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From: 88888 Dihedral <dihedral88888@googlemail.com>
Newsgroups: comp.lang.python
Subject: Re: Dynamic variable creation from string
Date: Mon, 12 Dec 2011 04:49:10 -0800 (PST)
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On Monday, December 12, 2011 3:11:18 PM UTC+8, alex23 wrote:
> On Dec 8, 3:09=A0am, Massi <mass...@msn.com> wrote:
> > in my script I have a dictionary whose items are couples in the form
> > (string, integer values), say
> >
> > D =3D {'a':1, 'b':2, 'c':3}
> >
> > This dictionary is passed to a function as a parameter, e.g. :
> >
> > def Sum(D) :
> > =A0 =A0 return D['a']+D['b']+D['c']
> >
> > Is there a way to create three variables dynamically inside Sum in
> > order to re write the function like this?
> >
> > def Sum(D) :
> > =A0 =A0 # Here some magic to create a,b,c from D
> > =A0 =A0 return a+b+c
>=20
> Okay, here's a possible solution that doesn't rely on exec, but does
> use the third-party module byteplay (which I believe limits it to
> Python 2.5-2.7) and tries to retain as much as possible your syntax
> (with some slight adjustments):
>=20
>     from byteplay import Code, opmap
>=20
>     class VariableInjector(dict):
>         def transmute(self, opcode, arg):
>             if (opcode =3D=3D opmap['LOAD_GLOBAL']) and (arg in self):
>                 self._transmuted.append(arg)
>                 return opmap['LOAD_FAST'], arg
>             return opcode, arg

>=20
>         def make_locals(self, args):
>             locals =3D []
>             for arg in args:
>                 locals.append((opmap['LOAD_CONST'], self[arg]))
>                 locals.append((opmap['STORE_FAST'], arg))
>             return locals
>=20
>         def bind_to(self, function):
>             function.ofunc_code =3D function.func_code
>             def _(*args, **kwargs):
>                 self._transmuted =3D []
>                 code =3D Code.from_code(function.ofunc_code)
>                 code.code =3D [self.transmute(op, arg) for op, arg in
> code.code]
>                 code.code =3D self.make_locals(self._transmuted) +
> code.code
>                 function.func_code =3D code.to_code()
>                 return function(*args, **kwargs)
>             return _
>=20
> For your example, you'd use it like this:
>=20
>     >>> def sum():
>     ...     return a + b + c
>     ...
>     >>> def product():
>     ...     return a * b * c
>     ...
>     >>> data =3D VariableInjector(a=3D1,b=3D2,c=3D3)
>     >>> sum =3D data.bind_to(sum)
>     >>> product =3D data.bind_to(product)
>     >>> sum()
>     6
>     >>> product()
>     6
>     >>> data
>     {'a': 1, 'c': 3, 'b': 2}
>     >>> data['a'] =3D 100
>     >>> sum()
>     105
>     >>> product()
>     600
>=20
> I'm not sure how rigorous this would be in real use but it's passed
> the few quick toy cases I've tried it out on.
>=20
> Any thanks should go to Michael Foord, as this borrows heavily from
> his self-less metaclass example:
> http://www.voidspace.org.uk/python/articles/metaclasses.shtml

This is the way to write an assembler or=20
to roll out a script language to be included in an app
by users.=20