pytest: assert almost equal


How to do assert almost equal with pytest 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 comparing pairs of float, without unpacking them:

assert (1.32, 2.4) == i_return_tuple_of_two_floats()
Asked By: Vladimir Keleshev



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())

pytest.approx was released as part of pytest v3.0.0 in 2016.
This answer predates it, use this if:

  • you don’t have a recent version of pytest AND
  • you understand floating point precision and it’s impact to your use case.

for practically all common scenarios, use
pytest.approx as suggested in this answer.

Answered By: yurib

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())
Answered By: John La Rooy

If you have access to NumPy it has great functions for floating point comparison that already do pairwise comparison with numpy.testing.

Then you can do something like:

numpy.testing.assert_allclose(i_return_tuple_of_two_floats(), (1.32, 2.4))
Answered By: jiffyclub

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
Answered By: KobeJohn

I’d use It plays well with py.test runner and have other equally useful asserts – assert_dict_equal(), assert_list_equal(), etc.

from import assert_almost_equals
assert_almost_equals(x, y, places=7) #default is 7 
Answered By: volodymyr

I noticed that this question specifically asked about pytest. pytest 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
Answered By: dbn

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)
Answered By: validname

Could just use round()

a, b = i_return_tuple_of_two_floats()
assert (1.32, 2.4) == round(a,2), round(b,1)
Answered By: Pluto
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