bpo-39019: Implement missing __class_getitem__ for SpooledTemporaryFile (GH-17560)

Author: isidentical

Lib/tempfile.py

@@ -643,6 +643,18 @@ def __init__(self, max_size=0, mode='w+b', buffering=-1,
                                    'encoding': encoding, 'newline': newline,
                                    'dir': dir, 'errors': errors}
 
+    def __class_getitem__(cls, type):
+        """Provide minimal support for using this class as generic
+        (for example in type annotations).
+
+        See PEP 484 and PEP 560 for more details. For example,
+        `SpooledTemporaryFile[str]` is a valid expression at runtime (type
+        argument `str` indicates whether the file is open in bytes or text
+        mode). Note, no type checking happens at runtime, but a static type
+        checker can be used.
+        """
+        return cls
+
     def _check(self, file):
         if self._rolled: return
         max_size = self._max_size

Lib/test/test_tempfile.py

@@ -1229,6 +1229,9 @@ def test_truncate_with_size_parameter(self):
         self.assertTrue(f._rolled)
         self.assertEqual(os.fstat(f.fileno()).st_size, 20)
 
+    def test_class_getitem(self):
+        self.assertIs(tempfile.SpooledTemporaryFile[bytes],
+                      tempfile.SpooledTemporaryFile)
 
 if tempfile.NamedTemporaryFile is not tempfile.TemporaryFile:
 

Misc/NEWS.d/next/Library/2019-12-10-21-11-05.bpo-39019.YIlgZ7.rst

@@ -0,0 +1 @@
+Implement dummy ``__class_getitem__`` for :class:`tempfile.SpooledTemporaryFile`.