Get key by value in dictionary

Question

I made a function which will look up ages in a Dictionary and show the matching name   dictionary     george    16   amber    19  search age   raw input  Provide age   for age in dictionary values        if age    search age          name   dictionary age          print name   I know how to compare and find the age I just don t know how to show the name of the person  Additionally  I am getting a KeyError because of line 5  I know it s not correct but I can t figure out how to make it search backwards

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here is my take on it  This is good for displaying multiple results just in case you need one  So I added the list as well   myList     george  16  amber  19   rachel  19               david  15                            Setting the dictionary result                                           Making ready of the result list search age   int input  Enter age      for keywords in myList keys        if myList keywords    search age      result append keywords                      This part  we are making list of results  for res in result                               We are now printing the results     print res    And that s it

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There is no easy way to find a key in a list by  looking up  the value  However  if you know the value  iterating through the keys  you can look up values in the dictionary by the element  If D element  where D is a dictionary object  is equal to the key you re trying to look up  you can execute some code   D     Ali   20   Marina   12   George  16  age   int input  enter age  t      for element in D keys        if D element     age          print element

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A simple way to do this could be   list     george  16  amber  19  search age   raw input  Provide age   for age in list values        name   list list  search age  key   tolist       print name   This will return a list of the keys with value that match search age  You can also replace  list  search age  with any other conditions statement if needed

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This is how you access the dictionary to do what you want   list     george   16   amber   19  search age   raw input  Provide age   for age in list      if list age     search age          print age   of course  your names are so off it looks like it would be printing an age  but it DOES print the name  Since you are accessing by name  it becomes more understandable if you write   list     george   16   amber   19  search age   raw input  Provide age   for name in list      if list name     search age          print name   Better yet    people     george     age   16    amber     age   19   search age   raw input  Provide age   for name in people      if people name   age      search age          print name

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it s answered  but it could be done with a fancy  map reduce  use  e g    def find key value  dictionary       return reduce lambda x  y  x if x is not None else y                    map lambda x  x 0  if x 1     value else None                         dictionary iteritems

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I realize it s been a long time and the original asker likely no longer has any need of an answer  but none of these are good answers if you actually have control over this code  You re just using the wrong data structure  This is a perfect illustration of the use case for a two-way dict   gt  gt  gt  from collections import defaultdict  UserDict  gt  gt  gt  class TwoWayDict UserDict           def   init   self   args    kwargs               super     init    args    kwargs              self val to keys   defaultdict list          def   setitem   self  key  value               super     setitem   key  value              self val to keys value  append key          def get keys for val self  value               return self val to keys value        gt  gt  gt  d   TwoWayDict    gt  gt  gt  d  a     1  gt  gt  gt  d  b     1  gt  gt  gt  d get keys for val 1    a    b    Adds miniscule overhead to insertions but you keep constant-time lookup  except now in both directions  No need to construct the reverse mapping from scratch every time you need it  Just store it as you go and access it as needed  Further  many of these answers are not even correct because clearly many people can have the same age but they re only returning the first matching key  not all of them

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def get Value dic value       for name in dic          if dic name     value              del dic name              return name

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my dict     A   19   B   28   carson   28  search age   28  take only one name   next  name for name  age in my dict items   if age    search age   None  print name      B   get multiple data name list    name for name  age in filter lambda item  item 1     search age  my dict items     print name list       B    carson

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I glimpsed all answers and none mentioned simply using list comprehension  This Pythonic one-line solution can return all keys for any number of given values  tested in Python 3 9 1    gt  gt  gt  dictionary     george    16   amber    19   frank   19   gt  gt  gt   gt  gt  gt  age   19  gt  gt  gt  name    k for k in dictionary keys   if dictionary k     age   name   george    frank    gt  gt  gt   gt  gt  gt  age    16  19   gt  gt  gt  name    k for k in dictionary keys   if dictionary k  in age   name   george    amber    frank    gt  gt  gt   gt  gt  gt  age    22  25   gt  gt  gt  name    k for k in dictionary keys   if dictionary k  in age   name

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Try this one-liner to reverse a dictionary   reversed dictionary   dict map reversed  dictionary items

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get key   lambda v  d  next k for k in d if d k  is v

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You need to use a dictionary and reverse of that dictionary  It means you need another data structure  If you are in python 3  use enum module but if you are using python 2 7 use enum34 which is back ported for python 2   Example   from enum import Enum  class Color Enum        red   1      green   2      blue   3   gt  gt  gt  print Color red   Color red   gt  gt  gt  print repr Color red     lt color red  1    gt     gt  gt  gt  type Color red    lt enum  color     gt    gt  gt  gt  isinstance Color green  Color   True    gt  gt  gt  member   Color red   gt  gt  gt  member name   red    gt  gt  gt  member value  1

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There is none  dict is not intended to be used this way   dictionary     george   16   amber   19  search age   input  Provide age   for name  age in dictionary items       for name  age in dictionary iteritems      for Python 2 x      if age    search age          print name

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I ended up doing it with a function  This way you might avoid doing the full loop  and the intuition says that it should be faster than other solutions presented  def get key from value my dict  to find        for k v in my dict items            if v  to find  return k      return None

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mydict     george   16   amber   19  print mydict keys   mydict values   index 16      Prints george   Or in Python 3 x   mydict     george   16   amber   19  print list mydict keys    list mydict values    index 16       Prints george   Basically  it separates the dictionary s values in a list  finds the position of the value you have  and gets the key at that position   More about keys   and  values   in Python 3  How can I get list of values from dict

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If you want both the name and the age  you should be using  items   which gives you key  key  value  tuples   for name  age in mydict items        if age    search age          print name   You can unpack the tuple into two separate variables right in the for loop  then match the age   You should also consider reversing the dictionary if you re generally going to be looking up by age  and no two people have the same age    16   george   19   amber     so you can look up the name for an age by just doing  mydict search age    I ve been calling it mydict instead of list because list is the name of a built-in type  and you shouldn t use that name for anything else   You can even get a list of all people with a given age in one line    name for name  age in mydict items   if age    search age    or if there is only one person with each age   next  name for name  age in mydict items   if age    search age   None    which will just give you None if there isn t anyone with that age   Finally  if the dict is long and you re on Python 2  you should consider using  iteritems   instead of  items   as Cat Plus Plus did in his answer  since it doesn t need to make a copy of the list

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d    george  16  amber  19   dict  v k  for k v in d items    get 16    The output is as follows   - gt  prints george

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If you want to find the key by the value  you can use a dictionary comprehension to create a lookup dictionary and then use that to find the key from the value   lookup    value  key for key  value in self data  lookup value

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I found this answer very effective but not very easy to read for me   To make it more clear you can invert the key and the value of a dictionary  This is make the keys values and the values keys  as seen here   mydict     george  16  amber  19  res   dict  v k  for k v in mydict iteritems    print res 16     Prints george   or  mydict     george  16  amber  19  dict  v k  for k v in mydict iteritems    16    which is essentially the same that this other answer

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for name in mydict      if mydict name     search age          print name            or do something else with it            if in a function append to a temporary list            then after the loop return the list

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Just my answer in lambda and filter   filter  lambda x  dictionary dictionary  search age int search age   dictionary x     search age    dictionary

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I thought it would be interesting to point out which methods are the quickest  and in what scenario   Here s some tests I ran  on a 2012 MacBook Pro    gt  gt  gt  def method1 list search age           for name age in list iteritems                    if age    search age                          return name       gt  gt  gt  def method2 list search age           return  name for name age in list iteritems   if age    search age        gt  gt  gt  def method3 list search age           return list keys   list values   index search age     Results from profile run   on each method 100000 times   Method 1    gt  gt  gt  profile run  for i in range 0 100000   method1 list 16         200004 function calls in 1 173 seconds   Method 2    gt  gt  gt  profile run  for i in range 0 100000   method2 list 16         200004 function calls in 1 222 seconds   Method 3    gt  gt  gt  profile run  for i in range 0 100000   method3 list 16         400004 function calls in 2 125 seconds   So this shows that for a small dict  method 1 is the quickest  This is most likely because it returns the first match  as opposed to all of the matches like method 2  see note below      Interestingly  performing the same tests on a dict I have with 2700 entries  I get quite different results  this time run 10000 times    Method 1    gt  gt  gt  profile run  for i in range 0 10000   method1 UIC CRS  7088380          20004 function calls in 2 928 seconds   Method 2    gt  gt  gt  profile run  for i in range 0 10000   method2 UIC CRS  7088380          20004 function calls in 3 872 seconds   Method 3    gt  gt  gt  profile run  for i in range 0 10000   method3 UIC CRS  7088380          40004 function calls in 1 176 seconds   So here  method 3 is much faster  Just goes to show the size of your dict will affect which method you choose   Notes  Method 2 returns a list of all names  whereas methods 1 and 3 return only the first match  I have not considered memory usage  I m not sure if method 3 creates 2 extra lists  keys   and values    and stores them in memory

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we can get the Key of dict by    def getKey dct value        return  key for key in dct if  dct key     value

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Consider using Pandas  As stated in William McKinney s  Python for Data Analysis      Another way to think about a Series is as a fixed-length  ordered   dict  as it is a mapping of index values to data values  It can be   used in many contexts where you might use a dict    import pandas as pd list     george  16  amber  19  lookup list   pd Series list    To query your series do the following   lookup list lookup list values    19    Which yields   Out 1    amber    19 dtype  int64   If you need to do anything else with the output transforming the  answer into a list might be useful   answer   lookup list lookup list values    19  index answer   pd Index tolist answer

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Cat Plus Plus mentioned that this isn t how a dictionary is intended to be used  Here s why   The definition of a dictionary is analogous to that of a mapping in mathematics  In this case  a dict is a mapping of K  the set of keys  to V  the values  - but not vice versa  If you dereference a dict  you expect to get exactly one value returned  But  it is perfectly legal for different keys to map onto the same value  e g    d     k1   v1  k2   v2  k3   v1    When you look up a key by it s corresponding value  you re essentially inverting the dictionary  But a mapping isn t necessarily invertible  In this example  asking for the key corresponding to v1 could yield k1 or k3  Should you return both  Just the first one found  That s why indexof   is undefined for dictionaries   If you know your data  you could do this  But an API can t assume that an arbitrary dictionary is invertible  hence the lack of such an operation

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I tried to read as many solutions as I can to prevent giving duplicate answer  However  if you are working on a dictionary which values are contained in lists and if you want to get keys that have a particular element you could do this   d     Adams    18  29  30         Allen    9  27         Anderson    24  26         Bailey    7  30         Baker    31  7  10  19         Barnes    22  31  10  21         Bell    2  24  17  26     Now lets find names that have 24 in their values   for key in d keys            if 24 in d key           print key    This would work with multiple values as well

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dictionary     george    16   amber    19  search age   raw input  Provide age   key    filter  lambda x  dictionary x     k    dictionary    None   0     key   None from  None  which is a safeguard for not found    For multiple occurrences use   keys    filter  lambda x  dictionary x     k    dictionary

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one line version   i is an old dictionary  p is a reversed dictionary   explanation   i keys   and i values   returns two lists with keys and values of the dictionary respectively  The zip function has the ability to tie together lists to produce a dictionary   p   dict zip i values   i keys       Warning   This will work only if the values are hashable and unique

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Here  recover key takes dictionary and value to find in dictionary  We then loop over the keys in dictionary and make a comparison with that of value and return that particular key   def recover key dicty value       for a key in dicty keys            if  dicty a key     value               return a key

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key   next  k for k in my dict if my dict k     val   None

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a     a  1  b  2  c  3   v k for k  v in a items    1    or better   k v for k  v in a items   if v    1

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Heres a truly  quot Reversible Dictionary quot   Based upon Adam Acosta s solution  but enforcing val-to-key calls to be unique and easily return key from value  from collections import UserDict   class ReversibleDict UserDict       def   init   self  enforce unique True   args    kwargs           super     init    args    kwargs          self val to keys              self check val   self check unique if enforce unique else lambda x  x      def   setitem   self  key  value           self check val value          super     setitem   key  value          self val to keys value    key      def   call   self  value           return self val to keys value       def check unique self  value           assert value not in self val to keys  f quot Non unique value   value   quot          return value  If you want to enforce uniqueness on dictionary values ensure to set enforce unique True  to get keys from values just do rev dict value   to call values from keys just do as usual dict  key    here s an example of usage  rev dict   ReversibleDict enforce unique True  rev dict  quot a quot     1 rev dict  quot b quot     2 rev dict  quot c quot     3 print  quot full dictinoary is   quot   rev dict  print  quot value for key  b  is   quot   rev dict  quot b quot    print  quot key for value  2  is   quot   rev dict 2   print  quot tring to set another key with the same value results in error   quot   rev dict  quot d quot     1

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Sometimes int   may be needed   titleDic             1           2   def categoryTitleForNumber self  num       search title          for title  titleNum in self titleDic items            if int titleNum     int num               search title   title     return search title

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You can get key by using dict keys    dict values   and list index   methods  see code samples below   names dict     george  16  amber  19  search age   int raw input  Provide age    key   names dict keys   names dict values   index search age

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Here is a solution which works both in Python 2 and Python 3   dict  v  k  for k  v in list items    search age    The part until  search age  constructs the reverse dictionary  where values are keys and vice-versa   You could create a helper method which will cache this reversed dictionary like so   def find name age   rev lookup dict  v  k  for k  v in ages by name items          return  rev lookup age    or even more generally a factory which would create a by-age name lookup method for one or more of you lists  def create name finder ages by name       names by age   dict  v  k  for k  v in ages by name items        def find name age         return names by age age    so you would be able to do   find teen by age   create name finder   george  16  amber  19       find teen by age search age    Note that I renamed list to ages by name since the former is a predefined type

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Here is my take on this problem     I have just started learning Python  so I call this    The Understandable for beginners  solution    Code without comments   list1     george  16  amber  19   Garry  19  search age   raw input  Provide age     print search age   int search age   listByAge       for name  age in list1 items        if age    search age          age   str age          results   name        age         print results          age2   int age          listByAge name    listByAge get name 0  age2  print print listByAge       Code with comments   I ve added another name with the same age to the list  list1     george  16  amber  19   Garry  19   Original code  search age   raw input  Provide age     print  Because raw input gives a string  we need to convert it to int   so we can search the dictionary list with it  search age   int search age    Here we define another empty dictionary  to store the results in a more   permanent way  listByAge        We use double variable iteration  so we get both the name and age   on each run of the loop  for name  age in list1 items         Here we check if the User Defined age   the age parameter       for this run of the loop      if age    search age           Here we convert Age back to string  because we will concatenate it           with the person s name           age   str age           Here we concatenate          results   name        age          If you want just the names and ages displayed you can delete          the code after  print results   If you want them stored  don t            print results           Here we create a second variable that uses the value of          the age for the current person in the list           For example if  Anna  is  10   age2   10           integer value which we can use in addition          age2   int age           Here we use the method that checks or creates values in dictionaries           We create a new entry for each name that matches the User Defined Age          with default value of 0  and then we add the value from age2          listByAge name    listByAge get name 0  age2   Here we print the new dictionary with the users with User Defined Age  print print listByAge       Results Running    test py  Thu Jun 06 05 10 02 2013   Provide age  19  amber 19 Garry 19    amber   19   Garry   19   Execution Successful

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In my case the easiest way is to instantiate disctionary in your code then you can call keys from it like below  here is my class having dictionary   class Config   def local self       return            temp dir     tmp dirtest             devops    Mansur           To instantiate your dictionary  config    vars Config   local config   config local     Finally calling your dictionary keys  patched   local config get  devops

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already been answered  but since several people mentioned reversing the dictionary  here s how you do it in one line  assuming 1 1 mapping  and some various perf data   python 2 6   reversedict   dict   value  key  for key  value in mydict iteritems       2 7    reversedict    value key for key  value in mydict iteritems      if you think it s not 1 1  you can still create a reasonable reverse mapping with a couple lines   reversedict   defaultdict list   reversedict value  append key  for key  value in mydict iteritems      how slow is this  slower than a simple search  but not nearly as slow as you d think - on a  straight  100000 entry dictionary  a  fast  search  i e  looking for a value that should be early in the keys  was about 10x faster than reversing the entire dictionary  and a  slow  search  towards the end  about 4-5x faster  So after at most about 10 lookups  it s paid for itself    the second version  with lists per item  takes about 2 5x as long as the simple version   largedict   dict  x x  for x in range 100000      Should be slow  has to search 90000 entries before it finds it In  26    timeit largedict keys   largedict values   index 90000   100 loops  best of 3  4 81 ms per loop    Should be fast  has to only search 9 entries to find it   In  27    timeit largedict keys   largedict values   index 9   100 loops  best of 3  2 94 ms per loop    How about using iterkeys   instead of keys      These are faster  because you don t have to create the entire keys array    You DO have to create the entire values array - more on that later   In  31    timeit islice largedict iterkeys    largedict values   index 90000   100 loops  best of 3  3 38 ms per loop  In  32    timeit islice largedict iterkeys    largedict values   index 9   1000 loops  best of 3  1 48 ms per loop  In  24    timeit reversedict   dict   value  key  for key  value in largedict iteritems     10 loops  best of 3  22 9 ms per loop  In  23     timeit       reversedict   defaultdict list         reversedict value  append key  for key  value in largedict iteritems          10 loops  best of 3  53 6 ms per loop   Also had some interesting results with ifilter  Theoretically  ifilter should be faster  in that we can use itervalues   and possibly not have to create go through the entire values list  In practice  the results were    odd     In  72     timeit       myf   ifilter lambda x  x 1     90000  largedict iteritems          myf next   0        100 loops  best of 3  15 1 ms per loop  In  73     timeit       myf   ifilter lambda x  x 1     9  largedict iteritems          myf next   0        100000 loops  best of 3  2 36 us per loop   So  for small offsets  it was dramatically faster than any previous version  2 36  u S vs  a minimum of 1 48  m S for previous cases   However  for large offsets near the end of the list  it was dramatically slower  15 1ms vs  the same 1 48mS   The small savings at the low end is not worth the cost at the high end  imho

[python] Get key by value in dictionary

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