How to Add Duplicate Keys in Dictionary - Python

In Python, dictionaries are used to store key-value pairs. However, dictionaries do not support duplicate keys. In this article, we will explore several techniques to store multiple values for a single dictionary key.

Understanding Dictionary Key Constraints

In Python, dictionary keys must be unique. This means that if we try to add a key that already exists in the dictionary, the new value will overwrite the existing value. This behavior ensures that each key maps to one and only one value.

d = {'name': 'Alice', 'age': 25}

# value 25 will be overwritten by 30
d['age'] = 30
print(d)

{'name': 'Alice', 'age': 30}

Workarounds for Duplicate Keys

Since dictionaries don’t allow duplicate keys, we use alternative methods to store multiple values under one key. Below are some common approaches.

Using list as the Dictionary Value

Instead of overwriting the value associated with a key, we can associate each key with a list. Whenever a new value needs to be added for the key, we append it to the list. This approach allows us to efficiently manage multiple values for the same key.

d = {}

# Add values to the dictionary
d.setdefault('Platform', []).append('GeeksForGeeks')
d.setdefault('Courses', []).extend(['MERN', 'DSA'])

print(d)

{'Platform': ['GeeksForGeeks'], 'Courses': ['MERN', 'DSA']}

Explanation:

• setdefault('Platform', []) ensures 'Platform' has a list as its value then append('GeeksForGeeks') adds the value 'GeeksForGeeks'.
• setdefault('Courses', []) ensures 'Courses' has a list as its value then extend(['MERN', 'DSA']) adds 'MERN' and 'DSA' to the list.

Using dictionary of lists

In this method, we initialize each key in the dictionary with an empty list if it doesn’t already exist . We can then add new values to these lists as needed. This requires explicit checks to verify whether a key exists before appending values.

d = {}

# Add values to the dictionary
if 'Platform' not in d:
    d['Platform'] = []
d['Platform'].append('GeeksForGeeks')

if 'Courses' not in d:
    d['Courses'] = []
d['Courses'].extend(['MERN', 'DSA'])

print(d)

{'Platform': ['GeeksForGeeks'], 'Courses': ['MERN', 'DSA']}

Explanation:

• if 'Platform' not in d checks if 'Platform' exists in d. If not, initializes it as an empty list: d['Platform'] = [].
• d['Platform'].append('GeeksForGeeks') adds 'GeeksForGeeks' to the 'Platform' list.
• if 'Courses' not in d checks if 'Courses' exists in d. If not, initializes it as an empty list: d['Courses'] = [].
• d['Courses'].extend(['MERN', 'DSA']) adds 'MERN' and 'DSA' to the 'Courses' list using extend().

Using defaultdict

defaultdict is a special type of dictionary that simplifies the handling of keys that need default values. It automatically initializes a key with a default value like an empty list if it doesn’t exist, removing the need for manual checks.

from collections import defaultdict

# Initialize the defaultdict
d = defaultdict(list)

# Add values to the dictionary
d['Platform'].append('GeeksForGeeks')
d['Courses'].extend(['MERN', 'DSA'])

print(dict(d))

{'Platform': ['GeeksForGeeks'], 'Courses': ['MERN', 'DSA']}

Explanation:

• d['Platform'].append('GeeksForGeeks') adds 'GeeksForGeeks' to the 'Platform' list. If the key doesn't exist, it is created with an empty list.
• d['Courses'].extend(['MERN', 'DSA']) adds 'MERN' and 'DSA' to the 'Courses' list. If the key doesn't exist, it is created with an empty list.

Using list of dictionaries

If we require true duplicate keys, storing each key-value pair as a separate dictionary inside a list allows multiple entries for the same key. Each dictionary represents one key-value pair, and multiple such dictionaries can coexist in the list.

d = []

# Add values as dictionaries
d.append({'Platform': 'GeeksForGeeks'})
d.append({'Courses': 'MERN'})
d.append({'Courses': 'DSA'})

print(d)

[{'Platform': 'GeeksForGeeks'}, {'Courses': 'MERN'}, {'Courses': 'DSA'}]

Explanation:

• d.append({'Platform': 'GeeksForGeeks'}) adds {'Platform': 'GeeksForGeeks'} to the list.
• d.append({'Courses': 'MERN'}) adds {'Courses': 'MERN'} to the list.
• d.append({'Courses': 'DSA'}) adds {'Courses': 'DSA'} to the list.