Make assertions to show a custom message when raising/warning

[benjello]

The message would be similar to the ones available through the show command.

[gdementen]

Would adding a "msg" argument to all assert* functions be enough, or do you want to include the values of both sides? In the later case, I guess we could handle that in *args)
eg. assertEqual(a, b, a, "is super different from", b)

I wonder if this would work:
assertEqual(a, b, "%s is super different from %s" % (a, b))

[benjello]

Something like

assertEqual(a, b, a, "is super different from", b)

would be very useful since it would be flexible enough to dump some information à la show !

[benjello]

I renew my interest in this feature. Something like

assertTrue(expression, message = show_args_tuple)

which would behave like

show(*show_args_tuple)

when the assertion fails

[gdementen]

Initially I thought I would rather have the signature be:

assertEqual(first, second, msg=None)

where msg is either None or a string. i.e., let users build the message string themselves. But this cannot work because when users type something like:

assertEqual(first, second, '%s abc %s' % (first, second))

the string is evaluated immediately (at the string "definition"), and so our expression evaluator only ever sees: assertEqual(first, second, 'first abc second') which we cannot do much with. That leaves us with either:

assertEqual(a, b, a, "is super different from", b)
# OR
assertEqual(a, b, (a, "is super different from", b))

I am ambivalent to either. As for the implementation, it should not be too hard to implement. The code is in actions.py. Here is a proof of concept of the later for ComparisonAssert:

    def eval_assertion(self, context, v1, v2, msg=None):
        result = self.compare(v1, v2)
        if isinstance(result, tuple):
            result, details = result
        else:
            details = ''
        if not result:
            op = self.inv_op
            if isinstance(msg, tuple):
                msg = ' '.join(str(v) for v in msg)
            msg = ': {}'.format(msg) if msg is not None else ''
            return "%s %s %s (%s %s %s)%s%s" % (self.args[0], op, self.args[1],
                                                v1, op, v2, details, msg)

If you could generalize this to all assertion functions, modify the corresponding documentation, add a note in the changelog and make a PR out of this, that would be nice :)

[benjello]

I will definitively try to implement it along the lines you described.
But I would also like to see it would be possible to dump some diagnostics when the assertion fails:

• Is it possible to evaluate an expression using the same context within eval_assertion ?
• Should I use liam2 control statements to catch failed assertion and dump diagnostics ?

 - assertion: 1 == 0
 - while(not(assertion):
    - show(dump(diagnostics))

[gdementen]

I think the above proof-of-concept should work. For the user, that would look like:

assertEqual(1, 0, msg=dump(diagnostics))