Confused by `contains_exactly` message

When asserting using contains_exactly I was a little confused by the error message. The fault was mine, I was passing the matcher a list rather than the elements of the list. However, the error message wasn’t immediately clear to me - although I’m not sure the error should be clearer if the API usage was incorrect.

        assert_that(degrees, contains_exactly([
>           (1, 2), (2, 1), (3, 1), (4, 0)
        ]))

tests/build/test_importer.py:53:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

value = [(1, 2), (2, 1), (3, 1), (4, 0)], matcher = <precisely.iterable_matchers.ContainsExactlyMatcher object at 0x10f6635f8>

    def assert_that(value, matcher):
        result = matcher.match(value)
        if not result.is_match:
>           raise AssertionError("\nExpected: {0}\nbut: {1}".format(matcher.describe(), result.explanation))
E           AssertionError:
E           Expected: iterable containing in any order:
E             * [(1, 2), (2, 1), (3, 1), (4, 0)]
E           but: was missing element:
E             * [(1, 2), (2, 1), (3, 1), (4, 0)]
E           mismatched elements:
E             * (1, 2): was (1, 2)
E             * (2, 1): was (2, 1)
E             * (3, 1): was (3, 1)
E             * (4, 0): was (4, 0)

[mwilliamson]

So far, I’ve changed the explanations so that your example would be:

Expected: iterable containing 1 element:
  * [(1, 2), (2, 1), (3, 1), (4, 0)]
but: was missing element:
  * [(1, 2), (2, 1), (3, 1), (4, 0)]
These elements were in the iterable, but did not match:
  * (1, 2): was (1, 2)
  * (2, 1): was (2, 1)
  * (3, 1): was (3, 1)
  * (4, 0): was (4, 0)

I’ve avoided removing the mismatch explanations, even when they’re just a repetition of the repr of the element, since it could be confusing if some elements have the explanation and others don’t. I was thinking of explicitly explaining that the suffix is the mismatch explanation, but I wonder if that would be overly verbose?

Expected: iterable containing 1 element:
  * [(1, 2), (2, 1), (3, 1), (4, 0)]
but: was missing element:
  * [(1, 2), (2, 1), (3, 1), (4, 0)]
These elements were in the iterable, but did not match:
  * (1, 2)
    * mismatch reason: was (1, 2)
  * (2, 1)
    * mismatch reason: was (2, 1)
  * (3, 1)
    * mismatch reason: was (3, 1)
  * (4, 0)
    * mismatch reason: was (4, 0)

[mwilliamson]

I think this can be closed now, but feel free to disagree.