pytest: assert almost equal

python unit-testing pytest
Vladimir Keleshev picture Vladimir Keleshev · Dec 19, 2011 · Viewed 61.7k times · Source

How to do assert almost equal with py.test for floats without resorting to something like:

assert x - 0.00001 <= y <= x + 0.00001

More specifically it will be useful to know a neat solution for quickly compare pairs of float, without unpacking them:

assert (1.32, 2.4) == i_return_tuple_of_two_floats()

Answer

dbn picture dbn · Sep 21, 2016

I noticed that this question specifically asked about py.test. py.test 3.0 includes an approx() function (well, really class) that is very useful for this purpose.

import pytest

assert 2.2 == pytest.approx(2.3)
# fails, default is ± 2.3e-06
assert 2.2 == pytest.approx(2.3, 0.1)
# passes

# also works the other way, in case you were worried:
assert pytest.approx(2.3, 0.1) == 2.2
# passes

The documentation is here: https://docs.pytest.org/en/latest/reference.html#pytest-approx

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