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

[Thomas Lee]

Option 4 just struck me: only optimize Name nodes if they have a Load 
ctx. This makes even more sense: in a Store context, we almost 
invariably want the name rather than the constant.

Cheers,
T

Thomas Lee wrote:
> 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 test_xmlrpc ... 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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