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() 

Answer 1

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.

Answer 2

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:

>>> d = dictlist.Dictlist() >>> d['test'] = 1 >>> d['test'] = 2 >>> d['test'] = 3 >>> d {'test': [1, 2, 3]} >>> d['other'] = 100 >>> d {'test': [1, 2, 3], 'other': [100]}