[Python-Dev] context manager - generator interaction? (original) (raw)

Guido van Rossum guido at python.org
Thu Apr 5 22:10:06 CEST 2007

Isn't this violating the rule that a try/except should only enclose the smallest expression where the exception is expected?

BTW this is probably more appropriate for c.l.py -- even if there's a change to the language to come out of the discussion, it should be held there until the use case is well established. But my suspicion is that you get what you ask for with an overly-wide try block.

--Guido

On 4/5/07, Bob Sidebotham <bob.sidebotham at gmail.com> wrote:

The interaction shown below feels like a bug, or at least very much a trap for the unwary. It's some sort of interaction between a context manager and a generator, both of which can raise StopIteration. The code is excised from a real application, so it's a bit artificial looking. Nevertheless, it represents an entirely natural evolution of some code (whereby the call to a context manager was added deep within the code). Since it seems to work with open as the context manager, but not with my own null context manager defined with contextmanager(), it feels like this is probably a bug in contextmanager().

I did look in the archives, and it's possible this has been raised before, but I wasn't clear on that. So apologies in advance if this is already understood. This is using the stock distribution of python 2.5 on Linux 2.4. Comments? Bob Sidebotham bash-2.05b$ cat test.py from future import withstatement def gen(): return# Should cause stop iteration yield # Just to make this a generator, for this example # This works as expected. try: gen().next() # Should immediately raise StopIteration except StopIteration: print "Part 1: should reach here!" else: print "Part 1: shouldn't reach here!" # Now try the same thing, but with a context manager wrapping the generator call. # This also works as expected. try: with open("/etc/passwd") as pw: gen().next() except StopIteration: print "Part 2: should reach here!" else: print "Part 2: shouldn't reach here!" # Finally, try this with our own home grown null context manager. # This does not work as expected. from contextlib import contextmanager @contextmanager def ctxt(): yield try: with ctxt(): gen().next() except StopIteration: print "Part 3: should reach here!" else: print "Part 3: shouldn't reach here!" bash-2.05b$ python test.py Part 1: should reach here! Part 2: should reach here! Part 3: shouldn't reach here! bash-2.05b$

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-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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