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Silly questions about True and False

Steve Holden steve at holdenweb.com
Mon Dec 20 14:00:34 EST 2004 
Peter Hansen wrote:

> drs wrote:
> 
>> I just upgraded my Python install, and for the first time have True and
>> False rather than 1 and 0.  I was playing around at the command line 
>> to test
>> how they work (for instance, "if 9:" and "if True:" both lead to the
>> conditional being executed, but True == 9 -> False, that this would be 
>> true
>> was not obvious to me -- "True is True" is True, while "9 is True" is 
>> false
>> even though 9 evaluates to True.)  
> 
> 
> What do you mean by "9 evalutes to True"?  That's not the
> case.  bool(9) does evaluate to True, and that's effectively
> what "if 9:" is doing...
> 
> Anyhow, in doing my tests, I accidentally
> 
>> typed
>>
>>
>>>>> False = 0
>>
>> rather than
>>
>>>>> False == 0
>>
>>
>> and I lost the False statement.
> 
I should point out a terminological inexactitude here: False is not a 
statement, it's an identifier. As Peter pointed out, that identifier can 
exist in several different namespaces, leading to your initial confusion.
> 
> To get it back, you should do "del False".  Remarkably,
> this actually just removes the local name False that
> you created which was "shadowing" the builtin name,
> allowing the builtin name (which you didn't change)
> to be seen again.
> 
> Note that this name you are creating is actually *local*
> to the module you are in, which at the interactive
> prompt is called __main__.  Thus you are not changing
> False from the point of view of any other module, or
> of the Python internals.  They are (for the most part)
> still getting it from the builtin module.
> 
> 
>> which seems to put False back to False, but this seems weird.
>>
>>>>> 1 = 0
>>
>>
>> throws an error (can't assign to literal), why doesn't False = 0 throw 
>> the
>> same error?  
> 
> 
> False is not a constant, it's merely a name.  1 and 0 are
> constants.  You can't change a constant, but you *can*
> "rebind" a name (that is, attach it to something else).
> That's all you're doing here.
> 
Note, however, that the 2.4 documentation does actually list True and 
False as constants (along with None) in section 2.5 of the Library 
Reference manual.

>> Also, why doesn't False = 0 make
>>
>>>>> 1 == 2
>>
>> 0
>> Instead of False?
> 
> 
> Because such comparisons are all effectively doing a bool()
> which continues to return the builtin False.
> 

Sadly, the builtin False and True can actually be overwritten (unlike 
None, which starting from 2.4 really *is* a constant):

$ python
Python 2.4 (#1, Dec  4 2004, 20:10:33)
[GCC 3.3.3 (cygwin special)] on cygwin
Type "help", "copyright", "credits" or "license" for more information.
  >>> None = 42
SyntaxError: assignment to None
  >>> True = 33
  >>> True
33
  >>> __builtins__.True
True
  >>> __builtins__.True = 42
  >>> True
33
  >>> del True
  >>> True
42
  >>>
Of course, even though you can change True's value, you can't make a 
comparison return __builtins__.True:

  >>> None is None
True
  >>> (None is None) + 1
2
  >>>

Here the comparison is returning the Python object to which 
__builtins__.True initially refers.

when-i-tell-you-three-times-it-is-forty-true-ly y'rs  - steve
-- 
Steve Holden               http://www.holdenweb.com/
Python Web Programming  http://pydish.holdenweb.com/
Holden Web LLC      +1 703 861 4237  +1 800 494 3119
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