while 1 vs while True

[Nick Coghlan]

Paul Rubin wrote:
> "Raymond Hettinger" <vze4rx4y at verizon.net> writes:
> 
>>It is unlike to before Py3.0.  Making them constants would break the
>>reams of compatability code: True, False = (1==1), (1!=1).
> 
> 
> I don't see why that particular statement needs to fail.  The
> interpreter could permit assigning True=True or False=False without
> raising an error.  Then True and False wouldn't really be constants,
> but they'd instead be variables whose value was always known to the
> compiler, which for optimization purposes is just as good.  The
> compiler could alternatively do some simple dataflow analysis to make
> sure the values of True and False haven't been changed in a particular
> module, before doing that particular optimization.

Until this code:

.>>> import pdb
.>>> pdb.True = 0
.>>> pdb.x = "Darn writeable module dictionaries"
.>>> from pdb import True
.>>> True
0
.>>> from pdb import x
.>>> x
'Darn writeable module dictionaries'


is illegal, the compiler is really limited in what it can safely assume about 
the contents of module dictionaries. There's such a thing as being *too* 
flexible - and in this case, the flexibility eliminates some potential 
optimisations (in this case, "I know what this is" compile time name binding).

I believe modifying a module's globals dictionary from outside the module is in 
the process of being deprecated - the catch is that distinguishing it from some 
legitimate accesses to a module's globals is not straightforward.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at email.com   |   Brisbane, Australia
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