[Python-ideas] Automatic context managers

[David Mertz]

On Apr 26, 2013, at 4:14 PM, Bruce Leban wrote:
> Yup, you're right. Clever. And I read the code a bit too quickly. :-) I think it's a bit more complicated to write a general wrapper though. For example, to handle chained calls that SQLAlchemy uses you have to know which calls should close the object and which ones shouldn't.
>     data = SafeSession(...).query(...).filter(...).order_by(...).values(...)

Yeah sure.  I was just demonstrating what's possible in a toy way.  I actually feel like simply using the 'with' statement is perfectly fine and perfectly clear.  But you *can* you could also wrap the bunch of chained methods if you wanted too.  I'd have to think about how to define that properly for a few minutes, but if you get to the point where there are various conditional exits to the chain, just use a 'with' block, for gosh sake.

  with session(...) as x:
      x.query(...)
      x.filter(...)
      if x.something():
          x.order_by(...)
      else:
          x.order_by(...)
      if not x.time_to_leave():
          data = x.values()

Or whatever details apply to your own program logic.  I don't really know SQLAlchemy, but I guess it must be that most of those chained methods return a mutated object, right? That would be easy enough to check for in the proxy, and I guess whenever it got to something that wasn't a mutated object but rather some "plain" values (a list, scalar, dict, etc), that would be time to leave the context manager.  I'll leave that as an exercise :-).


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