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gh-71385: add relative delta parameter for assertAlmostEqual and assertNotAlmostEqual #96881

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b8e619a
relative delta keyword added to assertAlmostEqual and assertNotAlmost…
wehlgrundspitze Sep 16, 2022
6eeb2b9
unittest added to relative delta keyword in assertAlmostEqual and ass…
wehlgrundspitze Sep 16, 2022
0797a24
📜🤖 Added by blurb_it.
blurb-it[bot] Sep 16, 2022
6151d7e
Clarify news message and add internal rereferences.
wehlgrundspitze Oct 8, 2022
fc545ec
rel_delta algorithm symmetrized in analogy to math.isclose
wehlgrundspitze Oct 8, 2022
ca09f2e
unittests adapted to symmetrization of rel_delta argument of unittest…
wehlgrundspitze Oct 8, 2022
1b01829
unittest.TestCase.assertAlmostEqual and assertNotAlmostEqual modified…
wehlgrundspitze Oct 16, 2022
640ba3b
unittest added to check +-inf and nan handling of unittest.TestCase.a…
wehlgrundspitze Oct 16, 2022
2d115d1
test duplication and typo removed
wehlgrundspitze Oct 16, 2022
835bea7
Merge branch 'main' into fix-issue-71385
ambv Feb 21, 2024
e319648
Merge branch 'main' into fix-issue-71385
skirpichev May 8, 2026
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60 changes: 55 additions & 5 deletions Lib/test/test_unittest/test_assertions.py
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,11 @@
from itertools import product


NAN = float('nan')
INF = float('inf')
NINF = float('-inf')


class Test_Assertions(unittest.TestCase):
def test_AlmostEqual(self):
self.assertAlmostEqual(1.00000001, 1.0)
Expand All @@ -26,11 +31,7 @@ def test_AlmostEqual(self):
self.assertRaises(self.failureException,
self.assertNotAlmostEqual, 0, .1+.1j, places=0)

self.assertAlmostEqual(float('inf'), float('inf'))
self.assertRaises(self.failureException, self.assertNotAlmostEqual,
float('inf'), float('inf'))

def test_AmostEqualWithDelta(self):
def test_AlmostEqualWithDelta(self):
self.assertAlmostEqual(1.1, 1.0, delta=0.5)
self.assertAlmostEqual(1.0, 1.1, delta=0.5)
self.assertNotAlmostEqual(1.1, 1.0, delta=0.05)
Expand All @@ -57,6 +58,55 @@ def test_AmostEqualWithDelta(self):
self.assertNotAlmostEqual(first, second,
delta=datetime.timedelta(seconds=5))

def test_AlmostEqualWithRelativeDelta(self):
self.assertAlmostEqual(1.0, 0.6, rel_delta=0.5)
self.assertAlmostEqual(0.6, 1.0, rel_delta=0.5)
with self.assertRaises(self.failureException):
self.assertAlmostEqual(1.0, 0.6, rel_delta=0.05)
with self.assertRaises(self.failureException):
self.assertAlmostEqual(0.6, 1.0, rel_delta=0.05)

self.assertNotAlmostEqual(1.0, 0.6, rel_delta=0.05)
self.assertNotAlmostEqual(0.6, 1.0, rel_delta=0.05)
with self.assertRaises(self.failureException):
self.assertNotAlmostEqual(1.0, 0.6, rel_delta=0.5)
with self.assertRaises(self.failureException):
self.assertNotAlmostEqual(0.6, 1.0, rel_delta=0.5)

with self.assertRaises(TypeError):
self.assertAlmostEqual(1.0, 1.1, places=0, rel_delta=0.5)
with self.assertRaises(TypeError):
self.assertAlmostEqual(1.0, 1.1, delta=1, rel_delta=0.5)
with self.assertRaises(TypeError):
self.assertNotAlmostEqual(1.0, 1.1, places=0, rel_delta=0.05)
with self.assertRaises(TypeError):
self.assertNotAlmostEqual(1.0, 1.1, delta=0.05, rel_delta=0.05)

first_dt = datetime.timedelta(seconds=20)
second_dt = datetime.timedelta(seconds=15)

self.assertAlmostEqual(first_dt, second_dt, rel_delta=0.5)

@sobolevn sobolevn Sep 21, 2022

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Let's add two datetime objects together with timedelta as rel_delta

@wehlgrundspitze wehlgrundspitze Oct 8, 2022

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From my point of view this seems not well defined. The clearest method to see are the units: Within the calculation, we multiply the relative delta with one of the objects: timedelta * datetime would have a unit of square seconds ...

self.assertNotAlmostEqual(first_dt, second_dt, rel_delta=0.1)

def test_AlmostEqual_inf_nan(self):
for criterion in [{"places": 2}, {"delta": 0.5}, {"rel_delta": 0.2}]:
for n1, n2 in [(INF, INF), (NINF, NINF)]:
self.assertAlmostEqual(n1, n2, **criterion)
with self.assertRaises(self.failureException,
msg="%s %s should be equal with %s" % (n1, n2, criterion)):
self.assertNotAlmostEqual(n1, n2, **criterion)

for n1, n2 in [(INF, NINF), (NINF, INF),
(INF, 1), (1, INF),
(NINF, 1), (1, NINF),
(NAN, NAN),
(NAN, 1), (1, NAN),
(NAN, INF), (INF, NAN),
(NAN, NINF), (NINF, NAN)]:
self.assertNotAlmostEqual(n1, n2, **criterion)
with self.assertRaises(self.failureException):
self.assertAlmostEqual(n1, n2, **criterion)

def test_assertRaises(self):
def _raise(e):
raise e
Expand Down
68 changes: 48 additions & 20 deletions Lib/unittest/case.py
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Original file line number Diff line number Diff line change
Expand Up @@ -958,24 +958,33 @@ def assertNotEqual(self, first, second, msg=None):
raise self.failureException(msg)

def assertAlmostEqual(self, first, second, places=None, msg=None,
delta=None):
delta=None, rel_delta=None):
"""Fail if the two objects are unequal as determined by their
difference rounded to the given number of decimal places
(default 7) and comparing to zero, or by comparing that the
difference between the two objects is more than the given
delta.
difference by one of the following methods:

Note that decimal places (from zero) are usually not the same
as significant digits (measured from the most significant digit).
1) Given number of decimal places. This is the default method
(default value 7 digits). Note that decimal places (from
zero) are usually not the same as significant digits
(measured from the most significant digit).

2) Comparing that the absolute difference between the two
objects is more than the given delta.

3) Comparing that the relative difference between the two
objects is more than the given rel_delta, using the same
algorithm as :meth:`math.isclose`.

If the two objects compare equal then they will automatically
compare almost equal.
compare almost equal. Comparison of floating point numbers
(espaicially inf, -inf, and NaN) is applied in the same way as
in :meth:`math.isclose` according to IEEE standard 754.
"""
if first == second:
# shortcut
return
if delta is not None and places is not None:
raise TypeError("specify delta or places not both")
if int(delta is not None) + int(places is not None) + \
int(rel_delta is not None) > 1:
raise TypeError("specify maximally one: delta, places or rel_delta")

diff = abs(first - second)
if delta is not None:
Expand All @@ -987,6 +996,15 @@ def assertAlmostEqual(self, first, second, places=None, msg=None,
safe_repr(second),
safe_repr(delta),
safe_repr(diff))
elif rel_delta is not None:
if (diff < rel_delta*abs(first)) or (diff < rel_delta*abs(second)):
return
Comment on lines +999 to +1001

@skirpichev skirpichev May 22, 2026

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As per Mark's commentary, I think that behavior should match math.isclose().

Namely, in the given case (when delta is None, that means delta=0.0): diff <= rel_delta * max(abs(first), abs(second)).

But for delta is not None case you should also check if rel_delta is not None and use diff <= max(rel_delta * max(abs(first), abs(second)), delta).


standardMsg = '%s != %s within relatively %s (%s rel. difference)' % (
safe_repr(first),
safe_repr(second),
safe_repr(rel_delta),
safe_repr(diff/abs(first)))
else:
if places is None:
places = 7
Expand All @@ -1003,28 +1021,38 @@ def assertAlmostEqual(self, first, second, places=None, msg=None,
raise self.failureException(msg)

def assertNotAlmostEqual(self, first, second, places=None, msg=None,
delta=None):
delta=None, rel_delta=None):
"""Fail if the two objects are equal as determined by their
difference rounded to the given number of decimal places
(default 7) and comparing to zero, or by comparing that the
difference between the two objects is less than the given delta.

Note that decimal places (from zero) are usually not the same
as significant digits (measured from the most significant digit).
difference by one of the methods analougsly described for
assertAlmostEqual().

Objects that are equal automatically fail.
"""
if delta is not None and places is not None:
raise TypeError("specify delta or places not both")
if int(delta is not None) + int(places is not None) + \
int(rel_delta is not None) > 1:
raise TypeError("specify maximally one: delta, places or rel_delta")

diff = abs(first - second)

if delta is not None:
if not (first == second) and diff > delta:
if not (first == second) and ((diff > delta) or diff != diff):
return
standardMsg = '%s == %s within %s delta (%s difference)' % (
safe_repr(first),
safe_repr(second),
safe_repr(delta),
safe_repr(diff))
elif rel_delta is not None:
if not (first == second) and \
(((diff > rel_delta*abs(first)) and (diff > rel_delta*abs(second)))
or (diff != diff) or (diff/2 == diff)):
return

standardMsg = '%s == %s within relatively %s (%s rel. difference)' % (
safe_repr(first),
safe_repr(second),
safe_repr(rel_delta),
safe_repr(diff/abs(first)))
else:
if places is None:
places = 7
Expand Down
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2022-09-16-21-51-41.gh-issue-71385.r5XUzt.rst
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@@ -0,0 +1 @@
A new parameter ``rel_delta`` was added to the :meth:`unittest.TestCase.assertAlmostEqual` and :meth:`unittest.TestCase.assertNotAlmostEqual`. It allows comparing two objects with a maximum relative difference to each other.
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