[pytest-dev] Custom reporting for asserts without comparison operators?

RonnyPfannschmidt opensource at ronnypfannschmidt.de
Mon Mar 19 14:02:55 EDT 2018

hi everyone,

**re-sent using the other sending address**

this is just about single value assertion helpers

i logged an feature request about that a few year back
see https://github.com/pytest-dev/pytest/issues/95 -

so basically this use-case was known since 2011 ^^ and doesn't require
ast rewriting lice macros,
just proper engineering of the representation and handling of single
values in the assertion rewriter.

-- Ronny

Am 19.03.2018 um 15:13 schrieb holger krekel:
> On Mon, Mar 19, 2018 at 15:03 +0100, Floris Bruynooghe wrote:
>> On Sun, Mar 18 2018, Shawn Brown wrote:
>>> Unfortunately, this does not solve my usecase. I'm trying to handle cases
>>> where the following statement would pass:
>>>     assert myfunc(failing_input) == False
>>> But where this next statement would fail using my custom report:
>>>     assert myfunc(failing_input)
>>> Calling myfunc() needs to return True or False (or at least Truthy or
>>> Falsy)--this is locked-in behavior.
>> I'm not sure if this is compatible with Python's semantics really.  If I
>> understand correctly you're asking for a full-on macro implementation on
>> Python or something.  Which in theory you could do with an AST
>> NodeVisitor, but really Python isn't made for this -- sounds like you'd
>> enjoy lisp! ;-)
>> The best thing I can suggest is to make use of the::
>>    assert myfunc(failing_input), repr(myfunc(failing_input()))
> i wonder if one could try to rewrite the ast for "assert myfunc(x)" to
> "assert __pytest_funcrepr_helper(myfunc(x), 'myfunc(x)')" with something like:
>     class __pytest_funcrepr_helper:
>         def __init__(self, val, source):
>             self.val = val
>             self.source = source
>         def __bool__(self):
>             return bool(self.val)
>         def __repr__(self):
>             return "{!r} returned non-true {!r}".format(self.source, self.val)
> but maybe i am not grasping all details involved. It's been a while since
> i looked into ast-rewriting ...
> holger
>> functionality to also get a custom error message.  Here your myfunc()
>> whould have to return some object which both implements __bool__ as well
>> as __repr__ I guess.
>> Maybe there's a feature request in here for something like this::
>>    class Foo:
>>        def __bool__(self):
>>            return False
>>        def __repr__(self):
>>            return 'multiline\nstring'
>>    assert Foo()
>> To actually show the repr in the error message, which it currently
>> doesn't.  I'd like to know what other people think of such a feature
>> though, and haven't thought through all the implications yet.  But I'm
>> curious, would something like that solve your case?
>> Cheers,
>> Floris
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