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You are probably right that the io classes are broken.
From https://docs.python.org/3/library/stdtypes.html#iterator-types:
Once an iterator’s __next__() method raises StopIteration, it must continue to do so on subsequent calls. Implementations that do not obey this property are deemed broken.
One consequence of __iter__ returning self is that the above is not guaranteed:
>>> with open('somefile', 'w') as f:
	f.write('some text')
	
9
>>> with open('somefile', 'r') as f:
	i = iter(f)
	assert f is i
	for line in i:
		print(line)
	try:
		next(i)
	except StopIteration:
		print('exhausted iterator')
	f.seek(0)
	print(next(i))
	
some text
exhausted iterator
0
some text
So the io classes are *officially* broken.