Error reporting in pytest.approx()

[peffjepin]

I've seen a couple test failures where pytest detects a relative tolerance < 0.

In the case I have a record of it was invoked like so:

def test_transform_base_case(transform_inputs):
    # assert that TransformComponent behaves in the same manner as Transform.
    regular = transforms.Transform(*transform_inputs)
    component = transforms.TransformComponent(*transform_inputs)
  
    vertex1 = np.array((1, 1, 1))
    vertex2 = np.array((1, 1, 1))
    regular.apply(vertex1)
    component.apply(vertex2)
  
    assert_approx(vertex1, vertex2)

Where assert_approx is defined as:

def assert_approx(iter1, iter2, rel=1e-6):
    for v1, v2 in zip(iter1, iter2):
        assert v1 == pytest.approx(v2, rel=rel)

Resulting in:

self = -9223372036854775808 ± ???
  
      @property
      def tolerance(self):
          """Return the tolerance for the comparison.
      
          This could be either an absolute tolerance or a relative tolerance,
          depending on what the user specified or which would be larger.
          """
      
          def set_default(x, default):
              return x if x is not None else default
      
          # Figure out what the absolute tolerance should be.  ``self.abs`` is
          # either None or a value specified by the user.
          absolute_tolerance = set_default(self.abs, self.DEFAULT_ABSOLUTE_TOLERANCE)
      
          if absolute_tolerance < 0:
              raise ValueError(
                  f"absolute tolerance can't be negative: {absolute_tolerance}"
              )
          if math.isnan(absolute_tolerance):
              raise ValueError("absolute tolerance can't be NaN.")
      
          # If the user specified an absolute tolerance but not a relative one,
          # just return the absolute tolerance.
          if self.rel is None:
              if self.abs is not None:
                  return absolute_tolerance
      
          # Figure out what the relative tolerance should be.  ``self.rel`` is
          # either None or a value specified by the user.  This is done after
          # we've made sure the user didn't ask for an absolute tolerance only,
          # because we don't want to raise errors about the relative tolerance if
          # we aren't even going to use it.
          relative_tolerance = set_default(
              self.rel, self.DEFAULT_RELATIVE_TOLERANCE
          ) * abs(self.expected)
      
          if relative_tolerance < 0:
  >           raise ValueError(
                  f"relative tolerance can't be negative: {absolute_tolerance}"
              )
  E           ValueError: relative tolerance can't be negative: 1e-12

I'm not familiar with the rest of the source for the class and can't reproduce the error with any consistency, but at the least it seems that

if relative_tolerance < 0:
    raise ValueError(
        f"relative tolerance can't be negative: {absolute_tolerance}"
    )

Should probably be:

if relative_tolerance < 0:
    raise ValueError(
        f"relative tolerance can't be negative: {relative_tolerance}"
    )