User defined lexical scoping... can I do this?

[Thomas Jollans]

On 09/18/2012 10:10 PM, porkfried wrote:
> I want to define a 'with' command that makes entries
> in dictionary available within the local scope, and
> stores new local variables into that dictionary.  The
> original scope should be restored on exit, and called
> functions should not see anything special.  Can I do this?

No.*

It is not possible to set locals by ways other than an assignment**, and
it is certainly not possible to set locals in a scope other than the one
you're in**.

You should simply type out the dict's name. This is a lot clearer. If
you want to minimize typing, you can give the variable a one-character name.

Also, Python scope simply doesn't work like that. There is no block
scope, only local (=function) scope, and global scope, with a dash of
non-locals to spice things up a bit.

> 
> my_dict = dict(a=1, b=2)
> with MyScope(my_dict):
>     print "A", a, "B", b
>     x = 3
> print my_dict["x"]
> print x # FAIL, unbound
> 

*You could set global variables, and remove them on exit, but this is
ugly for a number of reasons: this could overwrite existing globals if
you're not very careful, called functions would see these globals, and
they would also be exposed to other threads.

**I believe there is actually a way to edit a caller's locals, but this
is not documented, not portable across Python implementations and
versions, and you couldn't create new locals like this, so it'd be
fairly pointless here