missing evaluated value in report when asserting a boolean attribute (using --assert=rewrite)

[bagerard]

Hi there,

First of all, thank you for the amazing work you've done on pytest! It is really cool!
When asserting a boolean attribute of an object, the actual value isn't printed in the output as it would be with other types (int, str, etc). I'm using latest pytest 2.9.1

def test_a():
    class AA():
        pass
    a = AA()
    a.t = False
    assert a.t is True

Provides the following output (using with ---assert=rewrite):

...

  assert a.t is True

E assert <test.AA instance at 0x7fb9399169e0>.t is True

And if run with ---assert=reinterp:

...

  assert a.t is True

E assert False is True
E + where False = <test.AA instance at 0x7f24de4740e0>.t

Thanks

[tomviner]

This is caused by the _collapse_false function, which aims to trim away trivial explanations.

For example this test:

def test_false_value():
    x = 0
    assert bool(x)

Full explanation, with _collapse_false patched out:

    def test_false_value():
        x = 0
>       assert bool(x)
E       assert False
E        +  where False = bool(0)

Trimmed explanation, with _collapse_false back in place:

    def test_false_value():
        x = 0
>       assert bool(x)
E       assert bool(0)

I'm having trouble recreating an example where useful information is trimmed away.

[bagerard]

Hi tomviner, Thanks for your reply. I wasn't aware of such behaviour and in my case I wasn't sure if my asserted value was a boolean or not. In fact if you take the following test:

def test_foo2():
    mo = Foo(a=False)
#>       assert mo.a
#E       assert <test.Foo instance at 0x7f28a5ff0dd0>.a

The output isn't telling me if mo.a is False or if it's anything else that evalues to False (0 for instance). It turns out that the output is different if it is 0 but without knowing this, you can't tell...
I understand that you want to trim away trivial explanations but personally I find it confusing for newcomers.
This ticket can be switched to "proposal", I suggest to trim away the _collapse_false method :)