Make a dictionary with duplicate keys in Python
PythonDictionaryPython Problem Overview
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()
Python Solutions
Solution 1 - Python
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.
Solution 2 - Python
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]}
Solution 3 - Python
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]
Solution 4 - Python
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}
Solution 5 - Python
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.
Solution 6 - Python
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]}
.
Solution 7 - Python
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)
Solution 8 - Python
Dictionary does not support duplicate key, instead you can use defaultdict
Below is the example of how to use defaultdict in python3x to solve your problem
from collections import defaultdict
sdict = defaultdict(list)
keys_bucket = list()
data_list = [lines.split(",") for lines in contents.split("\n")]
for data in data_list:
key = data.pop(0)
detail = data
keys_bucket.append(key)
if key in keys_bucket:
sdict[key].append(detail)
else:
sdict[key] = detail
print("\n", dict(sdict))
Above code would produce output as follow:
{'EDF768': [[' Bill Meyer', ' 2456', ' Vet_Parking'], [' Jenny Meyer', ' 9987', ' Vet_Parking']], 'TY5678': [[' Jane Miller', ' 8987', ' AgHort_Parking'], [' Jo King', ' 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'], [' Mike Green', ' 3212', ' 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']]}
Solution 9 - Python
It's pertty old question but maybe my solution help someone.
by overriding __hash__
magic method, you can save same objects in dict.
Example:
from random import choices
class DictStr(str):
"""
This class behave exacly like str class but
can be duplicated in dict
"""
def __new__(cls, value='', custom_id='', id_length=64):
# If you want know why I use __new__ instead of __init__
# SEE: https://stackoverflow.com/a/2673863/9917276
obj = str.__new__(cls, value)
if custom_id:
obj.id = custom_id
else:
# Make a string with length of 64
choice_str = "abcdefghijklmopqrstuvwxyzABCDEFJHIJKLMNOPQRSTUVWXYZ1234567890"
obj.id = ''.join(choices(choice_str, k=id_length))
return obj
def __hash__(self) -> int:
return self.id.__hash__()
Now lets create a dict:
>>> a_1 = DictStr('a')
>>> a_2 = DictStr('a')
>>> a_3 = 'a'
>>> a_1
a
>>> a_2
a
>>> a_1 == a_2 == a_3
True
>>> d = dict()
>>> d[a_1] = 'some_data'
>>> d[a_2] = 'other'
>>> print(d)
{'a': 'some_data', 'a': 'other'}
NOTE: This solution can apply to any basic data structure like (int, float,...)
EXPLANATION :
We can use almost any object as key in dict
class (or mostly known as HashMap
or HashTable
in other languages) but there should be a way to distinguish between keys because dict have no idea about objects.
For this purpose objects that want to add to dictionary as key somehow have to provide a unique identifier number(I name it uniq_id, it's actually a number somehow created with hash algorithm) for themself.
Because dictionary structure widely use in most of solutions,
most of programming languages hide object uniq_id generation inside a hash
name buildin method that feed dict in key search
So if you manipulate hash
method of your class you can change behaviour of your class as dictionary key