outsmarting context managers with coroutines

Burak Arslan burak.arslan at arskom.com.tr
Sat Dec 28 23:13:56 CET 2013


Hi,

Have a look at the following code snippets:
https://gist.github.com/plq/8164035

Observations:

output2: I can break out of outer context without closing the inner one
in Python 2
output3: Breaking out of outer context closes the inner one, but the
closing order is wrong.
output3-yf: With yield from, the closing order is fine but yield returns
None before throwing.

All of the above seem buggy in their own way. And actually, Python 2
seems to leak memory when generators and context managers are used this way.

Are these behaviours intentional? How much of it is
implementation-dependent? Are they documented somewhere? Neither PEP-342
nor PEP-380 talk about context managers and PEP-343 talks about
generators but not coroutines.

My humble opinion:

1) All three should behave in the exact same way.
2) Throwing into a generator should not yield None before throwing.

Best,
Burak

ps: I have:

$ python -V; python3 -V
Python 2.7.5
Python 3.3.2



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