[Python-ideas] Re: Deprecate/change the behaviour of ~bool

[Steven D'Aprano]

On Wed, Feb 24, 2021 at 12:51:24PM -0500, Richard Damon wrote:

> Of the unary, python implements them all, the possible opererators for
> op a are:
> 
> True
> 
> False
> 
> a
> 
> not a


Those who have read my posts over the years will realise that I enjoy 
being pedantic as much as the next guy, but the trick to being pedantic 
is that you have to be *correct* in your pedantry :-)

The only operator in your list is `not`. True and False are values, not 
operators: you can't say "True x" to ignore the value of x and return 
True. And likewise `a` has no operator at all.

I'm sure that philosophers would like to debate whether the absense of 
an operator counts as an operator, but from the point of view of a 
programmer, it's hard to google for an operator that isn't there :-)

Of course, as you pointed out in another post, it's unlikely for any 
practical programming language to define such operators, although that's 
the sort of thing that esoteric languages might do. For the record, here 
are the four unary operators defined as functions:

def contradiction(a): return False
def tautology(a): return True
def identity(a): return a
def negation(a): return not a


> Of the 16 binary operators a op b you can get:
[...]

There is no question that we can get the results of calling all sixteen 
boolean operators, but whether Python defines them or not. Although I'll 
grant you that using the comparison operators is a very nice trick, 
e.g. using `==` to spell the "material biconditional" operator. I 
totally forgot that you could do that, so thank you. I guess Python does 
define more than `and` and `or` after all :-)

Speaking of esoteric languages, one can define the complete set of 
boolean 1- and 2-argument operators using only two functions, plus a 
third helper to display the results:


def true(x, y):
return x

def false(x, y):
return y

def print_bool(b):
print b("true", "false")

print_bool(true)
print_bool(false)

Using just those two functions, we can define all boolean operators.

def And(a, b):
return a(b, a)

def Or(a, b):
return a(a, b)


The others are left as an exercise :-)



-- 
Steve
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[Python-ideas] Re: Deprecate/change the behaviour of ~bool

[Chris Angelico]

On Thu, Feb 25, 2021 at 5:05 AM Richard Damon  wrote:
>
> On 2/24/21 12:34 PM, Chris Angelico wrote:
> > On Thu, Feb 25, 2021 at 4:28 AM Barry Scott  wrote:
> >>
> >>
> >> On 23 Feb 2021, at 22:10, Steven D'Aprano  wrote:
> >>
> >> There are exactly 2**4 = 16 boolean operators of two variables. Python
> >> only supports two: `and` and `or`. Plus a single unary operator `not`
> >> (out of four possible unary operators). What makes xnor so special that
> >> you want it to be an operator?
> >>
> >>
> >> Python implements more then 2 of them:
> >>
> >> True
> >> False
> >> not
> >> and
> >> or
> >>
> >> https://en.wikipedia.org/wiki/Boolean_algebras_canonically_defined#Truth_tables
> >>
> > True and False aren't operators in Python. Notionally you could say
> > that "take any input(s) and return True" could be considered an
> > operator in theory, but you can't write "x True y" to achieve that in
> > Python.
> >
> > ChrisA
>
> True, but you aren't really going to define real operators in a language
> that always ignore one or both of their arguments.
>
> Thus, of the 16 theoretical operators in the list, the 6 that don't
> depend on both values aren't going to get a real operator, and if you
> actually want that operation, you do have a 'spelling' in Python for it.
>

Sure. In any case, since it's easy to define operators in terms of
each other, Python needs just a handful to give the power to the
programmer. The others don't need any language support.

ChrisA
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[Python-ideas] Re: Deprecate/change the behaviour of ~bool

[Richard Damon]

On 2/24/21 12:34 PM, Chris Angelico wrote:
> On Thu, Feb 25, 2021 at 4:28 AM Barry Scott  wrote:
>>
>>
>> On 23 Feb 2021, at 22:10, Steven D'Aprano  wrote:
>>
>> There are exactly 2**4 = 16 boolean operators of two variables. Python
>> only supports two: `and` and `or`. Plus a single unary operator `not`
>> (out of four possible unary operators). What makes xnor so special that
>> you want it to be an operator?
>>
>>
>> Python implements more then 2 of them:
>>
>> True
>> False
>> not
>> and
>> or
>>
>> https://en.wikipedia.org/wiki/Boolean_algebras_canonically_defined#Truth_tables
>>
> True and False aren't operators in Python. Notionally you could say
> that "take any input(s) and return True" could be considered an
> operator in theory, but you can't write "x True y" to achieve that in
> Python.
>
> ChrisA

True, but you aren't really going to define real operators in a language
that always ignore one or both of their arguments.

Thus, of the 16 theoretical operators in the list, the 6 that don't
depend on both values aren't going to get a real operator, and if you
actually want that operation, you do have a 'spelling' in Python for it.

-- 
Richard Damon
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[Python-ideas] Re: Deprecate/change the behaviour of ~bool

[Richard Damon]

On 2/24/21 12:26 PM, Barry Scott wrote:
>
>
>> On 23 Feb 2021, at 22:10, Steven D'Aprano > > wrote:
>>
>> There are exactly 2**4 = 16 boolean operators of two variables. Python 
>> only supports two: `and` and `or`. Plus a single unary operator `not` 
>> (out of four possible unary operators). What makes xnor so special that 
>> you want it to be an operator?
>
> Python implements more then 2 of them:
>
> True
> False
> not
> and
> or
>
> https://en.wikipedia.org/wiki/Boolean_algebras_canonically_defined#Truth_tables
> 
>
> Barry
>
Of the unary, python implements them all, the possible opererators for
op a are:

True

False

a

not a

Of the 16 binary operators a op b you can get:

* True
* False
* a
* b
* not a
* not b
* a and b
* a or b

and for things that are already bools, you can use

* a < b
* a <= b
* a == b
* a != b
* a > b
* a >= b

The only combinations you can't get with a simple operator are 'not (a
and b)' and 'not (a or b)' (nand and nor) assuming you are starting with
true bools (or 0 and 1). If you only have 'truthy' values, you only get
1/2 of the possible options.

-- 
Richard Damon
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[Python-ideas] Re: Deprecate/change the behaviour of ~bool

[Chris Angelico]

On Thu, Feb 25, 2021 at 4:28 AM Barry Scott  wrote:
>
>
>
> On 23 Feb 2021, at 22:10, Steven D'Aprano  wrote:
>
> There are exactly 2**4 = 16 boolean operators of two variables. Python
> only supports two: `and` and `or`. Plus a single unary operator `not`
> (out of four possible unary operators). What makes xnor so special that
> you want it to be an operator?
>
>
> Python implements more then 2 of them:
>
> True
> False
> not
> and
> or
>
> https://en.wikipedia.org/wiki/Boolean_algebras_canonically_defined#Truth_tables
>

True and False aren't operators in Python. Notionally you could say
that "take any input(s) and return True" could be considered an
operator in theory, but you can't write "x True y" to achieve that in
Python.

ChrisA
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[Python-ideas] Re: Deprecate/change the behaviour of ~bool

[Barry Scott]

> On 23 Feb 2021, at 22:10, Steven D'Aprano  wrote:
> 
> There are exactly 2**4 = 16 boolean operators of two variables. Python 
> only supports two: `and` and `or`. Plus a single unary operator `not` 
> (out of four possible unary operators). What makes xnor so special that 
> you want it to be an operator?

Python implements more then 2 of them:

True
False
not
and
or

https://en.wikipedia.org/wiki/Boolean_algebras_canonically_defined#Truth_tables

Barry

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[Python-ideas] Re: Deprecate/change the behaviour of ~bool

[Steven D'Aprano]

I don't think we want to add a new dunder just to make it easier for 
people to abuse bitwise operators instead of using the `!=` operator.

-- 
Steve
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[Python-ideas] Re: Deprecate/change the behaviour of ~bool

[Soni L.]

On 2021-02-23 7:10 p.m., Steven D'Aprano wrote:

On Tue, Feb 23, 2021 at 01:57:08PM -0300, Soni L. wrote:


> What about getting the parser to recognize ^~ as an XNOR operator and 
> have __xnor__ on bools actually do xnor on bools? Make xnor a real 
> operator by fusing two operators together!


Let's not break existing code for the sake of an operator that hardly
anyone ever uses.

There are exactly 2**4 = 16 boolean operators of two variables. Python
only supports two: `and` and `or`. Plus a single unary operator `not`
(out of four possible unary operators). What makes xnor so special that
you want it to be an operator?

Do you have any use-cases for an xnor operator? What do you use it for?

These are not rhetorical questions. If you want to push this proposal
forward, you need to start with actual concrete use-cases, not "it would
be nice ..." and "how about ...".

Why can't you write a function to do what you want?




It's because someone was doing a comparison of a bool to the result of a 
comparison, a la (x == y) == z, and that looks kinda weird (especially 
because x == y == z has a completely different meaning). ^ works as a 
replacement for != but for == the only option is to either use "thing ^ 
(not foo)" or "not (thing ^ foo)". It's not that it's special - we 
"technically" already have it, if you abuse another language feature - 
it'd just make some code easier to understand.

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[Python-ideas] Re: Deprecate/change the behaviour of ~bool

[Steven D'Aprano]

On Tue, Feb 23, 2021 at 01:57:08PM -0300, Soni L. wrote:

> 64-bit bools were a mistake, tbh. Oh well, at least we tried.

What language are you talking about?

In Python, bool is a subclass of int, which is a BigNum, not 64 bits.

>>> False.bit_length()
0
>>> (~False).bit_length()
1


> What about getting the parser to recognize ^~ as an XNOR operator and 
> have __xnor__ on bools actually do xnor on bools? Make xnor a real 
> operator by fusing two operators together!

Let's not break existing code for the sake of an operator that hardly 
anyone ever uses.

There are exactly 2**4 = 16 boolean operators of two variables. Python 
only supports two: `and` and `or`. Plus a single unary operator `not` 
(out of four possible unary operators). What makes xnor so special that 
you want it to be an operator?

Do you have any use-cases for an xnor operator? What do you use it for?

These are not rhetorical questions. If you want to push this proposal 
forward, you need to start with actual concrete use-cases, not "it would 
be nice ..." and "how about ...".

Why can't you write a function to do what you want?


-- 
Steve
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[Python-ideas] Re: Deprecate/change the behaviour of ~bool

[Soni L.]

64-bit bools were a mistake, tbh. Oh well, at least we tried.

What about getting the parser to recognize ^~ as an XNOR operator and 
have __xnor__ on bools actually do xnor on bools? Make xnor a real 
operator by fusing two operators together!


On 2021-02-23 5:49 a.m., Steven D'Aprano wrote:

On Mon, Feb 22, 2021 at 10:11:14PM -0300, Soni L. wrote:

> Currently ~False is -1 and ~True is -2. Would be nicer if ~bool was the 
> same as not bool.


That's your opinion. I disagree.

Bitwise-not is not the same thing as boolean-not, and they should not be
spelled the same, especially not when that would break the invariant
that if `a == b` then `~a == ~b`.

>>> 0 == False
True
>>> ~0 == ~False
True


If you want boolean-not, then use the boolean-not operator `not`.


> In particular, this is nice for xnor operator: a ^~ b. This currently 
> works on ints, but not on bools


If it works on ints, of course it works on bools, because bools are
ints. You get identical results whether you use int (0, 1) or bools
(False, True) in every combination:


>>> def xnor(a, b):
... return a ^ ~b
...
>>> [xnor(a, b) for a in (0, 1) for b in (0, 1)]
[-1, -2, -2, -1]
>>> [xnor(a, b) for a in (0, 1) for b in (False, True)]
[-1, -2, -2, -1]
>>> [xnor(a, b) for a in (False, True) for b in (0, 1)]
[-1, -2, -2, -1]
>>> [xnor(a, b) for a in (False, True) for b in (False, True)]
[-1, -2, -2, -1]



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[Python-ideas] Re: Deprecate/change the behaviour of ~bool

[Steven D'Aprano]

On Mon, Feb 22, 2021 at 10:11:14PM -0300, Soni L. wrote:

> Currently ~False is -1 and ~True is -2. Would be nicer if ~bool was the 
> same as not bool.

That's your opinion. I disagree.

Bitwise-not is not the same thing as boolean-not, and they should not be 
spelled the same, especially not when that would break the invariant 
that if `a == b` then `~a == ~b`.

>>> 0 == False
True
>>> ~0 == ~False
True


If you want boolean-not, then use the boolean-not operator `not`.


> In particular, this is nice for xnor operator: a ^~ b. This currently 
> works on ints, but not on bools

If it works on ints, of course it works on bools, because bools are 
ints. You get identical results whether you use int (0, 1) or bools 
(False, True) in every combination:


>>> def xnor(a, b):
... return a ^ ~b
... 
>>> [xnor(a, b) for a in (0, 1) for b in (0, 1)]
[-1, -2, -2, -1]
>>> [xnor(a, b) for a in (0, 1) for b in (False, True)]
[-1, -2, -2, -1]
>>> [xnor(a, b) for a in (False, True) for b in (0, 1)]
[-1, -2, -2, -1]
>>> [xnor(a, b) for a in (False, True) for b in (False, True)]
[-1, -2, -2, -1]


-- 
Steve
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[Python-ideas] Re: Deprecate/change the behaviour of ~bool

[Marco Sulla]

On Tue, 23 Feb 2021 at 02:13, Soni L.  wrote:
>
> Currently ~False is -1 and ~True is -2. Would be nicer if ~bool was the
> same as not bool.

I suspect this is more for NumPy people. I remember that one of my
bosses used `~a` on a bool because he started with NumPy and had a
limited knowledge of Python.
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[Python-ideas] Re: Deprecate/change the behaviour of ~bool

[Josh Rosenberg]

> You could write it as a ^ (not b), as long as you don't mind it giving
back an integer rather than a bool.

Actually, that'll give back a bool if a is a bool (and (not b) produces a
bool); ^ is overridden for bool/bool operations and itself returns a bool.

On Tue, Feb 23, 2021 at 1:48 AM Chris Angelico  wrote:

> On Tue, Feb 23, 2021 at 12:14 PM Soni L.  wrote:
> >
> > Currently ~False is -1 and ~True is -2. Would be nicer if ~bool was the
> > same as not bool. Hopefully nobody actually relies on this but
> > nevertheless, it would be a backwards-incompatible change so the whole
> > deprecation warnings and whatnot would be required.
>
> There are quite a few ways in which bitwise operators are not the same
> as boolean operators. What would be the advantage of having them be
> the same in just this one case?
>
> > In particular, this is nice for xnor operator: a ^~ b. This currently
> > works on ints, but not on bools, while most other operators, including
> > xor, do successfully work on bools.
>
> You could write it as a ^ (not b), as long as you don't mind it giving
> back an integer rather than a bool. Fundamentally, you're doing
> bitwise operations on integers, and expecting them to behave as if
> they have only a single bit each, so another way to resolve this might
> be to mask it off at the end with "& 1".
>
> If this makes your code horrendously ugly, perhaps it would be better
> to create your own "one-bit integer" class, which responds to all
> bitwise operators by automatically masking down to one bit?
>
> ChrisA
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[Python-ideas] Re: Deprecate/change the behaviour of ~bool

[Guido van Rossum]

Also, code that is expecting an int should not behave differently when it
receives a bool.

On Mon, Feb 22, 2021 at 17:47 Chris Angelico  wrote:

> On Tue, Feb 23, 2021 at 12:14 PM Soni L.  wrote:
> >
> > Currently ~False is -1 and ~True is -2. Would be nicer if ~bool was the
> > same as not bool. Hopefully nobody actually relies on this but
> > nevertheless, it would be a backwards-incompatible change so the whole
> > deprecation warnings and whatnot would be required.
>
> There are quite a few ways in which bitwise operators are not the same
> as boolean operators. What would be the advantage of having them be
> the same in just this one case?
>
> > In particular, this is nice for xnor operator: a ^~ b. This currently
> > works on ints, but not on bools, while most other operators, including
> > xor, do successfully work on bools.
>
> You could write it as a ^ (not b), as long as you don't mind it giving
> back an integer rather than a bool. Fundamentally, you're doing
> bitwise operations on integers, and expecting them to behave as if
> they have only a single bit each, so another way to resolve this might
> be to mask it off at the end with "& 1".
>
> If this makes your code horrendously ugly, perhaps it would be better
> to create your own "one-bit integer" class, which responds to all
> bitwise operators by automatically masking down to one bit?
>
> ChrisA
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>
-- 
--Guido (mobile)
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[Python-ideas] Re: Deprecate/change the behaviour of ~bool

[Chris Angelico]

On Tue, Feb 23, 2021 at 12:14 PM Soni L.  wrote:
>
> Currently ~False is -1 and ~True is -2. Would be nicer if ~bool was the
> same as not bool. Hopefully nobody actually relies on this but
> nevertheless, it would be a backwards-incompatible change so the whole
> deprecation warnings and whatnot would be required.

There are quite a few ways in which bitwise operators are not the same
as boolean operators. What would be the advantage of having them be
the same in just this one case?

> In particular, this is nice for xnor operator: a ^~ b. This currently
> works on ints, but not on bools, while most other operators, including
> xor, do successfully work on bools.

You could write it as a ^ (not b), as long as you don't mind it giving
back an integer rather than a bool. Fundamentally, you're doing
bitwise operations on integers, and expecting them to behave as if
they have only a single bit each, so another way to resolve this might
be to mask it off at the end with "& 1".

If this makes your code horrendously ugly, perhaps it would be better
to create your own "one-bit integer" class, which responds to all
bitwise operators by automatically masking down to one bit?

ChrisA
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