ENH: GH34946 Check type of names argument to read_csv, read_table…

doc/source/whatsnew/v1.1.0.rst

@@ -1121,6 +1121,7 @@ Other
 - :class:`IntegerArray` now implements the ``sum`` operation (:issue:`33172`)
 - Bug in :class:`Tick` comparisons raising ``TypeError`` when comparing against timedelta-like objects (:issue:`34088`)
 - Bug in :class:`Tick` multiplication raising ``TypeError`` when multiplying by a float (:issue:`34486`)
+- Passing a `set` as `names` argument to :func:`pandas.read_csv`, :func:`pandas.read_table`, or :func:`pandas.read_fwf` will raise ``ValueError: Names should be an ordered collection.`` (:issue:`34946`)
 
 .. ---------------------------------------------------------------------------
 

pandas/io/parsers.py

@@ -397,7 +397,8 @@ def _validate_integer(name, val, min_val=0):
 
 def _validate_names(names):
     """
-    Raise ValueError if the `names` parameter contains duplicates.
+    Raise ValueError if the `names` parameter contains duplicates or has an
+    invalid data type.
 
     Parameters
     ----------
@@ -407,11 +408,13 @@ def _validate_names(names):
     Raises
     ------
     ValueError
-        If names are not unique.
+        If names are not unique or are not ordered (e.g. set).
     """
     if names is not None:
         if len(names) != len(set(names)):
             raise ValueError("Duplicate names are not allowed.")
+        if not is_list_like(names, allow_sets=False):
+            raise ValueError("Names should be an ordered collection.")
 
 
 def _read(filepath_or_buffer: FilePathOrBuffer, kwds):

pandas/tests/io/parser/test_common.py

@@ -2135,3 +2135,13 @@ def test_no_header_two_extra_columns(all_parsers):
     parser = all_parsers
     df = parser.read_csv(stream, header=None, names=column_names, index_col=False)
     tm.assert_frame_equal(df, ref)
+
+
+def test_read_csv_names_not_accepting_sets(all_parsers):
+    # GH 34946
+    data = """\
+    1,2,3
+    4,5,6\n"""
+    parser = all_parsers
+    with pytest.raises(ValueError, match="Names should be an ordered collection."):
+        parser.read_csv(StringIO(data), names=set("QAZ"))