Error handling

What you'll learn

How to catch errors with try / except, run cleanup with finally, raise your own errors and create custom exception classes.

Why it matters

Bad input, missing files and network drops are normal. A program that handles them can recover or explain the problem, instead of printing a long traceback and stopping.

Explanation

When something goes wrong, Python raises an exception. If nothing handles it, the program stops with a traceback. You handle it with:

• except runs only if that kind of exception happened. Catch specific exceptions, not everything.
• else runs only when no exception occurred.
• finally always runs, which is perfect for cleanup.
• Get the message with except ValueError as e: and str(e).
• Raise your own with raise ValueError("message").
• Common exceptions: ValueError, TypeError, KeyError, IndexError, ZeroDivisionError, FileNotFoundError.
• Make custom ones by subclassing Exception.

Example

Step-by-step walkthrough

1. NegativeAgeError is a custom exception, a class that inherits from Exception.
2. parse_age("30") works, so else runs: 30 -> ok 30. Then finally prints checked 30.
3. int("abc") raises ValueError, which prints abc -> not a number.
4. -5 converts fine but we raise NegativeAgeError, caught by the second except: -5 -> Age cannot be negative.
5. finally runs every time.
6. Dividing by zero raises ZeroDivisionError. The message is division by zero.

Try it yourself

Add a rule that ages over 150 raise a ValueError("too old") and test it with "200".

try / exceptPython

Editable codedef parse_age(text): age = int(text) if age < 0: raise ValueError("Age cannot be negative") return age for value in ["30", "abc", "-5"]: try: print(value, "->", parse_age(value)) except ValueError as e: print(value, "-> error:", e)

Run ResetCopyCtrl+Enter to run

Common mistakes

• A bare except: that hides every problem, even typos and Ctrl+C.
• Catching too much code inside try. Keep it small.
• Swallowing errors with pass. Log or handle them.
• Raising strings. Always raise exception objects.

Quick quiz

Summary

• try / except / else / finally handle exceptions; raise creates them.
• Catch specific exceptions and keep try blocks small.
• Subclass Exception for custom errors.

Next: splitting code into modules.

try:
    risky()
except ValueError:
    print("bad value")
else:
    print("no problem")
finally:
    print("always runs")

class NegativeAgeError(Exception):
    pass


def parse_age(text):
    age = int(text)
    if age < 0:
        raise NegativeAgeError("Age cannot be negative")
    return age


for value in ["30", "abc", "-5"]:
    try:
        age = parse_age(value)
    except ValueError:
        print(value, "-> not a number")
    except NegativeAgeError as e:
        print(value, "->", e)
    else:
        print(value, "-> ok", age)
    finally:
        print("checked", value)

try:
    print(10 / 0)
except ZeroDivisionError as e:
    print("Error:", e)