[Python-ideas] how rowexpr would be implemented

[Steve Howell]

--- Stargaming <stargaming at gmail.com> wrote:

> 
> > ?  Specifically, how would python know what to
> dereference and what not 
> > to?  What if there were two things, named the
> same, one in the row and 
> > one in the namespace?  (i.e. a variable named
> 'salary')  
> 
> That's a harder one, indeed. Perhaps, nothing should
> be dereferenced (at
> ``compile'' time, don't know if that's a valid term
> in CPython's
> interpreter chain tho), as it is in a
> lambda-expression.
> 
> 

Admitting to not fully understanding the Python
innards, here's some food for thought on how this
would be done:

  1) Not to diminish the parsing-into-AST stage, I
won't cover that here, as other problems are trickier.
 Despite pretty different semantics, I think rowexpr
would look a lot like lambda at the syntactic level.

  2) You'd add a case to compiler_visit_expr for
Rowexpr_kind in compile.c.

  3) There's a pretty small method in compile.c called
compiler_lambda that would be the model for
compiler_rowexpr.  The compiler_rowexp would generate
an opcode that caused the single (implied,
C++-"this"-like) arg to be popped, and possibly that
opcode would have to be different than a normal opcode
to pop one argument off the stack, so that ceval.c can
do the right thing, but I'm not so sure.

  4) You might need to add something to compiler_unit
that is analogous to u_varnames, u_cellvars,
u_freevars, etc., so that when you recursively call
down to evaluate the expression in the rowexpr, the
lower methods know to look in a different place to
scope the words "salary," "convert_to_euros," and
"dept" in the example expression 'where
convert_to_euros(salary) > 50000 an dept = "software
development"'.

  5) The behavior of compiler_expr would NOT change
for most operators, such as Num_kind, Str_kind,
Attribute_kind, Subscript_kind, etc., as rowexpr
doesn't change how any of those are intrerpreted
compared to an expression in other scopes.

  6) Compiler_expr eventually gets to building opcodes
that give you salary, convert_to_euros, and dept, and
those pieces of code need to look into compiler_unit
to determine if they're part of the rowexpr, and do
the right thing.

  7) I'm sure there's more.  I'm here to learn.







       
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