[issue21379] StopIteration is silenced when raised by generator within context manager

[Hannan Aharonov]

New submission from Hannan Aharonov:

If the code that defines a context manager performs some operations on a generator that cause it to raise StopIteration, this exception is propagated to the context manager's __exit__() method and there swallowed by a try..except block. 

This can cause unexpected/unintuitive behaviour.
The following simplified example shows an infinite loop caused by this:

-------
from contextlib import contextmanager

@contextmanager
def manager(gen):
    yield
    gen.next()

emptygen = (_ for _ in ())

while True:
    with manager(emptygen):
        print "Infinite Loop"
-------

In this case, even though the generator is empty, the loop will never stop, because the generator's StopIteration exception is swallowed by the context manager. (If `gen.next()` was wrapped by try..except that reraises exception as, say, RuntimeError, the program would stop.)

I've looked at the source (here: http://hg.python.org/cpython/file/tip/Lib/contextlib.py line 67)
and I understand why this is happening: the context manager can't tell the difference between the two kinds of StopIteration - one that is raised by its own completion, and one that is raised by a call in its body. 

I don't know if this is a bug or expected behavior of nested generators. In any case, I haven't seen this issue mentioned in the documentation, and the behavior is unintuitive.

I've searched the bug tracker and found a similar issue that handled swallowing of StopIteration in the code wrapped by the context manager (http://bugs.python.org/issue1705170), and another similar issue (http://bugs.python.org/issue16610) that was closed without resolution.

----------
components: Interpreter Core, Library (Lib)
messages: 217421
nosy: hannan.aha, ncoghlan
priority: normal
severity: normal
status: open
title: StopIteration is silenced when raised by generator within context manager
versions: Python 2.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21379>
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