pytest: assert almost equal

Question

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

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


        

Answer 1

Something like

assert round(x-y, 5) == 0

That is what unittest does

For the second part

assert all(round(x-y, 5) == 0 for x,y in zip((1.32, 2.4), i_return_tuple_of_two_floats()))

Probably better to wrap that in a function

def tuples_of_floats_are_almost_equal(X, Y):
    return all(round(x-y, 5) == 0 for x,y in zip(X, Y))

assert tuples_of_floats_are_almost_equal((1.32, 2.4), i_return_tuple_of_two_floats())

Answer 2

You will have to specify what is "almost" for you:

assert abs(x-y) < 0.0001

to apply to tuples (or any sequence):

def almost_equal(x,y,threshold=0.0001):
  return abs(x-y) < threshold

assert all(map(almost_equal, zip((1.32, 2.4), i_return_tuple_of_two_floats())

Answer 3

These answers have been around for a long time, but I think the easiest and also most readable way is to use unittest for it's many nice assertions without using it for the testing structure.

Get assertions, ignore rest of unittest.TestCase

(based on this answer)

import unittest

assertions = unittest.TestCase('__init__')

Make some assertions

x = 0.00000001
assertions.assertAlmostEqual(x, 0)  # pass
assertions.assertEqual(x, 0)  # fail
# AssertionError: 1e-08 != 0

Implement original questions' auto-unpacking test

Just use * to unpack your return value without needing to introduce new names.

i_return_tuple_of_two_floats = lambda: (1.32, 2.4)
assertions.assertAlmostEqual(*i_return_tuple_of_two_floats())  # fail
# AssertionError: 1.32 != 2.4 within 7 places

Answer 4

If you want something that works not only with floats but for example Decimals you can use python's math.isclose:

    # - rel_tol=0.01` is 1% difference tolerance.
    assert math.isclose(actual_value, expected_value, rel_tol=0.01)

Docs - https://docs.python.org/3/library/math.html#math.isclose

Answer 5

I'd use nose.tools. It plays well with py.test runner and have other equally useful asserts - assert_dict_equal(), assert_list_equal(), etc.

from nose.tools import assert_almost_equals
assert_almost_equals(x, y, places=7) #default is 7 

Answer 6

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