[Python-Dev] PEP 292, Simpler String Substitutions

Jeremy Hylton jeremy@zope.com
Wed, 19 Jun 2002 11:29:13 -0400

>>>>> "GvR" == Guido van Rossum <guido@python.org> writes:

  >> > BTW, you can't use locals() or globals() because you really
  >> > want globals()-overridden-with-locals(), i.e.
  >> >
  >> >     d = globals().copy() d.update(locals())
  >> What about free/cell vars?  Will these be used?  If not, is that
  >> a problem?

  GvR> Without compiler support for this construct we have no hope of
  GvR> getting references to outer non-global scopes right.  E.g.

  GvR> def f():
  GvR>     x = 12

  GvR>     def g():
  GvR>         return "x is $x".sub()

  GvR>     return g

  GvR> Here the compiler has no clue that g references x, so it
  GvR> wouldn't do the special treatment for x that's needed to make
  GvR> it work.

  GvR> I see no way to fix this in general without introducing new
  GvR> syntax; note that the string "x is $x" could have been an
  GvR> argument to g().

If Python had macros, then we could define the interpolation function
as a macro.  It would expand to explicit references to all the
variables in the block that called the macro.  Then the compiler could
do the right thing.

Of course, we ain't got macros, but whatever.  I think they would
provide cleaner support for interpolation than sys._getframe().