In Python, dictionaries are used to store key-value pairs. However, dictionaries do not support duplicate keys. In this article, we will see how to store multiple values for a single key using two simple approaches.

Understanding Dictionary Key Constraints

In Python, dictionary keys must be unique. This means that if you 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.

For Example:

my_dict = {'name': 'Alice', 'age': 25}
print(my_dict)

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

Output:

{'name': 'Alice', 'age': 25}
{'name': 'Alice', 'age': 30}

Workarounds for Duplicate Keys

Since dictionaries do not allow duplicate keys, you need to use alternative methods to achieve similar functionality. Here are a few common workarounds:

Use a List for Values

In this approach each key in the dictionary maps to a list of values. When we want to add a duplicate key, we append the new value to the existing list.

# Initialize the dictionary
duplicate_dict = {}

# Function to add a key-value pair
def add_to_dict(key, value):
    if key in duplicate_dict:
        duplicate_dict[key].append(value)
    else:
        duplicate_dict[key] = [value]

# Add values to the dictionary
add_to_dict('Platform', 'GeeksForGeeks')
add_to_dict('Courses', 'MERN')
add_to_dict('Courses', 'DSA')

print(duplicate_dict)

Output:

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

Use a Dictionary of Lists

In this approach each key in the main dictionary maps to another dictionary. The inner dictionary uses values as keys and the count of occurrences as values.

# Initialize the dictionary
multi_value_dict = {}

# Function to add a key-value pair
def add_to_dict(key, value):
    if key not in multi_value_dict:
        multi_value_dict[key] = {}
    if value in multi_value_dict[key]:
        multi_value_dict[key][value] += 1
    else:
        multi_value_dict[key][value] = 1

# Add values to the dictionary
add_to_dict('Platform', 'GeeksForGeeks')
add_to_dict('Courses', 'MERN')
add_to_dict('Courses', 'DSA')
add_to_dict('Courses', 'DSA')


# Print the dictionary
print(multi_value_dict)

Output:

{'Platform': {'GeeksForGeeks': 1}, 'Courses': {'MERN': 1, 'DSA': 2}}

Conclusion

While Python dictionaries do not support duplicate keys directly, there are several effective workarounds to store multiple values under a single key. By using lists, nested dictionaries, or defaultdict, you can achieve the desired functionality and efficiently manage your data.

FAQs

Q: Can I use sets instead of lists for storing values?

Yes, use sets if you don’t care about the order and don’t want duplicate values.

Q: How can I get all values for a key?

In Approach 1, access the list for the key. In Approach 2, iterate over the inner dictionary to get values and their counts.

Q: What if I need to remove a specific value for a key?

In Approach 1, remove the value from the list. In Approach 2, decrease the count or remove the entry from the inner dictionary.