How to Fix KeyError Exceptions in Python

The Python KeyError is an exception that occurs when an attempt is made to access an item in a dictionary that does not exist. The key used to access the item is not found in the dictionary, which leads to the KeyError.

What Causes KeyError

The Python KeyError is raised when a mapping key is not found in the set of existing keys of the mapping. In Python, the most common mapping is the dictionary. When an item in a dictionary is accessed using a key, and the key is not found within the set of keys of the dictionary, the KeyError is raised.

Python KeyError Example

Here’s an example of a Python KeyError raised when trying to access a dictionary item that does not exist:

employees = {1: "John", 2: "Darren", 3: "Paul"}
print(employees[4])

In the above example, the dictionary employees contains some key-value pairs. An attempt is then made to access an item from employees using the key 4. Since 4 does not exist in the set of keys of the dictionary, a KeyError is raised:

File "test.py", line 2, in <module>
    print(employees[4])
KeyError: 4

How to Fix KeyError in Python

To avoid the KeyError in Python, keys in a dictionary should be checked before using them to retrieve items. This will help ensure that the key exists in the dictionary and is only used if it does, thereby avoiding the KeyError. This can be done using the in keyword.

Another solution is to use the .get() function to either retrieve the item using the specified key, or None if it doesn’t exist.

Using the above approach, a check for the key can be added to the earlier example:

employees = {1: "John", 2: "Darren", 3: "Paul"}

if 4 in employees:
    print(employees[4])
else:
    print('Key not found')

Here, a check is performed to ensure that the key 4 exists in the employees dictionary before it is used to retrieve a value. If it does not exist, a message is displayed and the error is avoided:

 Key not found

Track, Analyze and Manage Errors With Rollbar

Managing errors and exceptions in your code is challenging. It can make deploying production code an unnerving experience. Being able to track, analyze, and manage errors in real-time can help you to proceed with more confidence. Rollbar automates error monitoring and triaging, making fixing Python errors easier than ever. Try it today!