Override Standard Assert Messaging in Pytest Assert

问题

I'm using Pytest to test some SQL queries my team runs programmatically over time.

My SQL queries are lists of JSONs - one JSON corresponds to one row of data.

I've got a function that diffs the JSON key:value pairs so that we can point to exactly which values are different for a given row. Ideally, I'd output a list of these diffs instead of the standard output of an assert statement, which ends up looking clunky and not-very-useful for the end user.

回答1:

Pytest give us the hook pytest_assertrepr_compare to add a custom explanation about why an assertion failed.

You can create a class to wrap the JSON string and implement your comparator algorithm overloading the equal operator.

class JSONComparator:
    def __init__(self, lst):
        self.value = value

    def __eq__(self, other):
        # Here your algorithm to compare two JSON strings
        ...

        # If they are different, save that information
        # We will need it later
        self.diff = "..."

        return True

# Put the hook in conftest.py or import it in order to make pytest aware of the hook.
def pytest_assertrepr_compare(config, op, left, right):

    if isinstance(left, JSONComparator) and op == "==":
        # Return the diff inside an array.
        return [left.diff]

# Create a reference as an alias if you want
compare = JSONComparator

Usage

def test_somethig():
    original = '{"cartoon": "bugs"}'
    expected = '{"cartoon": "bugs"}'

    assert compare(original) == expected

回答2:

You can use Python built-in capability to show custom exception message:

assert response.status_code == 200, "My custom message: actual status code {}".format(response.status_code)

Check it out: https://wiki.python.org/moin/UsingAssertionsEffectively

来源：https://stackoverflow.com/questions/52319116/override-standard-assert-messaging-in-pytest-assert

标签

python

testing

pytest

assert