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

[Brett Cannon]

On Fri, Nov 22, 2013 at 7:29 AM, Benjamin Peterson <benjamin at python.org>wrote:

> 2013/11/22 anatoly techtonik <techtonik at gmail.com>:
> > On Fri, Nov 15, 2013 at 5:43 PM, Benjamin Peterson <benjamin at python.org>
> wrote:
> >> 2013/11/15 anatoly techtonik <techtonik at gmail.com>:
> >>> On Tue, Nov 12, 2013 at 5:08 PM, Benjamin Peterson <
> benjamin at python.org> wrote:
> >>>> 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.
> >>>
> >>> Oh my. Now there is also CST in addition to AST. This stuff -
> >>> http://docs.python.org/devguide/ - badly needs diagrams about data
> >>> transformation toolchain from Python source code to machine
> >>> execution instructions. I'd like some pretty stuff, but raw blogdiag
> >>> hack will do the job http://blockdiag.com/en/blockdiag/index.html
> >>>
> >>> There is no set_expr_context in my copy of CPython code, which
> >>> seems to be some alpha of Python 3.4
> >>
> >> It's actually called set_context.
> >
> > Ok. So what is the process?
> >
> >  SOURCE --> TOKEN STREAM --> SENTENCE STREAM --> CST -->
> > --> AST --> BYTECODE
> >
> > Is that right?
>
> I don't know what sentence stream is, but otherwise looks right.


If you want more of an overview:
http://pyvideo.org/video/2331/from-source-to-code-how-cpythons-compiler-works
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