KeyError: conflicting key assignments in dictionary

A KeyError: conflicting key assignments in dictionary occurs when attempting to define or update a dictionary with duplicate or conflicting keys. In Python, dictionary keys must be unique, and if a duplicate key is added, it will overwrite the previous value without raising an error. However, conflicts may arise due to incorrect key usage, leading to unexpected results.

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1. Common Causes and Fixes

Cause 1: Using the Same Key Multiple Times in Dictionary Initialization

If a dictionary is defined with duplicate keys, Python only keeps the last occurrence.

Incorrect Code:

data = {"name": "Alice", "age": 25, "age": 30}  # Duplicate key
print(data["age"])  # Output: 30

Solution: Ensure that each key is unique.

data = {"name": "Alice", "age": 25, "dob": "1998-05-10"}  # No duplicate keys

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Cause 2: Using Mutable Objects as Dictionary Keys

Dictionary keys must be immutable (e.g., strings, numbers, tuples). If you try to use a list or another mutable object as a key, Python will raise a TypeError.

Incorrect Code:

data = {["name", "Alice"]: 25}  # Error: Lists are mutable

Solution: Use immutable types like tuples instead.

data = {("name", "Alice"): 25}  # Tuples are valid keys

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Cause 3: Key Conflicts in Dictionary Comprehensions

If a dictionary comprehension generates duplicate keys, only the last value is retained, leading to unexpected behavior.

Incorrect Code:

data = {x % 2: x for x in range(5)}
print(data)  # Output: {0: 4, 1: 3} (overwrites previous values)

Solution: Ensure keys are unique by adjusting the logic.

data = {x: x**2 for x in range(5)}  # Unique keys
print(data)  # Output: {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

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Cause 4: Conflicting Keys When Merging Dictionaries

If merging two dictionaries contains conflicting keys, the later dictionary overwrites the earlier values.

Incorrect Code:

dict1 = {"a": 1, "b": 2}
dict2 = {"b": 3, "c": 4}
merged = {**dict1, **dict2}  # 'b' key gets overwritten
print(merged)  # Output: {'a': 1, 'b': 3, 'c': 4}

Solution: Use collections.ChainMap to keep both values.

from collections import ChainMap

dict1 = {"a": 1, "b": 2}
dict2 = {"b": 3, "c": 4}
merged = ChainMap(dict2, dict1)  # Keeps older values when possible
print(merged["b"])  # Output: 3 (latest value)

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Cause 5: Accidentally Overwriting Dictionary Keys in a Loop

When looping and assigning dictionary keys dynamically, it’s possible to overwrite previous entries.

Incorrect Code:

data = {}
for i in range(5):
    key = i % 2  # Generates only 0 or 1 as keys
    data[key] = i

print(data)  # Output: {0: 4, 1: 3} (earlier values overwritten)

Solution: Use a list or dictionary with lists as values.

data = {}
for i in range(5):
    key = i % 2
    data.setdefault(key, []).append(i)

print(data)  # Output: {0: [0, 2, 4], 1: [1, 3]}

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