Python handles globals badly.

Rustom Mody rustompmody at gmail.com
Fri Sep 11 18:08:24 CEST 2015


On Friday, September 11, 2015 at 9:27:46 PM UTC+5:30, rand... at fastmail.us wrote:
> On Fri, Sep 11, 2015, at 11:55, Chris Angelico wrote:
> > On Sat, Sep 12, 2015 at 1:49 AM, Ian Kelly  wrote:
> > > Ah, that makes sense. It's writing into the dict that is created and
> > > returned by locals(), but not actually updating the frame locals which
> > > are the source of truth.
> > 
> > Yeah... but it only makes sense to people who understand the
> > implementation. It's certainly not a logical and sane behaviour that
> > would be worth documenting and using.
> 
> What else would you document? Reading from them is a reasonable thing to
> do, and works. Writing to them is a reasonable thing to want to do, but
> won't work, so you need to document that it doesn't work.

This is actually an old elusive holy grail -- first class environments.
In denotational semantics the two tools used to model variables and control-flow
respectively are environments and continuations.
The Scheme inventors were brave enough to mandate first-class continuations
but could not make the courage for first-class environments.
So every Scheme dialect introduces 1½ class envs in a ½-assed inconsistent way

Likewise python's locals-dicts and (I am guessing) most languages with some
commitment to first-classness


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