This bug is very similar to #18879, the only difference is that _TemporaryFileWrapper.__iter__ is the problem (in #18879, __getattr__ was fixed, but __iter__ was not). The real world use case that helped me find this bug is at the bottom of this report, this is a simple reproducer:


>>> import tempfile
>>> for l in tempfile.NamedTemporaryFile(mode='a+b'):
...     print(l)
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: readline of closed file


I'm attaching a patch that fixes this (+ testcase).

Note: I actually discovered this while using


>>> from urllib.request import urlopen
>>> for l in urlopen('ftp://<some_ftp>'):
...     print(l)
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: readline of closed file


Opening FTP uses urllib.response, which in turn uses tempfile._TemporaryFileWrapper, which makes this example fail.