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

[Georg Brandl]

Remember that it must still be possible to write (in 2.6)

True = 0
assert not True

Georg

Thomas Lee schrieb:
> 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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