the values in my map aren t unique  inv map      for k  v in my map iteritems        inv map v    inv map get v         k

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I wrote this with the help of cycle  for  and method   get    and I changed the name  map  of the dictionary to  map1  because  map  is a function   def dict invert map1       inv map        new dictionary     for key in map1 keys            inv map map1 get key     key     return inv map

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A case where the dictionary values is a set  Like  some dict     quot 1 quot    quot a quot   quot b quot   quot c quot             quot 2 quot    quot d quot   quot e quot   quot f quot             quot 3 quot    quot g quot   quot h quot   quot i quot     The inverse would like  some dict    vi  k  for k  v in some dict items   for vi in v   The output is like this    c    1     b    1     a    1     f    2     d    2     e    2     g    3     h    3     i    3

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If values aren t unique AND may be a hash  one dimension    for k  v in myDict items        if len v   gt  1          for item in v              invDict item    invDict get item                  invDict item  append k      else          invDict v    invDict get v              invDict v  append k    And with a recursion if you need to dig deeper then just one dimension   def digList lst       temp          for item in lst          if type item  is list              temp append digList item           else              temp append item      return set temp   for k  v in myDict items        if type v  is list          items   digList v          for item in items              invDict item    invDict get item                  invDict item  append k      else          invDict v    invDict get v              invDict v  append k

User · Answer

Another  more functional  way   my map      a   1   b  2   dict map reversed  my map items

User · Answer

This handles non-unique values and retains much of the look of the unique case    inv map    v  k for k in my map if my map k     v  for v in my map itervalues      For Python 3 x  replace itervalues with values

User · Answer

I would do it that way in python 2   inv map    my map x    x for x in my map

User · Answer

Fast functional solution for non-bijective maps  values not unique    from itertools import imap  groupby  def fst s       return s 0   def snd s       return s 1   def inverseDict d               input d  a - gt  b     output   b - gt  set a              return           v   set imap fst  kv iter           for  v  kv iter  in groupby              sorted d iteritems                       key snd               key snd                   In theory this should be faster than adding to the set  or appending to the list  one by one like in the imperative solution   Unfortunately the values have to be sortable  the sorting is required by groupby

User · Answer

Try this   inv map   dict zip my map values    my map keys        Note that the Python docs on dictionary views explicitly guarantee that  keys   and  values   have their elements in the same order  which allows the approach above to work    Alternatively   inv map   dict  my map k   k  for k in my map    or using python 3 0 s dict comprehensions  inv map    my map k    k for k in my map

User · Answer

Not something completely different  just a bit rewritten recipe from Cookbook  It s futhermore optimized by retaining setdefault method  instead of each time getting it through the instance   def inverse mapping               A function to inverse mapping  collecting keys with simillar values     in list  Careful to retain original type and to be fast       gt  gt  d   dict a 1  b 2  c 1  d 3  e 2  f 1  g 5  h 2       gt  gt  inverse d       1    f    c    a    2    h    b    e    3    d    5    g                res          setdef   res setdefault     for key  value in mapping items            setdef value      append key      return res if mapping   class    dict else mapping   class   res    Designed to be run under CPython 3 x  for 2 x replace mapping items   with mapping iteritems    On my machine runs a bit faster  than other examples here

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def invertDictionary d       myDict        for i in d       value   d get i       myDict setdefault value     append i      return myDict  print invertDictionary   a  1   b  2   c  3    d    1     This will provide output as    1    a    d    2    b    3    c

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I think the best way to do this is to define a class  Here is an implementation of a  symmetric dictionary    class SymDict      def   init   self           self aToB              self bToA           def assocAB self  a  b             Stores and returns a tuple  a b  of overwritten bindings         currB   None         if a in self aToB  currB   self bToA a          currA   None         if b in self bToA  currA   self aToB b           self aToB a    b         self bToA b    a         return  currA  currB       def lookupA self  a           if a in self aToB              return self aToB a          return None      def lookupB self  b           if b in self bToA              return self bToA b          return None   Deletion and iteration methods are easy enough to implement if they re needed   This implementation is way more efficient than inverting an entire dictionary  which seems to be the most popular solution on this page   Not to mention  you can add or remove values from your SymDict as much as you want  and your inverse-dictionary will always stay valid -- this isn t true if you simply reverse the entire dictionary once

User · Answer

We can also reverse a dictionary with duplicate keys using defaultdict   from collections import Counter  defaultdict  def invert dict d       d inv   defaultdict list      for k  v in d items            d inv v  append k      return d inv  text    aaa bbb ccc ddd aaa bbb ccc aaa   c   Counter text split      Counter   aaa   3   bbb   2   ccc   2   ddd   1   dict invert dict c      1    ddd    2    bbb    ccc    3    aaa        See here      This technique is simpler and faster than an equivalent technique using dict setdefault

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I would do it that way in python 2   inv map    my map x    x for x in my map

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def reverse dictionary input dict         out            for v in input dict values                for value in v                if value not in out                    out value lower                for i in input dict            for j in out                if j in map  lambda x   x lower   input dict i                      out j  append i lower                      out j  sort         return out   this code do like this   r   reverse dictionary   Accurate     exact    precise     exact     precise     astute     Smart    clever     smart     clever    bright    talented      print r     precise     accurate    exact     clever     astute    smart     talented     smart     bright     smart     exact     accurate     smart     astute

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Assuming that the values in the dict are unique   dict  v  k  for k  v in my map iteritems

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Since dictionaries require one unique key within the dictionary unlike values  we have to append the reversed values into a list of sort to be included within the new specific keys    def r maping dictionary       List z        Map         for z  x in dictionary iteritems     iterate through the keys and values         Map setdefault x List z  append z   Setdefault is the same as dict key  default  The method returns the key value available in the dictionary and if given key is not available then it will return provided default value  Afterward  we will append into the default list our new values for the specific key      return Map

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Python 3   inv map    v  k for k  v in my map items     Python 2  inv map    v  k for k  v in my map iteritems

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I wrote this with the help of cycle  for  and method   get    and I changed the name  map  of the dictionary to  map1  because  map  is a function   def dict invert map1       inv map        new dictionary     for key in map1 keys            inv map map1 get key     key     return inv map

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Function is symmetric for values of type list  Tuples are coverted to lists when performing reverse dict reverse dict dictionary    def reverse dict dictionary       reverse dict          for key  value in dictionary iteritems            if not isinstance value   list  tuple                value    value          for val in value              reverse dict val    reverse dict get val                  reverse dict val  append key      for key  value in reverse dict iteritems            if len value     1              reverse dict key    value 0      return reverse dict

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This expands upon the answer by Robert  applying to when the values in the dict aren t unique   class ReversibleDict dict        def reversed self                       Return a reversed dict  with common values in the original dict         grouped into a list in the returned dict           Example           gt  gt  gt  d   ReversibleDict   a   3   c   2   b   2   e   3   d   1   f   2            gt  gt  gt  d reversed            1    d    2    c    b    f    3    a    e                         revdict              for k  v in self iteritems                revdict setdefault v      append k          return revdict   The implementation is limited in that you cannot use reversed twice and get the original back  It is not symmetric as such  It is tested with Python 2 6  Here is a use case of how I am using to print the resultant dict   If you d rather use a set than a list  and there could exist unordered applications for which this makes sense  instead of setdefault v      append k   use setdefault v  set    add k

User · Answer

If the values aren t unique  and you re a little hardcore   inv map   dict       v   k for  k  xx  in filter lambda  key  value   value    v  my map items           for v in set my map values        Especially for a large dict  note that this solution is far less efficient than the answer Python reverse   invert a mapping because it loops over items   multiple times

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def invertDictionary d       myDict        for i in d       value   d get i       myDict setdefault value     append i      return myDict  print invertDictionary   a  1   b  2   c  3    d    1     This will provide output as    1    a    d    2    b    3    c

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Python 3   inv map    v  k for k  v in my map items     Python 2  inv map    v  k for k  v in my map iteritems

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Assuming that the values in the dict are unique   dict  v  k  for k  v in my map iteritems

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Here is another way to do it  my map     a   1   b   2   inv map     for key in my map keys         val   my map key      inv map val    key

[python] Reverse / invert a dictionary mapping

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