Exceptions signal contract violations cleanly

Beginner Defensive APIs: validation, sanitization & exceptions

Created by Pavel · 29.04.2026 at 19:10 UTC

Return codes like -1 or sentinel strings collide with legitimate data: a temperature of -1 might be real, and the string "none" might be a valid category label. Exceptions separate happy-path returns from failure signalling and carry stack context when uncaught.

In data pipelines, parse JSON, CSV, and query parameters at boundaries; raise ValueError or domain-specific errors when contracts break. Catch narrowly at integration points (try / except ValueError) instead of bare except: which also traps KeyboardInterrupt and SystemExit in Python 2 style habits.

Libraries like Pydantic turn validation failures into structured ValidationError objects—same idea: fail loudly with diagnostic information.

Errors and exceptions tutorial: [1].

Sources

[1]https://docs.python.org/3/tutorial/errors.html Return to text

University approvals: 0

Tasks

Question 1

Why is return -1 a poor universal error signal for parse_age(text) in public APIs?

Hint

Sentinel return values collide with real data.

-1 is never a valid integer

Callers cannot distinguish error from a legitimate negative or sentinel business value

Python forbids negative ints

Exceptions are slower than return codes in all cases

Question 2

What does this pattern risk?

try:
    load_dataset(path)
except:
    return None

Hint

Bare except is overbroad.

Nothing—it is best practice

It catches only IOError

It masks unrelated bugs (KeyboardInterrupt, AssertionError, etc.)

It guarantees transactional rollback

Question 3

parse_positive_int(raw: str) -> int: strip whitespace; require all digits after strip; value must be > 0. Else ValueError('bad'). No leading sign.

Hint

Reject empty, non-digit, and non-positive after parsing.

def parse_positive_int(raw: str) -> int:
    pass

Starter code is prefilled; replace TODO blocks with your solution.

Runtime output (stdout/stderr)

1 test case will be used for grading

Run checks runtime behavior only. Final correctness is evaluated when you submit.

Card Info

Topic: Defensive APIs: validation, sanitization & exceptions
Difficulty: Beginner
Completed: 0 users

Creator

Pavel