maths/largest_of_very_large_numbers.py: res() missing documented ValueError for negative input

[thejesh23]

Repository commit

c0db072 (master)

Python version (python --version)

Python 3.12.13

Dependencies version (pip freeze)

Standard library only (math).

Expected behavior

The docstring of maths.largest_of_very_large_numbers.res documents the following doctest for a negative x:

>>> res(-1, 5)
Traceback (most recent call last):
...
ValueError: expected a positive input

So res(-1, 5) should raise ValueError with the message expected a positive input, and this doctest should pass under pytest --doctest-modules.

Actual behavior

There is no input validation in res(). When x < 0 and y != 0 the function reaches math.log10(x), which raises ValueError: math domain error (wrong message) instead of the documented expected a positive input. The doctest as written therefore fails:

$ python3 -m doctest maths/largest_of_very_large_numbers.py -v
...
Failed example:
    res(-1, 5)
Expected:
    Traceback (most recent call last):
    ...
    ValueError: expected a positive input
Got:
    Traceback (most recent call last):
        ...
        return y * math.log10(x)
                   ^^^^^^^^^^^^^
    ValueError: math domain error
1 items had failures:
   1 of   4 in maths.largest_of_very_large_numbers.res
***Test Failed*** 1 failures.

Suggested fix

Add the missing check at the top of res():

if x < 0:
    raise ValueError("expected a positive input")

This makes the code match the documented contract, and turns the currently-failing doctest into a passing one. A PR is being opened.