REGR: ValueError raised when both prefix and names are set to None

doc/source/whatsnew/v1.3.2.rst

@@ -19,7 +19,7 @@ Fixed regressions
 - Regression in :meth:`DataFrame.from_records` with empty records (:issue:`42456`)
 - Fixed regression in :meth:`DataFrame.shift` where TypeError occurred when shifting DataFrame created by concatenation of slices and fills with values (:issue:`42719`)
 - Regression in :meth:`DataFrame.agg` when the ``func`` argument returned lists and ``axis=1`` (:issue:`42727`)
--
+- Fixed regression where :meth:`pandas.read_csv` raised a ``ValueError`` when parameters ``names`` and ``prefix`` were both set to None (:issue:`42387`)
 
 .. ---------------------------------------------------------------------------
 

pandas/io/parsers/readers.py

@@ -1302,7 +1302,8 @@ def _refine_defaults_read(
     if delimiter and (sep is not lib.no_default):
         raise ValueError("Specified a sep and a delimiter; you can only specify one.")
 
-    if names is not lib.no_default and prefix is not lib.no_default:
+    allowed_names_prefix = {None, lib.no_default}
+    if names not in allowed_names_prefix and prefix not in allowed_names_prefix:
         raise ValueError("Specified named and prefix; you can only specify one.")
 
     kwds["names"] = None if names is lib.no_default else names

pandas/tests/io/parser/common/test_common_basic.py

@@ -764,15 +764,24 @@ def test_read_table_delim_whitespace_non_default_sep(all_parsers, delimiter):
 
 
 @pytest.mark.parametrize("func", ["read_csv", "read_table"])
-@pytest.mark.parametrize("prefix", [None, "x"])
-@pytest.mark.parametrize("names", [None, ["a"]])
-def test_names_and_prefix_not_lib_no_default(all_parsers, names, prefix, func):
+def test_names_and_prefix_not_None_raises(all_parsers, func):
     # GH#39123
     f = StringIO("a,b\n1,2")
     parser = all_parsers
     msg = "Specified named and prefix; you can only specify one."
     with pytest.raises(ValueError, match=msg):
-        getattr(parser, func)(f, names=names, prefix=prefix)
+        getattr(parser, func)(f, names=["a", "b"], prefix="x")
+
+
+@pytest.mark.parametrize("func", ["read_csv", "read_table"])
+@pytest.mark.parametrize("prefix, names", [(None, ["x0", "x1"]), ("x", None)])
+def test_names_and_prefix_explicit_None(all_parsers, names, prefix, func):
+    # GH42387
+    f = StringIO("a,b\n1,2")
+    expected = DataFrame({"x0": ["a", "1"], "x1": ["b", "2"]})
+    parser = all_parsers
+    result = getattr(parser, func)(f, names=names, sep=",", prefix=prefix, header=None)
+    tm.assert_frame_equal(result, expected)
 
 
 def test_dict_keys_as_names(all_parsers):