[Types-sig] type declaration syntax

[Guido van Rossum]

[John Skaller]
> 1) Ban rebinding module variables after importing is done
> 	
> 2) Specify that 'def' creates an immutable binding. You can't
>    rebind (or del) the name. Same for 'class'. [This includes
>     defs inside classes]
> 
> 3) It isn't necessary to introduce 'const', at least for modules and
> classes:
>    Here's why:
>    Given (1), the only ways to change a module level variable are:
> 
> 	a) a module level assignment -- easy to detect
> 	b) an assignement in a function defined in the same module,
>            which has a global directive for that variable
> 	c) exec statement [if you see one, abandon hope of optimisation]
> 	d) __dict__ hackery from outside the module
> 
> The only hard case is (d). This cannot be statically checked.
> It can be banned all the same (and on the programmers head be it),
> 
> I.e. TWO bans fix most problems. The ban on module level rebindings
> is a significant restriction. The ban on changing 'def' and 'class'
> bindings can be worked around [use another variable] and so
> no functionality is lost, provided that the author takes
> this into account. EG:
> 
> 	class X:
> 		def f(..): pass
> 		g = f # g can be rebound, f cannot be
> 
> I think this covers:
> 
> 	caching all module and class level variables
> 	caching instance methods
> 
> what is not covered is caching instance attributes.

Agreed.  I proposed most of this a week ago.  However, I don't see why
you propose to disallow rebinding def and class.  I proposed to have
a warning for this.  If they are rebound, it is easily detected, so
the effects can simply be calculated by the type checker.

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