How Can Python's `defaultdict` Handle Duplicate Keys in Dictionaries?

Overcoming Duplicate Keys in Python Dictionaries

In Python, dictionaries are data structures that map unique keys to associated values. However, what happens when you encounter duplicate keys in your data?

Consider a text file with duplicate car registration numbers and related information, like:

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

You might want to create a dictionary with the registration numbers as keys and the data as values. However, simply using the dict[key] = value assignment overwrites existing values associated with duplicate keys.

The Solution: defaultdict

Python's defaultdict from the collections module allows you to overcome this limitation. It is a subclass of dict that provides default values for unspecified keys.

To use it, simply replace:

data_dict = {}

with:

from collections import defaultdict
data_dict = defaultdict(list)

Now, instead of assigning values directly, use the append() method:

data_dict[regNumber].append(details)

This will create a list of values associated with each duplicate key, effectively storing all the data related to those keys.

Example:

The following code uses defaultdict to create a dictionary from the sample data:

from collections import defaultdict

data_dict = defaultdict(list)

for line in data_list:
    regNumber = line[0]
    # ... (rest of the code unchanged)
    data_dict[regNumber].append(details)

This will result in a dictionary where each registration number (key) maps to a list of associated data (value).

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