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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<http://bugs.python.org/issue21379>
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