`KeyError`

In Python, a dictionary can cause a KeyError to occur. This happens due to incorrect use of the dictionary—like when we access a nonexistent key.

It is possible to prevent the KeyError in most cases by using the get() method on the dictionary. A default value returned, when the key does not exist, can also help.

Error example

This program causes a KeyError to be thrown. The dictionary contains 2 entries—these have the keys "cat" and "book." We try to access a key "python" but it does not exist.

# Get started.
values = {"cat" : 1, "book" : 2}

# There is no python in the dictionary.
print(values["python"])Traceback (most recent call last):
  File "C:\programs\program.py", line 8, in <module>
    print(values["python"])
KeyError: 'python'

Fixed example

A KeyError can be avoided. Usually, calling get() on the dictionary is the best solution, as this is a standard approach to using dictionaries in Python.

Tip We fix the problem by using a safe method, such as get(), instead of directly accessing the key.

Tip 2 If any direct accesses occur in your program, using a try-except block may be worthwhile if your code is new or untested.

Here We trap the KeyError in a try-except construct. We print an error message in the except-block.

Finally After handling exceptions, we access key "d" with the get() method. This is safe—no exception is raised.

# Create dictionary with three entries.
values = {"a" : 1, "b" : 2, "c" : 3}

# Using the value directly can cause an error.
try:
    print(values["d"])
except KeyError:
    print("KeyError encountered")

# We use get to safely get a value.
print(values.get("d"))KeyError encountered
None

Summary

The KeyError is an avoidable exception in Python. It occurs when a dictionary is incorrectly used. We saw two ways to prevent this error.

Some fixes

We used a try-except statement. And we replaced the value access with a get() method call. In most programs, calling get() is the best option.