WIP: compare datetimes and timedeltas with approx

src/_pytest/python_api.py

@@ -4,6 +4,7 @@
 import pprint
 import sys
 import warnings
+from datetime import timedelta, datetime
 from decimal import Decimal
 from numbers import Number
 
@@ -347,6 +348,43 @@ class ApproxDecimal(ApproxScalar):
     DEFAULT_RELATIVE_TOLERANCE = Decimal("1e-6")
 
 
+class ApproxDateTime(ApproxBase):
+    """
+    Perform approximate comparisons where the expected value is a
+    datetime or timedelta.
+    """
+
+    DEFAULT_ABSOLUTE_TOLERANCE = timedelta(seconds=1)
+
+    def __init__(self, expected, rel=None, abs=None, nan_ok=False):
+        if abs is None:
+            abs = self.DEFAULT_ABSOLUTE_TOLERANCE
+
+        if not isinstance(abs, timedelta):
+            raise TypeError("absolute tolerance must be a timedelta type")
+        if abs < timedelta(0):
+            raise ValueError(
+                "absolute tolerance can't be negative: {}".format(abs))
+
+        if rel is not None:
+            raise ValueError("datetime objects cannot be compared with a "
+                             "relative tolerance")
+        if nan_ok:
+            raise ValueError("datetime objects cannot be compared to NaN")
+
+        super(ApproxDateTime, self).__init__(expected, rel=None, abs=abs,
+                                             nan_ok=False)
+
+    def __repr__(self):
+        if sys.version_info[0] == 2:
+            return "approx({!r} +- {!r})".format(self.expected, self.abs)
+        else:
+            return u"approx({!r} \u00b1 {!r})".format(self.expected, self.abs)
+
+    def __eq__(self, actual):
+        return abs(self.expected - actual) <= self.abs
+
+
 def approx(expected, rel=None, abs=None, nan_ok=False):
     """
     Assert that two numbers (or two sets of numbers) are equal to each other
@@ -531,6 +569,8 @@ def approx(expected, rel=None, abs=None, nan_ok=False):
         and not isinstance(expected, STRING_TYPES)
     ):
         cls = ApproxSequencelike
+    elif isinstance(expected, (datetime, timedelta)):
+        cls = ApproxDateTime
     else:
         raise _non_numeric_type_error(expected, at=None)
 

testing/python/approx.py

@@ -2,6 +2,7 @@
 import doctest
 import operator
 import sys
+from datetime import datetime, timezone, timedelta
 from decimal import Decimal
 from fractions import Fraction
 from operator import eq
@@ -60,6 +61,17 @@ def test_repr_string(self, plus_minus):
             ),
         )
 
+        expected = datetime(1970, 1, 1, tzinfo=timezone.utc)
+        tolerance = timedelta(minutes=1)
+        assert repr(
+            approx(expected, abs=tolerance)
+        ) == 'approx({!r} {} {!r})'.format(expected, plus_minus, tolerance)
+        expected = timedelta(hours=1)
+        tolerance = timedelta(minutes=1)
+        assert repr(
+            approx(expected, abs=tolerance)
+        ) == 'approx({!r} {} {!r})'.format(expected, plus_minus, tolerance)
+
     @pytest.mark.parametrize(
         "value, repr_string",
         [
@@ -507,3 +519,47 @@ def __len__(self):
 
         expected = MySizedIterable()
         assert [1, 2, 3, 4] == approx(expected)
+
+    def test_datetime(self):
+        a_little_earlier = datetime.now()
+        a_little_later = datetime.now()
+        a_minute_later = a_little_earlier + timedelta(seconds=30)
+        assert a_little_later == approx(a_little_earlier)
+        assert a_minute_later != approx(a_little_earlier)
+        assert a_minute_later == approx(a_little_earlier,
+                                        abs=timedelta(minutes=1))
+
+        with pytest.raises(TypeError):
+            approx(a_little_earlier, abs=1)
+        with pytest.raises(ValueError):
+            approx(a_little_earlier, abs=-timedelta(seconds=1))
+        with pytest.raises(ValueError):
+            approx(a_little_earlier, rel=1)
+        with pytest.raises(ValueError):
+            approx(a_little_earlier, rel=timedelta(seconds=1))
+        with pytest.raises(ValueError):
+            approx(a_little_earlier, nan_ok=True)
+        with pytest.raises(TypeError):
+            assert timedelta(seconds=1) == approx(a_little_earlier)
+
+    def test_timedelta(self):
+        an_hour = timedelta(hours=1)
+        an_hour_and_a_bit = timedelta(hours=1, milliseconds=1)
+        an_hour_and_a_bit_more = timedelta(hours=1, seconds=30)
+        assert an_hour_and_a_bit == approx(an_hour)
+        assert an_hour_and_a_bit_more != approx(an_hour)
+        assert an_hour_and_a_bit_more == approx(an_hour,
+                                                abs=timedelta(minutes=1))
+
+        with pytest.raises(TypeError):
+            approx(an_hour, abs=1)
+        with pytest.raises(ValueError):
+            approx(an_hour, abs=-timedelta(seconds=1))
+        with pytest.raises(ValueError):
+            approx(an_hour, rel=1)
+        with pytest.raises(ValueError):
+            approx(an_hour, rel=timedelta(seconds=1))
+        with pytest.raises(ValueError):
+            approx(an_hour, nan_ok=True)
+        with pytest.raises(TypeError):
+            assert datetime.now() == approx(an_hour)