predeq 0.0.3

Predicate-based equivalence testing

predeq

PredEq is a utility for testing objects using an equivalence predicate. It facilitates writing declarative-style tests for complex requirements, and for cases where the default __eq__ is inapplicable or missing.

At its core, predeq(predicate) is an object which tests equal to X if predicate(X) returns True.

from predeq import predeq, instanceof


def test_tuple_of_2_ints():
    """Test that value is a tuple of an int and a string."""

    value = (123, 'abc')

    # without predeq
    assert isinstance(value, tuple) and len(value) == 2
    assert isinstance(value[0], int)
    assert isinstance(value[1], str)

    # with predeq
    assert value == (
        predeq(lambda x: isinstance(x, int)),  # provide your own predicate
        instanceof(str),                       # or use one of the recipes
    )

Since PredEq is primarily designed for tests, it also tries to be helpful when they fail. If lambda is used as a predicate, predeq attempts to locate its source code and show it in pytest output:

>   assert value == (
        predeq(lambda x: isinstance(x, int)),  # provide your own predicate
        instanceof(str),                       # or use one of the recipes
    )
E   AssertionError: assert (123.0, 'abc') == (<predeq to m...stanceof(str))
E
E     At index 0 diff: 123.0 != <predeq to meet lambda x: isinstance(x, int)>
E     Use -v to get more diff

Each built-in recipe also provides a readable representation, which typically looks like the corresponding code:

>   assert value == (
        predeq(lambda x: isinstance(x, int)),  # provide your own predicate
        instanceof(str),                       # or use one of the recipes
    )
E   AssertionError: assert (123, b'abc') == (<predeq to m...stanceof(str))
E
E     At index 1 diff: b'abc' != instanceof(str)
E     Use -v to get more diff