gh-76728: Coerce DictReader and DictWriter fieldnames argument to a list

Doc/library/csv.rst

@@ -167,6 +167,9 @@ The :mod:`csv` module defines the following classes:
    All other optional or keyword arguments are passed to the underlying
    :class:`reader` instance.
 
+   If the argument passed to *fieldnames* is not a :term:`sequence`,
+   a :exc:`TypeError` is raised.
+
    .. versionchanged:: 3.6
       Returned rows are now of type :class:`OrderedDict`.
 
@@ -209,6 +212,9 @@ The :mod:`csv` module defines the following classes:
    Note that unlike the :class:`DictReader` class, the *fieldnames* parameter
    of the :class:`DictWriter` class is not optional.
 
+   If the argument passed to *fieldnames* is not a
+   :mod:`sequence <collections.abc>`, a :exc:`TypeError` is raised.
+
    A short usage example::
 
        import csv

Lib/csv.py

@@ -10,6 +10,7 @@
                  QUOTE_MINIMAL, QUOTE_ALL, QUOTE_NONNUMERIC, QUOTE_NONE, \
                  __doc__
 from _csv import Dialect as _Dialect
+from _collections_abc import Sequence
 
 from io import StringIO
 
@@ -80,6 +81,8 @@ class unix_dialect(Dialect):
 class DictReader:
     def __init__(self, f, fieldnames=None, restkey=None, restval=None,
                  dialect="excel", *args, **kwds):
+        if fieldnames is not None and not isinstance(fieldnames, Sequence):
+            raise TypeError(f"Expected a sequence, got {type(fieldnames)}")
         self._fieldnames = fieldnames   # list of keys for the dict
         self.restkey = restkey          # key to catch long rows
         self.restval = restval          # default value for short rows
@@ -130,6 +133,8 @@ def __next__(self):
 class DictWriter:
     def __init__(self, f, fieldnames, restval="", extrasaction="raise",
                  dialect="excel", *args, **kwds):
+        if not isinstance(fieldnames, Sequence):
+            raise TypeError(f"Expected a sequence, got {type(fieldnames)}")
         self.fieldnames = fieldnames    # list of keys for the dict
         self.restval = restval          # for writing short dicts
         if extrasaction.lower() not in ("raise", "ignore"):

Lib/test/test_csv.py

@@ -736,6 +736,36 @@ def test_write_field_not_in_field_names_ignore(self):
         csv.DictWriter.writerow(writer, dictrow)
         self.assertEqual(fileobj.getvalue(), "1,2\r\n")
 
+    def test_dict_reader_accepts_only_sequences(self):
+        fieldnames = ["a", "b", "c"]
+        f = StringIO()
+        self.assertRaises(TypeError, csv.DictReader, f, iter(fieldnames))
+
+        try:
+            reader = csv.DictReader(f, fieldnames)
+        except:
+            self.fail("csv.DictReader(f, fieldnames) threw an error")
+
+    def test_dict_writer_accepts_only_sequences(self):
+        fieldnames = ["a", "b", "c"]
+        f = StringIO()
+        self.assertRaises(TypeError, csv.DictWriter, f, iter(fieldnames))
+
+        try:
+            writer = csv.DictWriter(f, fieldnames)
+        except:
+            self.fail("csv.DictWriter(f, fieldnames) threw an error")
+
+
+    def test_dict_reader_fieldnames_is_optional(self):
+        fieldnames = ["a", "b", "c"]
+        f = StringIO()
+
+        try:
+            reader = csv.DictReader(f, fieldnames=None)
+        except:
+            self.fail("csv.DictReader(f, fieldnames=None) threw an error")
+
     def test_read_dict_fields(self):
         with TemporaryFile("w+", encoding="utf-8") as fileobj:
             fileobj.write("1,2,abc\r\n")

Misc/NEWS.d/next/Library/2022-04-01-09-43-54.bpo-32547.NIUiNC.rst

@@ -0,0 +1,2 @@
+The constructors for :class:`~csv.DictWriter` and :class:`~csv.DictReader` raise :exc:`TypeError` when
+the ``fieldnames`` argument is not a sequence.