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

Doc/library/csv.rst

@@ -153,7 +153,9 @@ The :mod:`csv` module defines the following classes:
    information in each row to a :class:`dict` whose keys are given by the
    optional *fieldnames* parameter.
 
-   The *fieldnames* parameter is a :term:`sequence`.  If *fieldnames* is
+   The *fieldnames* parameter is a :term:`sequence`.  If the argument passed
+   to *fieldnames* is not a :term:`sequence` then a :exc:`TypeError` is
+   raised.  If *fieldnames* is
    omitted, the values in the first row of file *f* will be used as the
    fieldnames.  Regardless of how the fieldnames are determined, the
    dictionary preserves their original ordering.
@@ -195,7 +197,9 @@ The :mod:`csv` module defines the following classes:
    onto output rows.  The *fieldnames* parameter is a :mod:`sequence
    <collections.abc>` of keys that identify the order in which values in the
    dictionary passed to the :meth:`writerow` method are written to file
-   *f*.  The optional *restval* parameter specifies the value to be
+   *f*.  If the argument passed to *fieldnames* is not a
+   :mod:`sequence <collections.abc>`, a :exc:`TypeError` is raised.  The optional
+   *restval* parameter specifies the value to be
    written if the dictionary is missing a key in *fieldnames*.  If the
    dictionary passed to the :meth:`writerow` method contains a key not found in
    *fieldnames*, the optional *extrasaction* parameter indicates what action to

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 not isinstance(fieldnames, Sequence):
+            raise TypeError("fieldnames must be a sequence")
         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("fieldnames must be a sequence")
         self.fieldnames = fieldnames    # list of keys for the dict
         self.restval = restval          # for writing short dicts
         if extrasaction.lower() not in ("raise", "ignore"):

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

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