How Do I Handle Duplicate Keys When Creating a Python Dictionary?

Handling Duplicate Keys in Python Dictionaries

In Python, dictionaries are an essential data structure for mapping keys to values. However, dictionaries do not inherently support duplicate keys, which can be an issue when working with certain types of data.

Problem:

Suppose you have a text file containing duplicate car registration numbers with varying values, as shown below:

EDF768, Bill Meyer, 2456, Vet_Parking
TY5678, Jane Miller, 8987, AgHort_Parking
GEF123, Jill Black, 3456, Creche_Parking
ABC234, Fred Greenside, 2345, AgHort_Parking
...
EDF768, Jenny Meyer, 9987, Vet_Parking
TY5678, Jo King, 8987, AgHort_Parking

You want to create a dictionary where the registration numbers are keys and the data on each line is the value. However, using the standard dictionary syntax will overwrite the old value with the new one when duplicate keys are encountered.

Solution:

To handle duplicate keys, one common solution is to use a defaultdict from the collections module:

from collections import defaultdict

data_dict = defaultdict(list)

Instead of assigning a single value to the duplicate key, you append the new value to a list associated with that key:

data_dict[regNumber].append(details)

This will create a dictionary where each key corresponds to a list of values. For example:

data_dict = {
    'EDF768': [
        ('Bill Meyer', '2456', 'Vet_Parking'),
        ('Jenny Meyer', '9987', 'Vet_Parking'),
    ],
    'TY5678': [
        ('Jane Miller', '8987', 'AgHort_Parking'),
        ('Jo King', '8987', 'AgHort_Parking'),
    ],
    ...
}

This way, you can access all the data associated with a given registration number by accessing the corresponding list in the dictionary.

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