[Python-Dev] Iterator version of contextlib.nested

Hagen Fürstenau hfuerstenau at gmx.net
Fri Jun 12 17:18:18 CEST 2009

contextlib.nested has recently been deprecated on grounds of being 
unnecessary now that the with statement accepts multiple context 
managers. However, as has been mentioned before 
(http://mail.python.org/pipermail/python-dev/2009-May/089359.html), that 
doesn't cover the case of a variable number of context managers, i.e.

with contextlib.nested(*list_of_managers) as list_of_results:


with contexlib.nested(*iterator_of_managers):

It was suggested that in these use cases a custom context manager should 
be implemented. However, it seems that such an implementation would be 
an almost exact copy of the present code for "nested".

I'm proposing to add an iterator version of "nested" to contextlib 
(possibly called "inested"), which takes an iterable of context managers 
instead of a variable number of parameters. The implementation could be 
taken over from the present "nested", only changing "def 
nested(*managers)" to "def inested(managers)".

This has the advantage that an iterator can be passed to "inested", so 
that each context managers is created in the context of all previous 
ones, which was one of the reasons for introducing the multi-with 
statement in the first place. "contextlib.inested" would therefore be 
the generalization of the multi-with statement to a variable number of 
managers (and "contextlib.nested" would stay deprecated).

- Hagen

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