KeyError: accessing a non-existent dictionary key” in Python

The error message:

KeyError: 'some_key'

occurs when you try to access a key in a dictionary that does not exist.

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1. Causes and Solutions

Cause 1: Accessing a Missing Key

If the key does not exist in the dictionary, Python raises a KeyError.

Incorrect Code:

data = {"name": "Alice", "age": 25}
print(data["gender"])  # 'gender' key does not exist

Error Output:

KeyError: 'gender'

Solution 1: Use .get() to Avoid KeyError

print(data.get("gender", "Not specified"))  # Returns 'Not specified' if key is missing

Solution 2: Check if Key Exists Before Accessing

if "gender" in data:
    print(data["gender"])
else:
    print("Key not found!")

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Cause 2: Using the Wrong Key Name

Dictionary keys are case-sensitive and must be spelled correctly.

Incorrect Code:

data = {"Name": "Alice"}
print(data["name"])  # Wrong case

Error Output:

KeyError: 'name'

Solution: Use the Correct Key Name

print(data["Name"])  # Correct key

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Cause 3: Accessing a Key in a Nested Dictionary Without Checking

If a dictionary has nested dictionaries, accessing a missing key inside a sub-dictionary raises a KeyError.

Incorrect Code:

data = {"user": {"name": "Alice"}}
print(data["user"]["age"])  # 'age' key does not exist

Error Output:

KeyError: 'age'

Solution: Use .get() on Nested Dictionaries

print(data.get("user", {}).get("age", "Age not found"))  # Avoids error

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Cause 4: Using defaultdict to Avoid Missing Keys

If you frequently work with missing keys, use collections.defaultdict.

Solution: Use defaultdict to Return a Default Value

from collections import defaultdict
data = defaultdict(lambda: "Not found")
data["name"] = "Alice"
print(data["age"])  # Returns 'Not found' instead of raising KeyError