You’ve likely encountered errors in your programming journey such as IndexError, KeyError and ValueError.
Knowing how to handle these properly in your code so that these errors don’t show up in production is essential to being able to write clean, efficient code.
To handle errors effectively, you’ll leverage the try/except keywords.
An Example of Error Handling
Take this snippet of what seems to be clean code:
def fetch_user_score(data, user_id):
return int(data[user_id]["score"])It looks okay because you’re assuming that user_id is in the dictionary and that score is in there, and casting it to an integer. But what if score isn’t in the dictionary?
We can handle it using try/except:
def fetch_user_score(data: dict, user_id: int):
try:
return int(data[user_id]["score"])
except:
return f"Score not found for user ID: {user_id}"While this is acceptable for handling errors, one major fundamental (and non-best practice) issue is that this handles every single error that can be thrown.
» For more information on this topic, check out The Dangers of Overgeneralizing Exceptions.
Instead, we’re going to want to try and identify what errors can be raised and then identify how we want to handle it.
Looking at the code, there’s 4 errors that can be raised:
TypeError: score is either None or a list.
ValueError: score is “NA” or “bad”.
2 KeyError: user_id is missing or score is missing.
To fix this, we’ll modify our try/except statement appropriately:
def fetch_user_score(data, user_id):
try:
return int(data[user_id]["score"])
except KeyError:
return 0 # Default score if missing
except (TypeError, ValueError):
return -1 # Invalid score formatFrom here, we can insert logging statements and other logic to be able to handle the cases where these errors can be thrown.
Best Practices to Handle Errors
Catch only what you can handle. Don’t catch exceptions unless you can meaningfully recover from them.
Be specific. Use the most specific exception type possible.
Avoid bare try/except statements. These catch everything, including KeyboardInterrupt
Don’t silence errors. If something must be silenced, leave at least a comment explaining why.
Commonly used Exceptions
There’s an overwhelmingly high amount of exceptions that Python can throw. To help alleviate this, I’ve included a table of the most commonly used exceptions that Python gives you:
Exception | When to use it |
|---|---|
AttributeError | Calling a method or attribute that doesn’t exist on an object. |
FileNotFoundError | Missing file in open(), os, or path-related operations. |
ImportError | Base exception for failed imports: dynamic imports, optional deps, etc. |
IndexError | Accessing a list or tuple index that doesn’t exist. |
KeyError | Dict key lookup failed (e.g., my_dict["missing_key"]) |
OSError | OS-level failure; disk full, permission denied, file busy, etc. |
RuntimeError | Generic error; use when no other specific exception fits. |
TypeError | Invalid operation between mismatched types (e.g., str + int). |
ValueError | Right type, wrong content (e.g., int("abc"), math domain errors) |
Happy coding!
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