[Python-ideas] Loop manager syntax

[Andrew Barnert]

On Jul 28, 2015, at 15:28, Todd <toddrjen at gmail.com> wrote:
> 
> Following the discussion of the new "async" keyword, I think it would be useful to provide a generic way to alter the behavior of loops.  My idea is to allow a user to take control over the operation of a "for" or "while" loop.  

It strikes me that allowing control of comprehensions rather than loop statements might get you some of the desired benefits, while avoiding most of the problems I described in my previous email.

The major difference is that comprehensions can only contain expressions, not statements, so all the issues of scopes, break and friends, etc. go away (or are already handled by the way comprehension functions are compiled). While yield is allowed inside a comprehension, it's very weird to do so. (You're essentially turning the list-building function into a generator function that returns the built list as the argument to its StopIteration; I suspect this is only legal because nobody felt the need to write the code to make it illegal, not because anyone found it useful?)

You could come up with a syntax like this:

    values = [with pool spam(x) for x in iterable]

The semantics are still a bit complicated (and still need to be defined, because what method(s) this should call with what arguments and what they should do is still not obvious), but they might not require any new kinds of objects or any new compilation modes or anything like that.

I'm not sure how useful this would be, because you can always just wrap the expression in a lambda (or, in this case, use spam as-is) and call pool.map instead.