But the visitor pattern is pretty grim, really. It would be nice (tm) to have something like:
match node in: Assign(lhs=Var(_), rhs=_): # lhs, rhs bound in here Assign(lhs=Subscr(_,_), rhs=_): # ditto Assign(lhs=Slice(*_), rhs=_): # ditto Assign(lhs=_, rhs=_): raise SyntaxError
in Lib/compiler.
FWIW, I do intend to add this sort of thing to PyProtocols' predicate dispatch system. Actually, I can dispatch on rules like the above now, it's just that you have to spell out the cases as e.g.:
@do_it.when("isinstance(node, Assign) and isinstance(node.lhs, Subscr)")
def do_subscript_assign(node, ...):
...I'd like to create a syntax sugar for pattern matching though, that would let you 1) use a less verbose way of saying the same thing, and 2) let you bind the intermediate values to variables that then become accessible in the function body as locals.
Anyway, the main holdup on this is deciding what sort of Python syntax abuse should represent variable bindings. :) Maybe something like this will be suitably horrific:
@do_it.when("node in Assign.match(lhs=`lhs` in Subscr,rhs=`rhs`)")
def do_subscript_assign((lhs,rhs), node, ...):
...But I think maybe here the cure is worse than the disease. :) Pushed this far, it seems to beg for new syntax to accommodate in-expression variable bindings, something like 'var:=value'. Really, though, the problem is probably just that inline variable binding is downright unpythonic. The only time Python does anything vaguely similar is with the 'except type,var:' syntax.
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