Extending

Add your own assertions to assertpy2 with add_extension().

A custom assertion

from assertpy2 import add_extension, assert_that

def is_5(self):
    if self.val != 5:
        return self.error(f"{self.val} is NOT 5!")
    return self

add_extension(is_5)

assert_that(5).is_5()
assert_that(6).is_5()  # fails!

remove_extension() takes the same function and unregisters it, so a temporary assertion does not leak into other tests:

from assertpy2 import remove_extension

remove_extension(is_5)

Project-wide reuse

is_5() is only available in the file where add_extension() is called. To share extensions across all test files, register them in a pytest fixture in conftest.py:

import pytest
from assertpy2 import add_extension

def is_5(self):
    if self.val != 5:
        return self.error(f"{self.val} is NOT 5!")
    return self

@pytest.fixture(scope="module")
def my_extensions():
    add_extension(is_5)

Then request the fixture in any test that needs the extensions:

from assertpy2 import assert_that

def test_foo(my_extensions):
    assert_that(5).is_5()
    assert_that(6).is_5()  # fails!

Writing custom assertions

A few conventions keep custom assertions consistent with the built-ins:

1. Use self as the first parameter, as if the function were an instance method.
2. Read the actual value from self.val.
3. Test the negative case and fail if it holds.
4. Fail via self.error(...), which raises AssertionError and prints your message.
5. Raise TypeError/ValueError for bad input (a programming error), not self.error().
6. Always return self so the assertion chains.

def is_multiple_of(self, other):
    if isinstance(self.val, numbers.Integral) is False or self.val <= 0:
        raise TypeError("val must be a positive integer")
    if isinstance(other, numbers.Integral) is False or other <= 0:
        raise TypeError("given arg must be a positive integer")

    _, rem = divmod(self.val, other)
    if rem > 0:
        return self.error(f"Expected <{self.val}> to be multiple of <{other}>, but was not.")
    return self