TST: Test inequality w/ complex dtypes (GH28050) (pandas-dev#43159)

* TST: Test inequality w/ complex dtypes (GH28050)

Should raise TypeError

* Add test for complex dtype comparison using '<'

* Refactor code to reduce repetition

Remove https link to issue (keep issue number)
Save match message in variable for multiple use

* Combine arange call with DataFrame call in test

Author: calvh

pandas/tests/dtypes/test_dtypes.py

@@ -1095,3 +1095,15 @@ def test_period_dtype_compare_to_string():
     dtype = PeriodDtype(freq="M")
     assert (dtype == "period[M]") is True
     assert (dtype != "period[M]") is False
+
+
+def test_compare_complex_dtypes():
+    # GH 28050
+    df = pd.DataFrame(np.arange(5).astype(np.complex128))
+    msg = "'<' not supported between instances of 'complex' and 'complex'"
+
+    with pytest.raises(TypeError, match=msg):
+        df < df.astype(object)
+
+    with pytest.raises(TypeError, match=msg):
+        df.lt(df.astype(object))