KeyError

KeyError is raised when you try to access a dictionary key that doesn't exist. This happens with bracket notation (dict[key]) but not with the .get() method.

Common causes

• Accessing a dictionary with a key that doesn't exist
• Key has different type than expected (e.g., int vs string)
• Key was removed or was never added
• Case sensitivity in string keys
• Using a variable that holds a wrong key value

How to fix it

• Use dict.get(key, default) to provide a fallback value
• Check key existence with "if key in dict" before accessing
• Use collections.defaultdict for automatic defaults
• Use try/except KeyError for explicit handling
• Use dict.setdefault(key, default) for get-or-set patterns

Example

# Error example
user = {"name": "Alice", "email": "alice@example.com"}
print(user["age"])
# KeyError: 'age'

# Fix with .get()
print(user.get("age", 0))  # 0 (no error)

# Fix with check
if "age" in user:
    print(user["age"])

# Fix with try/except
try:
    print(user["age"])
except KeyError:
    print("Age not set")

# Fix with defaultdict
from collections import defaultdict
counts = defaultdict(int)
counts["missing"] += 1  # Works, defaults to 0

Related errors