py.test: format failed assert AND print custom message

Question

The py.test assert docs say

... if you specify a message with the assertion like this:

assert a % 2 == 0, "value was odd, should be even"

then no assertion introspection takes places at all and the message will be simply shown in the traceback.

Python's builtin unittest module does this too, unless your TestCase sets longMessage = True.

Having the nice assert formatting is test developer friendly, while the custom message is more business-requirement / human friendly. The custom message is especially helpful when you're not in the context of the test, i.e. it answers what that assert is doing there, without having to look at the code. So, I'd like to have both messages.

Is there any way to get py.test's nice assert introspection and formatting AND print a custom message?

Answer 1

There isn't a way to enable assertion extra info plus a message currently. I think it could be added but not sure how much effort is needed. So far this issue hasn't come to my knowledge. Feel free to file an issue or try a pull request. Note, however, that if you go through the effort of writing a custom human-readable message, you can probably also put in some interesting values from the expression.