Python doctests: test for None

Question

Using Python 2.7 I'm trying to test that the result of a particular function call is None

I would expect these tests to pass (excuse the rather silly example)

def six_or_none(val):
    """
    >>> six_or_none(6)
    6
    >>> six_or_none(4)
    None
    """
    if val == 6:
        return 6
    return None

However they yield the following result

Failed example:
    six_or_none(4)
Expected:
    None
Got nothing

What's the correct way to test for None in doctests?

Lobell

Answer 1

The Python interpreter ignores None return values, so doctests do the same.

Test for is None instead:

>>> six_or_none(4) is None
True

Individually

Martijn, do you know the motivations for ignoring None? – Rudyrudyard

@EdgarAroutiounian: None is the default "I have to return something but if I were allowed not to return anything I'd do so" return value. – Individually

@EdgarAroutiounian: You'd never be done reading past all the None outputs if you did echo those. – Individually

I wonder why doctest doesn't just treat as empty an expected output solely consisting of the line "None"; perhaps the principle is to avoid surprising a user who manually checks the documented example in an interactive interpreter? – Scepter

@Scepter "... manually checks the documented example in an interactive interpreter" - Exactly. Doctests are supposed to look as much like an interactive session as possible. – Begonia

Answer 2

Other option would be a direct check for None:

def six_or_none(val):
    """
    >>> six_or_none(6)
    6
    >>> six_or_none(4)
    """
    if val == 6:
        return 6
    return None

Prevalent

Answer 3

Another alternative if you want something that looks like you might expect in your docs is:

>>> print(six_or_none(4))
None

Interchangeable