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

Benjamin Peterson benjamin at python.org
Tue Nov 12 15:08:34 CET 2013

2013/11/12 anatoly techtonik <techtonik at gmail.com>:
> 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?

Only valid expression targets are allowed during AST construction. See
set_expr_context in ast.c.

> 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?

I'm not sure what you mean by an `expr` subset.


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