TST + DOC: test_compression_warning docstring

Author: dhimmel

pandas/tests/test_common.py

@@ -251,8 +251,18 @@ def test_compression_size_fh(obj, method, compression_only):
         assert uncompressed > compressed
 
 
-# GH 21227
 def test_compression_warning(compression_only):
+    """
+    Assert that passing a file object to to_csv while explicitly specifying a
+    compression protocol triggers a RuntimeWarning, as per
+    https://github.com/pandas-dev/pandas/issues/21227.
+
+    Note that pytest has an issue that causes assert_produces_warning to fail
+    in Python 2 if the warning has occurred in previous tests
+    (see https://git.io/fNEBm & https://git.io/fNEBC). Hence, should this test
+    fail in just Python 2 builds, it likely indicates that other tests are
+    producing RuntimeWarnings, thereby triggering the pytest bug.
+    """
     df = DataFrame(100 * [[0.123456, 0.234567, 0.567567],
                           [12.32112, 123123.2, 321321.2]],
                    columns=['X', 'Y', 'Z'])