Ask a local coding model for a change you can test

Reproduce the bug

The library uses 24-hour times for reservations. Its helper currently accepts inputs such as 24:00 and 9:05. The contract is exactly two ASCII digits, a colon, and two ASCII digits. Hours must be 00 through 23 and minutes 00 through 59.

Save both files in the same folder. The tests use Python’s standard library.

def minutes_since_midnight(value: str) -> int:
    hours, minutes = map(int, value.split(":"))
    return hours * 60 + minutes

import unittest

from library_slots import minutes_since_midnight


class LibrarySlotTests(unittest.TestCase):
    def test_valid_times(self):
        for value, expected in (("00:00", 0), ("09:05", 545), ("23:59", 1439)):
            with self.subTest(value=value):
                self.assertEqual(minutes_since_midnight(value), expected)

    def test_rejects_bad_shape(self):
        for value in ("9:05", "09:5", " 09:05", "09:05\n", "０９:０５", "09-05"):
            with self.subTest(value=value):
                with self.assertRaises(ValueError):
                    minutes_since_midnight(value)

    def test_rejects_out_of_range_values(self):
        for value in ("24:00", "12:60", "-1:15"):
            with self.subTest(value=value):
                with self.assertRaises(ValueError):
                    minutes_since_midnight(value)

python -m unittest -v

Give the model a contract

Open a chat with the 1.5B coding model in Ollama. The first run downloads about 1 GB. Send the prompt as one message. It includes the full function and the required behavior.

ollama run qwen2.5-coder:1.5b

Fix this Python function using only the standard library.
Accept exactly HH:MM using ASCII digits, with hours 00–23 and minutes 00–59.
Return the number of minutes since midnight. Raise ValueError for any
invalid string, including extra whitespace. Keep the function signature.
Return only the updated Python code, including any import it needs.

def minutes_since_midnight(value: str) -> int:
    hours, minutes = map(int, value.split(":"))
    return hours * 60 + minutes

Check the proposed change

Read the proposal and apply the replacement to library_slots.py if it fits the contract. Run the same tests. When a case fails, give the model that input, the expected behavior, and the current function. Re-run the complete suite after each revision.

python -m unittest -v

In one run, the model added numeric range checks but still accepted 9:05. It also caught its own error and returned None. The tests caught both issues.

import re


def minutes_since_midnight(value: str) -> int:
    if re.fullmatch(r"[0-9]{2}:[0-9]{2}", value) is None:
        raise ValueError("Use HH:MM.")
    hours, minutes = map(int, value.split(":"))
    if hours >= 24 or minutes >= 60:
        raise ValueError("Use a 24-hour time.")
    return hours * 60 + minutes