How do I catch a KeyError in Python?

Product Support
1

How do I catch a KeyError in Python?

When I run this code:

connection = manager.connect("I2Cx")

The program crashes and reports a KeyError because "I2Cx" doesn’t exist (it should be "I2C").

But when I do this:

try:
    connection = manager.connect("I2Cx")
except Exception as e:
    print(e)

It doesn’t print anything for e. I would like to be able to print the exception that was thrown. If I try the same thing with a divide by zero operation, it is caught and reported properly in both cases. What am I missing here?

2

If you’re encountering a KeyError with no message, you might want to ensure it actually prints something useful. A simple way to do that is by using repr() to get a full representation of the exception:

try:
    connection = manager.connect("I2Cx")
except Exception as e:
    print(repr(e))

This will give you the exception class name along with the actual error message, which is super helpful for debugging.

3

That’s a good start, but catching exceptions in a more structured way is even better. Instead of a generic except Exception, handling specific exceptions like KeyError and IndexError makes your code more readable and avoids masking unexpected issues:

try:
    connection = manager.connect("I2Cx")
except KeyError as e:
    print(f'I got a KeyError - reason "{str(e)}"')
except IndexError as e:
    print(f'I got an IndexError - reason "{str(e)}"')

This way, you’re explicitly handling different errors separately, which makes debugging easier and keeps your error handling clean.

4

Right, and if you need to catch all exceptions for logging or cleanup, it’s still a good practice to re-raise unexpected ones. Otherwise, you might accidentally swallow something critical like a KeyboardInterrupt. Here’s how you can do it properly:

try:
    connection = manager.connect("I2Cx")
except KeyError as e:
    print(f'I got a KeyError - reason "{str(e)}"')
except Exception as e:
    print(f'I got another exception: {repr(e)}. Re-raising it!')
    raise

This ensures that while you handle known errors, unexpected ones don’t get silently ignored, which can save you from nasty debugging surprises later on.