True inconsistency in Python

**Erik Max Francis** · Jul 18 '05, 05:32 AM

Re: True inconsistency in Python

Scott Chapman wrote:
[color=blue]
> Historically Python has allowed <> 0 to equal true in evaluations.
> Now
> <> 0 still evaluates to true in evaluations. However it doesn't equal
> True. They are not interchangable. (Same with empty lists, etc.)[/color]

That's because the proper way to test for truth does not use the True
value at all. It is this:

if x:
...

not this:

if x == True:
...

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Erik Max Francis && max@alcyone.com && http://www.alcyone.com/max/
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**David C. Fox** · Jul 18 '05, 05:32 AM

Re: True inconsistency in Python

Scott Chapman wrote:
[color=blue]
>
> if var == True: # only works if var is 1
> blah
>
> ' Must use:
>
> if var: # works if var is not 0
> blah[/color]

Just because something isn't True doesn't mean it isn't true.

David

**Ben Finney** · Jul 18 '05, 05:32 AM

Re: True inconsistency in Python

On Wed, 12 Nov 2003 19:42:27 -0800, Scott Chapman wrote:[color=blue]
> There seems to be an inconsistency here:[/color]

Yes. The inconsistency is in expecting all Boolean truths to be the
same value.

The object True will evaluate as a Boolean truth. The object False will
not evaluate as a Boolean truth. This doesn't mean that there are no
other values that will or won't evaluate as Boolean truth.

You many want to read the PEP that led to the creation of the 'bool'
type (and True and False objects):

<http://www.python.org/peps/pep-0285.html>

In short: Testing the Boolean truth of an expression is done with 'if',
not with value-comparison operators.

--
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**Ron Adam** · Jul 18 '05, 05:32 AM

Re: True inconsistency in Python

On Wed, 12 Nov 2003 19:42:27 -0800, Scott Chapman
<scott_list@mis chko.com> wrote:
[color=blue]
>
>Historically Python has allowed <> 0 to equal true in evaluations. Now
><> 0 still evaluates to true in evaluations. However it doesn't equal
>True.[/color]

[color=blue][color=green][color=darkred]
>>>a = (2 != 0)
>>>a == True[/color][/color][/color]
True

Seems to work for me. Am I missing something?

The only thing that surprises me in all of this is the "if var:"
evaluating to true for numbers other than 1. That's new to me, I
would have expected an exception in that case. But it still makes
since if I look at it as a shortcut for "if (var != 0):". This
only proves I'm still relatively new to Python I think.

_Ron Adam

**Ben Finney** · Jul 18 '05, 05:32 AM

Re: True inconsistency in Python

On Thu, 13 Nov 2003 06:26:09 GMT, Ron Adam wrote:[color=blue]
> The only thing that surprises me in all of this is the "if var:"
> evaluating to true for numbers other than 1. That's new to me, I
> would have expected an exception in that case.[/color]

Python has only recently gained a Boolean type ('bool').

<http://www.python.org/peps/pep-0285.html>

Before that, Boolean logic was done with integer values. Zero equated
to Boolean false, non-zero equated to Boolean true; and the default
Boolean true value was simply the integer 1.

This conflation of types is confusing, and (like many other languages)
Python has now "grown a Boolean type" to distinguish integer 0 and 1
from Boolean False and True. However, the previous behaviour is still
supported -- for how long, I don't know.

--
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**Ron Adam** · Jul 18 '05, 05:32 AM

Re: True inconsistency in Python

On 13 Nov 2003 17:08:13 +1050, Ben Finney
<bignose-hates-spam@and-benfinney-does-too.id.au> wrote:
[color=blue]
>On Thu, 13 Nov 2003 06:26:09 GMT, Ron Adam wrote:[color=green]
>> The only thing that surprises me in all of this is the "if var:"
>> evaluating to true for numbers other than 1. That's new to me, I
>> would have expected an exception in that case.[/color]
>
>Python has only recently gained a Boolean type ('bool').
>
> <http://www.python.org/peps/pep-0285.html>
>
>Before that, Boolean logic was done with integer values. Zero equated
>to Boolean false, non-zero equated to Boolean true; and the default
>Boolean true value was simply the integer 1.
>
>This conflation of types is confusing, and (like many other languages)
>Python has now "grown a Boolean type" to distinguish integer 0 and 1
>from Boolean False and True. However, the previous behaviour is still
>supported -- for how long, I don't know.[/color]

That's good to know. I've always explicitly defined my bool values as
0 and 1 so it's never been a problem.

Thanks for the PEP link, it was informative.

_Ron Adam

**Michael Geary** · Jul 18 '05, 05:32 AM

Re: True inconsistency in Python

Ron Adam:[color=blue]
> The only thing that surprises me in all of this is the
> "if var:" evaluating to true for numbers other than 1.
> That's new to me, I would have expected an
> exception in that case. But it still makes since if I
> look at it as a shortcut for "if (var != 0):". This only
> proves I'm still relatively new to Python I think.[/color]

It's handy, and natural in many real life situations, to treat any nonzero
value as "true". For example:

Do you have any money in your wallet?

Do you have children?

I could ask how much money or how many children you have, but if I just ask
the yes-or-no question, you'll naturally answer "yes" for any nonzero value.

Of course, if you have children, you won't have money, but that's a separate
problem...

-Mike

**Borcis** · Jul 18 '05, 05:32 AM

Re: True inconsistency in Python

Scott Chapman wrote:
[color=blue]
>
> if var == True: # only works if var is 1
> blah
>
> ' Must use:
>
> if var: # works if var is not 0
> blah[/color]

there's the equivalent, and more explicit :

if bool(var)==True : blah

**Dang Griffith** · Jul 18 '05, 05:32 AM

Re: True inconsistency in Python

On Wed, 12 Nov 2003 21:21:03 -0800, Erik Max Francis <max@alcyone.co m>
wrote:
[color=blue]
>Scott Chapman wrote:
>[color=green]
>> Historically Python has allowed <> 0 to equal true in evaluations.
>> Now
>> <> 0 still evaluates to true in evaluations. However it doesn't equal
>> True. They are not interchangable. (Same with empty lists, etc.)[/color]
>
>That's because the proper way to test for truth does not use the True
>value at all. It is this:
>
> if x:
> ...
>
>not this:
>
> if x == True:
> ...[/color]
Sort of like (stretchy) saying that if these are both True:
"a snake is green"
"a pearl is white"
that they are the same as each other.
They are both true, yet unrelated. You can say:
if "a snake is green": print 1
if "a pearl is white": print 2
and the prints would happen, but if you said
if "a snake is green" == "a pearl is white": print 3
the print would not happen.

--dang

**Alex Martelli** · Jul 18 '05, 05:33 AM

Re: True inconsistency in Python

Borcis wrote:
[color=blue]
> Scott Chapman wrote:
>[color=green]
>>
>> if var == True: # only works if var is 1
>> blah
>>
>> ' Must use:
>>
>> if var: # works if var is not 0
>> blah[/color]
>
> there's the equivalent, and more explicit :
>
> if bool(var)==True : blah[/color]

Why stop there? If adding one useless and redundant check is
better, surely having more will be merrier and merrier...:

if ( bool(var) == True) == True: doubleblah

oh wait, we should add ANOTHER level of uselessness...:

if (( bool(var) == True) == True) == True: tripleblah

oh wait...

"if var:" is the Pythonic way. You could argue that each
level of "== True" makes it ``more explicit'', but I just
consider them all equally silly in their utter redundancy.

[[ we TOLD Guido people would start on this absurd path, when
he added bool, True, and False, but he wouldn't listen...
]]

Alex

**Alex Martelli** · Jul 18 '05, 05:33 AM

Re: True inconsistency in Python

Ben Finney wrote:
...[color=blue]
> This conflation of types is confusing, and (like many other languages)
> Python has now "grown a Boolean type" to distinguish integer 0 and 1
> from Boolean False and True. However, the previous behaviour is still
> supported -- for how long, I don't know.[/color]

I've never heard of any plans (not even for the totally mythical
"Python 3000") to remove or even deprecate Python's extremely useful
and practical feature that:
-- any value can be used as a true-or-false condition,
-- zero numbers, empty containers, and user-coded objects defining
__nonzero__ (or else __len__) and returning 0 are taken as false,
-- all other values are taken as true.

Alex

**Emile van Sebille** · Jul 18 '05, 05:33 AM

Re: True inconsistency in Python

Ben Finney[color=blue]
> You many want to read the PEP that led to the creation of the 'bool'
> type (and True and False objects):[/color]

.... or vise versa

Emile van Sebille
emile@fenx.com

**Alex Martelli** · Jul 18 '05, 05:33 AM

Re: True inconsistency in Python

Scott Chapman wrote:
...[color=blue]
> It seems that maybe Python should throw a warning (perhaps if a flag is
> set) any time it bumps into code comparing a variable to True or False.[/color]

It's a bit hard to do it just for variables, but doing it for _any_
comparison would _almost_ be reasonable -- testing if something
"== True" is hardly ever sensible. Unfortunately "a == b" CAN
be perfectly sensible and either or both of the variables MIGHT
just happen to be set to True.

However, this looks to me like a good suggestion for PyChecker
and the like, which should be able to catch and flag the actual
explicit use or the constant True in code involving == or != ...

Alex

**Gerrit Holl** · Jul 18 '05, 05:33 AM

Re: True inconsistency in Python

Alex Martelli wrote:
(about x vs. bool(x))[color=blue]
> [[ we TOLD Guido people would start on this absurd path, when
> he added bool, True, and False, but he wouldn't listen...
> ]][/color]

What do double square brackets mean?

(btw, I _do_ use bool ocasionally: I implemented an anytrue() function
using sum([bool(i) for i in L]), is this wrong?)

Gerrit.

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True inconsistency in Python

True inconsistency in Python

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