BUG: use cmath to test complex number equality in pandas._testing

[arw2019]

• closes pandas._lib.testing.assert_almost_equal seem to not use approximate equality for Series with complex doubles #28235
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

Adds cmath-based equality testing for complex numeric types (complex, np.complex64 and np.complex128)

[jbrockmendel]

we have lib.is_complex for this

[arw2019]

Thanks! Switched to using that

[arw2019]

Travis failure looks unrelated

[WillAyd]

Suggested change

	if not cmath.isclose(a, b, rel_tol=rtol, abs_tol=atol):
	if cmath.isclose(a, b, rel_tol=rtol, abs_tol=atol):

I think should just test for True here and return accordingly, otherwise let fall to the standard assertionerror, unless there is a reason for things to be different here?

[arw2019]

I copied what we do for reals, where we cite the rtol and atol if the comparison fails:

pandas/pandas/_libs/testing.pyx

Lines 208 to 210 in da1c6de

	if not math.isclose(fa, fb, rel_tol=rtol, abs_tol=atol):
	assert False, (f"expected {fb:.5f} but got {fa:.5f}, "
	f"with rtol={rtol}, atol={atol}")

whereas the message at the bottom is just:

pandas/pandas/_libs/testing.pyx

Line 213 in da1c6de

raise AssertionError(f"{a} != {b}")

[jreback]

can you add a whatsnew note, bug fixes in numeric for 1.2 (and @WillAyd comments); ping on green.

[arw2019]

Green barring unrelated doctest failure:

=================================== FAILURES ===================================
_________________ [doctest] pandas.core.reshape.pivot.crosstab _________________
549     Examples
550     --------
551     >>> a = np.array(["foo", "foo", "foo", "foo", "bar", "bar",
552     ...               "bar", "bar", "foo", "foo", "foo"], dtype=object)
553     >>> b = np.array(["one", "one", "one", "two", "one", "one",
554     ...               "one", "two", "two", "two", "one"], dtype=object)
555     >>> c = np.array(["dull", "dull", "shiny", "dull", "dull", "shiny",
556     ...               "shiny", "dull", "shiny", "shiny", "shiny"],
557     ...              dtype=object)
558     >>> pd.crosstab(a, [b, c], rownames=['a'], colnames=['b', 'c'])
Differences (unified diff with -expected +actual):
    @@ -1,5 +1,6 @@
    -b   one        two
    +Index(['__dummy__'], dtype='object')
    +b    one        two      
     c   dull shiny dull shiny
    -a
    +a                        
     bar    1     2    1     0
     foo    2     2    1     2

/home/runner/work/pandas/pandas/pandas/core/reshape/pivot.py:558: DocTestFailure

[WillAyd]

lgtm

[WillAyd]

Thanks @arw2019

[arw2019]

Thanks @WillAyd @jreback @jbrockmendel for reviewing!