The error message:
KeyError: 'nonexistent_key'
occurs when you try to access a key in a dictionary that does not exist.
1. Causes and Solutions
Cause 1: Accessing a Missing Key in a Dictionary
When you try to retrieve a value using a key that is not present in the dictionary, Python raises a KeyError.
Incorrect Code:
data = {"name": "Alice", "age": 25}
print(data["city"]) # Error: 'city' key does not exist
Solution: Use .get() to return a default value instead of raising an error.
print(data.get("city", "Key not found")) # Output: "Key not found"
Cause 2: Using an Incorrect Key Data Type
Keys in a dictionary must be hashable (e.g., strings, numbers, tuples). If you use an incorrect type, Python raises a KeyError.
Incorrect Code:
data = {{1, 2, 3}: "Invalid key"} # Error: Sets are unhashable and cannot be dictionary keys
Solution: Use hashable data types (strings, integers, or tuples).
data = {(1, 2, 3): "Valid key"} # Tuples are hashable and can be used as keys
Cause 3: Dictionary Key Contains Unexpected Whitespace or Case Differences
If the key you try to access is slightly different due to extra spaces or case sensitivity, Python won’t find it.
Incorrect Code:
data = {"Name": "Alice"}
print(data["name"]) # Error: 'name' is different from 'Name'
Solution: Normalize keys using .lower().strip().
key = " name ".strip().lower()
data = {"name": "Alice"} # Convert keys to lowercase
print(data.get(key, "Key not found")) # Output: Alice
Cause 4: Using pop() with a Missing Key
If you use .pop() on a key that doesn’t exist, it raises a KeyError.
Incorrect Code:
data = {"name": "Alice"}
data.pop("age") # Error: 'age' key does not exist
Solution: Provide a default value to .pop().
data.pop("age", "Key not found") # Output: "Key not found"
Cause 5: Looping Through a Dictionary Without Checking for a Key
If you’re iterating through a dictionary and assuming a key always exists, Python may raise an error.
Incorrect Code:
data = [{"name": "Alice"}, {"age": 25}]
for item in data:
print(item["name"]) # Error: The second dictionary has no 'name' key
Solution: Check if the key exists before accessing it.
for item in data:
print(item.get("name", "Key not found"))