[Python-Dev] More on contextlib - adding back a contextmanager decorator

Mon May 1 02:36:35 CEST 2006

On 4/30/06, Guido van Rossum <guido at python.org> wrote:
> On 4/30/06, Nick Coghlan <ncoghlan at iinet.net.au> wrote:
> > A few things from the pre-alpha2 context management terminology review have
> > had a chance to run around in the back of my head for a while now, and I'd
> > like to return to a topic Paul Moore brought up during that discussion.
>
> I believe the context API design has gotten totally out of hand.
> Regardless of the merits of the "with" approach to HTML generation
> (which I personally believe to be an abomination), I don't see why the
> standard library should support every possible use case with a
> custom-made decorator. Let the author of that tag library provide the
> decorator.
>
> I have a counter-proposal: let's drop __context__. Nearly all use
> cases have __context__ return self. In the remaining cases, would it
> really be such a big deal to let the user make an explicit call to
> some appropriately named method? The only example that I know of where
> __context__ doesn't return self is the decimal module. So the decimal
> users would have to type
>

+1.

-Brett

>   with mycontext.some_method() as ctx:    # ctx is a clone of mycontext
>       ctx.prec += 2
>       <BODY>
>
> The implementation of some_method() could be exactly what we currently
> have as the __context__ method on the decimal.Context object. Its
> return value is a decimal.WithStatementContext() instance, whose
> __enter__() method returns a clone of the original context object
> which is assigned to the variable in the with-statement (here 'ctx').
>
> This even has an additional advantage -- some_method() could have
> keyword parameters to set the precision and various other context
> parameters, so we could write this:
>
>   with mycontext.some_method(prec=mycontext.prec+2):
>       <BODY>
>
> Note that we can drop the variable too now (unless we have another
> need to reference it). An API tweak for certain attributes that are
> often incremented or decremented could reduce writing:
>
>   with mycontext.some_method(prec_incr=2):
>       <BODY>
>
> --
> --Guido van Rossum (home page: http://www.python.org/~guido/)
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