try / except / finally & raise

15 min 30 XP

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Lesson 2 of 5 · View course roadmap

Step 1

Learn the idea

Exception handling lets your program survive failures instead of crashing:

try:: the risky code
except ValueError:: runs only if that error occurred (always catch specific exceptions)
else:: runs if no error happened
finally:: always runs (cleanup: closing files, connections)
raise ValueError("message"): throw your own errors when input is invalid

Never write a bare except: that silently swallows everything: it hides real bugs. Catch what you expect, let the rest crash loudly.

Where you'll use this

Production services wrap every network call, file read and JSON parse in try/except: a payment API that crashes on one malformed record instead of logging and continuing loses real money.

Common mistakes

Bare except: swallowing everything including typo-NameErrors and Ctrl-C: always name the exception you expect.
Wrapping 50 lines in one try: narrow the try block to the single risky call so you know what failed.
Using exceptions for normal flow control when an if would do: check if key in d rather than catching KeyError you expect half the time.

Pro tip

except ValueError as e: print(f"bad input: {e}"): binding the exception gives you its message for logs. Multiple types in one clause: except (ValueError, KeyError):.

Step 2

Try it yourself

Blank · autosaved

The lesson example is loaded and ready: press Run, then change something and run it again. Breaking it is part of learning. Want a clean slate? Tap “New blank”.

PYexample.py

⌘ + Enter to run

Output appears here…

Step 3

Pass the challenge +30 XP

Blank · autosaved

Write to_number(text) that returns int(text), but returns -1 if conversion raises ValueError. Print to_number("42") and to_number("oops").

Target output

42
-1

PYchallenge.py

def to_number(text):
    try:
        return int(text)
    except ValueError:
        return -1

print(to_number("42"))
print(to_number("oops"))

Run your code to check it…

Step 4

Check your understanding

1. When does finally run? Only on success Only on error Always Never

2. Why is a bare except: bad practice? Slow It catches and hides every error, including bugs and typos Syntax error It isn't

Last step

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