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?

score 66 · Accepted Answer · answered Nov 18 '13 at 12:13

66

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

Test for is None instead:

>>> six_or_none(4) is None
True

answered Nov 18 '13 at 12:13

Martijn Pieters

Martijn, do you know the motivations for ignoring None? – Nov 18 '13 at 12:24
1

@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. – Martijn Pieters Nov 18 '13 at 12:25
@EdgarAroutiounian: You'd never be done reading past all the `None` outputs if you did echo those. – Martijn Pieters Nov 18 '13 at 12:26
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? – benjimin Aug 18 '20 at 03:40

score 9 · Answer 2 · answered Dec 03 '13 at 20:27

9

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

answered Dec 03 '13 at 20:27

alko

score 1 · Answer 3 · answered Dec 25 '20 at 10:03

1

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

>>> print(six_or_none(4))
None

answered Dec 25 '20 at 10:03

Sam Martin

3 Answers3