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I think this is a consequence of PEP380 and its decision to finalize the subgenerator when the delegating generator is closed.
Consider this simple example without tempfile:
def yielder (fileobj):
 yield from fileobj
with open('some_test_file', 'w') as f:
 f.write('line one\nline two\nline three')
with open('some_test_file', 'r') as f:
 line = next(yielder(f))
 nline = next(f)
==>
Traceback (most recent call last):
 File "<pyshell#11>", line 3, in <module>
 nline = next(f)
ValueError: I/O operation on closed file.
I think test_csv does the file-closing operation on lines 626/627 when it creates the temporary csv.reader(fileobj).
 def test_read_dict_fieldnames_from_file(self):
 with TemporaryFile("w+") as fileobj:
 fileobj.write("f1,f2,f3\r\n1,2,abc\r\n")
 fileobj.seek(0)
 reader = csv.DictReader(fileobj,
 fieldnames=next(csv.reader(fileobj)))
 self.assertEqual(reader.fieldnames, ["f1", "f2", "f3"])
 self.assertEqual(next(reader), {"f1": '1', "f2": '2', "f3": 'abc'})