True inconsistency in Python

Ron Adam radam2 at tampabay.rr.com
Mon Nov 17 18:40:20 CET 2003


On Mon, 17 Nov 2003 00:19:11 -0800, Erik Max Francis <max at alcyone.com>
wrote:

>Tim Roberts wrote:
>
>> Python is certainly not the only language in which this occurs.  Many
>> are
>> the C programmers who have been burned by:
>> 
>>   int SomeFunction();
>>     ...
>>   if( SomeFunction == TRUE )
>>   {
>>   }
>
>I think you're getting burned by something else, there :-).
>
>> Visual Basic has exactly the same problem.
>
>It's not a problem, explicitly comparison with a Boolean literal,
>whether or not Booleans are not distinct types (Lisp, C89), are distinct
>types with implicit conversion (C++, Python), or are distinct types in
>which implicit conversions are disallowed (Java) is completely
>superfluous.  The proper way to test whether an expression is true is
>
>	if (expression)
>	    ...


To me the usefulness of using True and False is that it is defined to
values consistent with the programming language that you are using.
So using them to assign x = True,  or x = False.  insures that when I
do:

	x = True
	if x:
		....

So I get consistent results for the language and platform I'm using
now and in the future.  If down the road someone decided to make True
= 3, and False = -5,  and they change the language so that any boolean
comparisons return 3 and -5 respectively,  my use of True and False
will still work.  If I tested for 1 or 0 using the 'if x:' method,
then my program will break.

	x = 1
	if x:		# returned value undefined,  x is not 3 or -5

 So I do an explicit test 'if (x=1)' to make sure it will work even if
'True' gets redefined.

So we should either use explicit comparisons of values, or use only
True and False,  but not mix them.  


>
>What is the point of the explicit test with the true constant?  What
>about the result of _that_ comparison, shouldn't that be tested with
>true as well -- ((expression == TRUE) == TRUE)?  But what about _that_
>result, shouldn't it be tested too?
>
>Explicit comparison with the true constant (or false constant)
>necessarily degenerates into complete uselessness.
>
>> Further, until very
>> recently,
>> True in VB actually evaulated to -1, so even comparing to "1" would
>> fail.
>
>That's because in traditional BASIC, "true" is all-bits on.  The true
>constant wasn't 1 in the first place.
>
>> >Assuming the old behavior is desired, programmers need to be careful
>> >not to compare a variable with True as in:
>> >
>> >if var == True: # only works if var is 1
>> >  blah
>> 
>> Your statement is absolutely true.  End of story.
>
>It's true in that that might cause a problem, but it's not true that
>that's undesirable language behavior.  It's programmer error -- in any
>language that has a Boolean true (or false) literal.


It looks to me that 'True' in python is a combination of the boolean
binary logic of 1 or 0, and as an "exists" test of 0 or not 0.  

If python had an exists operator, you could do.

if  x exists:
	'do something'

This could serve two options  also...  does the object exist?  or does
a value exist where the value is not equal to zero or None.  There
would need to be a syntax difference between the two.

Using the 'if x:'  form does this for values and is clear enough.  So
using an 'exists()' function to check for the presence of an object
would be useful if there isn't a simple way to do it already.  Right
now it you do this without first assigning a value to x you get:

>>> if x:
	pass


Traceback (most recent call last):
  File "<pyshell#2>", line 1, in -toplevel-
    if x:
NameError: name 'x' is not defined
>>> 

There is an os.path.exists() function for checking if a file path
exists.   A more  general exists() function or method my be useful and
still be consistent with it.   

With pythons dynamic variables,  I think an exists function would be
useful to check if an object exists.  And to use 'if x:'  to check if
a (value!=0) exists.  It's clear what is being tested for from the
context.  

I can't do:

def exists(object):
	try object:
		return True
	return False

if not exists(object):
	object = 0


This generates a not defined error if object isn't defined when exists
is called.


So I have to do:


try:
	if object:
		exist = True
except:
	exist = False
if not exist:
	object = 0

That can be shortened to:

try:
	if object:
		pass
except:
	object = 0

This isn't to bad,  but I would prefer something like,

if not object.exists:	# does the abject already exist?
	object = 0	# if not, create and set initial value
			# else use the existing value

I think I diverged from True or False a bit here.  And I'm not sure if
this functionality already exists or not?

_Ron Adam




	

















  




There's probably a way to do this in python already.  












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