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

Bob Sidebotham bob.sidebotham at gmail.com
Thu Apr 5 21:54:22 CEST 2007

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 context_manager(), 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 with_statement

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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