[Python-Dev] Assign(expr* targets, expr value) - why targetS?

[anatoly techtonik]

On Sun, Nov 10, 2013 at 8:34 AM, Benjamin Peterson <benjamin at python.org> wrote:
> 2013/11/10 anatoly techtonik <techtonik at gmail.com>:
>> http://hg.python.org/cpython/file/1ee45eb6aab9/Parser/Python.asdl
>>
>> In Assign(expr* targets, expr value), why the first argument is a list?
>
> x = y = 42

Thanks.

Speaking of this ASDL. `expr* targets` means that multiple entities of
`expr` under the name 'targets' can be passed to Assign statement.
Assign uses them as left value. But `expr` definition contains things
that can not be used as left side assignment targets:

    expr = BoolOp(boolop op, expr* values)
         | BinOp(expr left, operator op, expr right)
         ...
         | Str(string s) -- need to specify raw, unicode, etc?
         | Bytes(bytes s)
         | NameConstant(singleton value)
         | Ellipsis

         -- the following expression can appear in assignment context
         | Attribute(expr value, identifier attr, expr_context ctx)
         | Subscript(expr value, slice slice, expr_context ctx)
         | Starred(expr value, expr_context ctx)
         | Name(identifier id, expr_context ctx)
         | List(expr* elts, expr_context ctx)
         | Tuple(expr* elts, expr_context ctx)

If I understand correctly, this is compiled into C struct definitions
(Python-ast.c), and there is a code to traverse the structure, but
where is code that validates that the structure is correct? Is it done
on the first level - text file parsing, before ASDL is built? If so,
then what is the role of this ADSL exactly that the first step is
unable to solve?

Is it possible to fix ADSL to move `expr` that are allowed in Assign
into `expr` subset? What effect will it achieve? I mean - will ADSL
compiler complain about wrong stuff on the left side, or it will still
be a role of some other component. Which one?