gh-76728: Coerce DictReader and DictWriter fieldnames argument to a list (GH-32225)

Author: dignissimus

Doc/library/csv.rst

@@ -167,6 +167,8 @@ 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 an iterator, it will be coerced to a :class:`list`.
+
    .. versionchanged:: 3.6
       Returned rows are now of type :class:`OrderedDict`.
 
@@ -209,6 +211,8 @@ 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 an iterator, it will be coerced to a :class:`list`.
+
    A short usage example::
 
        import csv

Lib/csv.py

@@ -81,6 +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 iter(fieldnames) is fieldnames:
+            fieldnames = list(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
@@ -133,6 +135,8 @@ def __next__(self):
 class DictWriter:
     def __init__(self, f, fieldnames, restval="", extrasaction="raise",
                  dialect="excel", *args, **kwds):
+        if fieldnames is not None and iter(fieldnames) is fieldnames:
+            fieldnames = list(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 +736,34 @@ 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_fieldnames_accepts_iter(self):
+        fieldnames = ["a", "b", "c"]
+        f = StringIO()
+        reader = csv.DictReader(f, iter(fieldnames))
+        self.assertEqual(reader.fieldnames, fieldnames)
+
+    def test_dict_reader_fieldnames_accepts_list(self):
+        fieldnames = ["a", "b", "c"]
+        f = StringIO()
+        reader = csv.DictReader(f, fieldnames)
+        self.assertEqual(reader.fieldnames, fieldnames)
+
+    def test_dict_writer_fieldnames_rejects_iter(self):
+        fieldnames = ["a", "b", "c"]
+        f = StringIO()
+        writer = csv.DictWriter(f, iter(fieldnames))
+        self.assertEqual(writer.fieldnames, fieldnames)
+
+    def test_dict_writer_fieldnames_accepts_list(self):
+        fieldnames = ["a", "b", "c"]
+        f = StringIO()
+        writer = csv.DictWriter(f, fieldnames)
+        self.assertEqual(writer.fieldnames, fieldnames)
+
+    def test_dict_reader_fieldnames_is_optional(self):
+        f = StringIO()
+        reader = csv.DictReader(f, fieldnames=None)
+
     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 @@
+The constructors for :class:`~csv.DictWriter` and :class:`~csv.DictReader` now coerce the ``fieldnames`` argument to a :class:`list` if it is an iterator.