[Python-Dev] PEP 377 - allow __enter__() methods to skip the statement body

[glyph at divmod.com]

On 12:56 pm, ncoghlan at gmail.com wrote:
>PEP 377 is a proposal to allow context manager __enter__() methods to
>skip the body of the with statement by raising a specific (new) flow
>control exception.
>
>Since there is a working reference implementation now, I thought it was
>time to open it up for broader discussion.

Why not allow a context manager to implement some other method, for the 
sake of argument let's say "__start__", which was invoked with a 
callable object and could choose to evaluate or not evaluate the 
statement body by simply not calling that object (or perhaps iterable, 
in the case of a generator)?

This PEP proposes that we have two ways to deal with the body of a 
'with' statement: either the body is run or not.  I have always wanted 
to have another option: run the body later.

Passing around an object representing the body of the with statement 
would allow for this use-case, as well as removing the ugly protrusion 
of yet another control-flow exception (which, as has already been noted, 
creates difficulties for some other python implementations).