Add __class_getitem__ to csv.DictReader and csv.DictWriter (#92393)

Lib/csv.py

@@ -4,6 +4,7 @@ csv.py - read/write/investigate CSV files
 """
 
 import re
+import types
 from _csv import Error, __version__, writer, reader, register_dialect, \
                  unregister_dialect, get_dialect, list_dialects, \
                  field_size_limit, \
@@ -126,6 +127,8 @@ class DictReader:
                 d[key] = self.restval
         return d
 
+    __class_getitem__ = classmethod(types.GenericAlias)
+
 
 class DictWriter:
     def __init__(self, f, fieldnames, restval="", extrasaction="raise",
@@ -156,6 +159,8 @@ class DictWriter:
     def writerows(self, rowdicts):
         return self.writer.writerows(map(self._dict_to_list, rowdicts))
 
+    __class_getitem__ = classmethod(types.GenericAlias)
+
 # Guard Sniffer's type checking against builds that exclude complex()
 try:
     complex

Lib/test/test_genericalias.py

@@ -11,6 +11,7 @@ from concurrent.futures import Future
 from concurrent.futures.thread import _WorkItem
 from contextlib import AbstractContextManager, AbstractAsyncContextManager
 from contextvars import ContextVar, Token
+from csv import DictReader, DictWriter
 from dataclasses import Field
 from functools import partial, partialmethod, cached_property
 from graphlib import TopologicalSorter
@@ -122,7 +123,8 @@ class BaseTest(unittest.TestCase):
                      WeakSet, ReferenceType, ref,
                      ShareableList,
                      Future, _WorkItem,
-                     Morsel]
+                     Morsel,
+                     DictReader, DictWriter]
     if ctypes is not None:
         generic_types.extend((ctypes.Array, ctypes.LibraryLoader))
     if ValueProxy is not None:

Misc/ACKS

@@ -1228,6 +1228,7 @@ Alessandro Moura
 Pablo Mouzo
 Mher Movsisyan
 Ruslan Mstoi
+Marc Mueller
 Valentina Mukhamedzhanova
 Michael Mulich
 Sape Mullender

Misc/NEWS.d/next/Library/2022-05-06-13-00-57.gh-issue-92391.s-Lase.rst

@@ -0,0 +1,3 @@
+Add :meth:`~object.__class_getitem__` to  :class:`csv.DictReader` and
+:class:`csv.DictWriter`, allowing them to be parameterized at runtime.
+Patch by Marc Mueller.