[Python-Dev] Is outlawing-nested-import-* only an implementation issue?

[Ka-Ping Yee]

On Thu, 22 Feb 2001, Jeremy Hylton wrote:
> I can't think of another lexically scoped language that
> allows an exec or eval to create a new variable binding that can later
> be used via a plain-old reference.

I tried STk Scheme, guile, and elisp, and they all do this.

> One of the reasons I am strongly in favor of making import * and exec
> errors is that it stymies the efforts of a reader to understand the
> code.

Yes, i look forward to the day when no one will ever use import-*
any more.  I can see good reasons to discourage the use of import-*
and bare-exec in general anywhere.  But as long as they *do* have
a meaning, they had better mean the same thing at the top level as
internally.

> If we look at your examples:
>     >>> def f():                      # x = 3 inside, no g()
[...]
>     >>> def f():                      # x = 3 inside, nested g()
[...]
>     >>> def f():                      # x = 3 outside, nested g()
> 
> In these examples, it isn't at all obvious to the reader of the code
> whether the module foo contains a binding for x or whether the
> programmer intended to import that name and stomp on the exist
> definition.

It's perfectly clear -- since we expect the reader to understand
what happens when we do exactly the same thing at the top level.

> Another key difference between Scheme and Python is that in Scheme,
> each binding operation creates a new scope.

Scheme separates 'define' and 'set!', while Python only has '='.
In Scheme, multiple defines rebind variables:

    (define a 1)
    (define a 2)
    (define a 3)

just as in Python, multiple assignments rebind variables:

    a = 1
    a = 2
    a = 3

The lack of 'set!' prevents Python from rebinding variables
outside of the local scope, but it doesn't prevent Python from
being otherwise consistent and having "a = 2" do the same thing
inside or outside of a function: it binds a name in the current
scope.



-- ?!ng

"The only `intuitive' interface is the nipple.  After that, it's all learned."
    -- Bruce Ediger, on user interfaces