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

doc/source/whatsnew/v1.3.1.rst

@@ -27,7 +27,7 @@ Fixed regressions
 - Bug in :class:`Series` constructor not accepting a ``dask.Array`` (:issue:`38645`)
 - Fixed regression for ``SettingWithCopyWarning`` displaying incorrect stacklevel (:issue:`42570`)
 - Fixed regression in :func:`to_datetime` returning pd.NaT for inputs that produce duplicated values, when ``cache=True`` (:issue:`42259`)
-
+- 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

@@ -505,11 +505,11 @@ def read_csv(
     delimiter=None,
     # Column and Index Locations and Names
     header="infer",
-    names=lib.no_default,
+    names=None,
     index_col=None,
     usecols=None,
     squeeze=False,
-    prefix=lib.no_default,
+    prefix=None,
     mangle_dupe_cols=True,
     # General Parsing Configuration
     dtype: DtypeArg | None = None,
@@ -603,11 +603,11 @@ def read_table(
     delimiter=None,
     # Column and Index Locations and Names
     header="infer",
-    names=lib.no_default,
+    names=None,
     index_col=None,
     usecols=None,
     squeeze=False,
-    prefix=lib.no_default,
+    prefix=None,
     mangle_dupe_cols=True,
     # General Parsing Configuration
     dtype: DtypeArg | None = None,
@@ -1224,8 +1224,8 @@ def _refine_defaults_read(
     error_bad_lines: bool | None,
     warn_bad_lines: bool | None,
     on_bad_lines: str | None,
-    names: ArrayLike | None | object,
-    prefix: str | None | object,
+    names: ArrayLike | None,
+    prefix: str | None,
     defaults: dict[str, Any],
 ):
     """Validate/refine default values of input parameters of read_csv, read_table.
@@ -1302,11 +1302,11 @@ 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:
+    if names is not None and prefix is not None:
         raise ValueError("Specified named and prefix; you can only specify one.")
 
-    kwds["names"] = None if names is lib.no_default else names
-    kwds["prefix"] = None if prefix is lib.no_default else prefix
+    kwds["names"] = names
+    kwds["prefix"] = prefix
 
     # Alias sep -> delimiter.
     if delimiter is None:

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):