Make a dictionary with duplicate keys in Python

Question

I have the following list which contains duplicate car registration numbers with different values  I want to convert it into a dictionary which accepts this multiple keys of car registration numbers   So far when I try to convert list to dictionary it eliminates one of the keys  How do I make a dictionary with duplicate keys   The list is   EDF768  Bill Meyer  2456  Vet Parking TY5678  Jane Miller  8987  AgHort Parking GEF123  Jill Black  3456  Creche Parking ABC234  Fred Greenside  2345  AgHort Parking GH7682  Clara Hill  7689  AgHort Parking JU9807  Jacky Blair  7867  Vet Parking KLOI98  Martha Miller  4563  Vet Parking ADF645  Cloe Freckle  6789  Vet Parking DF7800  Jacko Frizzle  4532  Creche Parking WER546  Olga Grey  9898  Creche Parking HUY768  Wilbur Matty  8912  Creche Parking EDF768  Jenny Meyer  9987  Vet Parking TY5678  Jo King  8987  AgHort Parking JU9807  Mike Green  3212  Vet Parking   The code I have tried is   data dict      data list       def createDictionaryModified filename     path    C  Users user Desktop    basename    ParkingData Part3 txt    filename   path          basename   file   open filename    contents   file read     print contents   n    data list    lines split      for lines in contents split   n      for line in data list      regNumber   line 0      name   line 1      phoneExtn   line 2      carpark   line 3  strip       details    name phoneExtn carpark      data dict regNumber    details   print data dict   n    print data dict items     n    print data dict values

User · Answer

I just posted an answer to a question that was subequently closed as a duplicate of this one (for good reasons I think), but I'm surprised to see that my proposed solution is not included in any of the answers here.

Rather than using a defaultdict or messing around with membership tests or manual exception handling, you can easily append values onto lists within a dictionary using the setdefault method:

results = {}                              # use a normal dictionary for our output
for k, v in some_data:                    # the keys may be duplicates
    results.setdefault(k, []).append(v)   # magic happens here!

This is a lot like using a defaultdict, but you don't need a special data type. When you call setdefault, it checks to see if the first argument (the key) is already in the dictionary. If doesn't find anything, it assigns the second argument (the default value, an empty list in this case) as a new value for the key. If the key does exist, nothing special is done (the default goes unused). In either case though, the value (whether old or new) gets returned, so we can unconditionally call append on it, knowing it should always be a list.

User · Answer

Python dictionaries don t support duplicate keys  One way around is to store lists or sets inside the dictionary   One easy way to achieve this is by using defaultdict   from collections import defaultdict  data dict   defaultdict list    All you have to do is replace  data dict regNumber    details   with  data dict regNumber  append details    and you ll get a dictionary of lists

User · Answer

You can t have a dict with duplicate keys for definition  Instead you can use a single key and  as value  a list of elements that had that key   So you can follow these steps    See if the current element s  of your initial set  key is in the final dict  If it is  go to step 3 Update dict with key Append the new value to the dict key  list  Repeat  1-3

User · Answer

You can refer to the following article  http   www wellho net mouth 3934 Multiple-identical-keys-in-a-Python-dict-yes-you-can- html  In a dict  if a key is an object  there are no duplicate problems   For example   class p object       def   init   self  name           self name   name     def   repr   self           return self name     def   str   self           return self name d    p  k    1  p  k    2

User · Answer

If you want to have lists only when they are necessary  and values in any other cases  then you can do this   class DictList dict       def   setitem   self  key  value           try                Assumes there is a list on the key             self key  append value          except KeyError    If it fails  because there is no key             super DictList  self    setitem   key  value          except AttributeError    If it fails because it is not a list             super DictList  self    setitem   key   self key   value     You can then do the following   dl   DictList   dl  a      1 dl  b      2 dl  b     3   Which will store the following   a   1   b    2  3       I tend to use this implementation when I want to have reverse inverse dictionaries  in which case I simply do   my dict    1   a   2   b   3   b   rev   DictList   for k  v in my dict items        rev med v    k   Which will generate the same output as above    a   1   b    2  3       CAVEAT  This implementation relies on the non-existence of the append method  in the values you are storing   This might produce unexpected results if the values you are storing are lists  For example   dl   DictList   dl  a      1 dl  b       2  dl  b     3   would produce the same result as before   a   1   b    2  3    but one might expected the following    a   1   b     2   3

User · Answer

You can change the behavior of the built in types in Python  For your case it s really easy to create a dict subclass that will store duplicated values in lists under the same key automatically   class Dictlist dict       def   setitem   self  key  value           try              self key          except KeyError              super Dictlist  self    setitem   key              self key  append value    Output example    gt  gt  gt  d   dictlist Dictlist    gt  gt  gt  d  test     1  gt  gt  gt  d  test     2  gt  gt  gt  d  test     3  gt  gt  gt  d   test    1  2  3    gt  gt  gt  d  other     100  gt  gt  gt  d   test    1  2  3    other    100

User · Answer

You can t have duplicated keys in a dictionary  Use a dict of lists   for line in data list    regNumber   line 0    name   line 1    phoneExtn   line 2    carpark   line 3  strip     details    name phoneExtn carpark    if not data dict has key regNumber       data dict regNumber     details    else      data dict regNumber  append details

[python] Make a dictionary with duplicate keys in Python

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