lesson 86 Exceptions & Validation · intermediate

Catch one specific error

`int("twelve")` stops a program with ValueError. `try` and `except ValueError` let you decide what a bad value means, without hiding any other mistake.

You'll be able to

Put only the line that can fail inside try, and handle its error in except ValueError:
Explain why naming the error type matters: other errors still stop the program
Parse a list of texts, keep the good values and count the rejected ones

The idea

A CSV of invoice lines delivers every value as text, and people type what they like: "12", " 6 ", "twelve", "". int("12") works. int("twelve") can't, so Python raises a ValueError, and an error nobody handles stops the program.

Run each example to see what it prints. Running examples doesn't count toward progress.

example-1.py · One bad quantity stops everything

quantities = ["12", "6", "twelve", "24"]
for text in quantities:
    print(int(text))

try: marks lines that might fail. If one of them raises a ValueError, Python jumps straight to the matching except ValueError: block and then carries on after it; if nothing fails, the except block is skipped.

Always name the error. except ValueError: handles exactly the problem you expect (text that isn't a number). Every other error, such as a typo that raises NameError, still stops the program and shows you where. A bare except: would swallow those too, and the bug would hide behind your fallback value.

example-2.py · Skip what can't be read

quantities = ["12", " 6 ", "twelve", "", "24"]
total = 0
for text in quantities:
    try:
        total += int(text)
    except ValueError:
        print("skipped", repr(text))
print("total:", total)

Inside a function, the except block decides what a bad value means: here, None for "no usable price". The specific type also protects you: float(None) raises TypeError, not ValueError, so a missing value is not silently treated like a typo.

example-3.py · A price, or None

def to_price(text):
    try:
        return float(text)
    except ValueError:
        return None

for raw in ["6.95", "1,234.50", "  2.55", "N/A"]:
    print(repr(raw), "->", to_price(raw))

When to use it: when a failure is expected and you know what to do about it (skip the value, count it, use a default). Mistakes to avoid: wrapping many lines in one try, so you can't tell which one failed; a bare except:, which also hides real bugs; and catching an error you can't do anything useful about.

Review

try: runs code that may fail; except ValueError: handles that one error and carries on.
Name the error. Other errors still stop the program, which is how you find real bugs.
Keep try blocks small: one risky operation, one decision about what failure means.
Next: read the error itself, and handle different errors in different ways.

Practise this