[Python-Dev] Deprecate doctest?

[Paul Moore]

Aahz <aahz@pythoncraft.com> writes:

>> How can a test break when you change the error message text ? I'd
>> say that the test was broken: you should never rely on a particular
>> message text since these can and do change rather often.
>
> That's tricky, given that doctest is in the standard library, and
> that's exactly what doctest does.  Are you advocating deprecating
> doctest?

You can write the doctest to avoid the dependency on the exact
test. As a (none too realistic) example,

    >>> 1/0
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    ZeroDivisionError: integer division or modulo by zero

is a bad test, because it depends on an exact text. (This one probably
isn't "bad" in practice, as I can't imagine the text changing very
often). On the other hand,

    >>> try:
    ...     1/0
    ... except ZeroDivisionError:
    ...     print "Divide by zero"
    ...
    Divide by zero

is a good test, because it protects against such a dependency. This is
a fairly well-known type of issue with doctest, which usually shows up
in the form of reliance on a particular order of printing a
dictionary. See section 5.2.7 of the manual (Warnings).

Paul.
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