[Python-Dev] Example for PEP 343

[Guido van Rossum]

[Raymond Hettinger]
> The sin() example is correct.  The precision is changed and restored in
> the current context.

I got that eventually. :-)

> However, for a general purpose wrapper, it is preferable to make a
> context copy and then restore the context after the enclosed is run.
> That guards against the enclosed block making any unexpected context
> changes.

(Although if people get in the habit of using the provided wrappers
and the do-statement, there won't be any unexpected changes.)

> Also, since the wrapper is intended to work like a try/finally, it will
> make sure the context gets restored even if an exception is raised at
> some unexpected point in the middle of the computation.

Yes, that's the point of the do-statement. :-

Anyway, perhaps we should provide this most general template:

  @do_template
  def with_decimal_context():
      oldctx = decimal.getcontext()
      newctx = oldctx.copy()
      decimal.setcontext(newctx)
      yield newctx
      decimal.setcontext(oldctx)

To be used like this:

  do with_decimal_context() as ctx:
      ctx.prec += 2
      # change other settings
      # algorithm goes here

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)