[Python-Dev] xmlrpclib.{True, False} (was Re: Assignment to None) (original) (raw)

Thomas Lee tom at vector-seven.com
Sun Jun 15 15:11:36 CEST 2008

My work on the AST optimizer has led me down the path of attempting to replace things like Name("True") with Const(Py_True) nodes. This works fine most of the time, with the exception of the xmlrpclib module, where True and False are actually redefined:

True, False = True, False

As I stated in an earlier email, the optimizer tries to replace the tuple of Name nodes on the LHS with Py_True and Py_False respectively, which has the net effect of removing xmlrpclib.{True, False}. Obviously undesirable.

The simplest options I can think of to remedy this:

  1. A setattr hack: setattr(import(name), "True", True)
  2. Remove all optimization of Name("True") and Name("False")
  3. Skip AST optimization entirely for the LHS of Assignment nodes (effectively removing any optimization of the "targets" tuple)

I'm leaning towards #3 at the moment as it seems like it's going to be the cleanest approach and makes a lot of sense -- at least on the surface. Can anybody think of problems with this approach?

Cheers, T

Thomas Lee wrote:

Martin v. Löwis wrote:

The question is, what is the specification for Python.

Now, that's a more interesting question than the question originally asked (which I interpreted as "why does it work the way it works"). The only indication in the specification of that feature I could find was: http://docs.python.org/dev/library/constants.html "Changed in version 2.4: Assignments to None are illegal and raise a SyntaxError." Now, given that this talks about the built-in namespace, this doesn't specify that foo.None=1 should also raise a syntax error. So the implementation apparently deviates from the specification. In Python 3, None, True, and False are keywords, so clearly, the intended semantics is also the implemented one (and the language description for 2.x needs to be updated/clarified). Interestingly enough, the semantics of True, False and None are different from one another in 2.6: True = "blah" and False = 6 are perfectly legal in Python <=2.6. Funny, I just ran into this. I was trying to figure out why the AST optimization code was breaking testxmlrpc ... turns out xmlrpclib defines xmlrpclib.True and xmlrpclib.False and the optimizer was trying to resolve them as constants while compiling the module. Ouch. What happened in 3k? Were the constants in xmlrpclib renamed/removed? Cheers, T

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