"0 in [True,False]" returns True

[Pierre Quentel]

Hi all,

In some program I was testing if a variable was a boolean, with this
test : if v in [True,False]

My script didn't work in some cases and I eventually found that for v =
0 the test returned True

So I changed my test for the obvious "if type(v) is bool", but I still
find it confusing that "0 in [True,False]" returns True

By the way, I searched in the documentation what "obj in list" meant and
couldn't find a precise definition (does it test for equality or
identity with one of the values in list ? equality, it seems) ; did I
miss something ?

Regards,
Pierre

[Fredrik Lundh]

Re: "0 in [True,False]" returns True

Pierre Quentel wrote:
[color=blue]
> In some program I was testing if a variable was a boolean, with this
> test : if v in [True,False]
>
> My script didn't work in some cases and I eventually found that for v =
> 0 the test returned True
>
> So I changed my test for the obvious "if type(v) is bool", but I still
> find it confusing that "0 in [True,False]" returns True
>
> By the way, I searched in the documentation what "obj in list" meant and
> couldn't find a precise definition (does it test for equality or
> identity with one of the values in list ? equality, it seems) ; did I
> miss something ?[/color]
[color=blue][color=green][color=darkred]
>>> issubclass(bool , int)[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>> isinstance(Fals e, int)[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>> False == 0[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>> int(False)[/color][/color][/color]
0

but seriously, unless you're writing an introspection tool, testing for
bool is pretty silly. just use "if v" or "if not v", and leave the rest to
Python.

</F>

[Carsten Haese]

Re: &quot;0 in [True,False]&quot; returns True

On Mon, 2005-12-12 at 16:26, Pierre Quentel wrote:[color=blue]
> Hi all,
>
> In some program I was testing if a variable was a boolean, with this
> test : if v in [True,False]
>
> My script didn't work in some cases and I eventually found that for v =
> 0 the test returned True
>
> So I changed my test for the obvious "if type(v) is bool", but I still
> find it confusing that "0 in [True,False]" returns True
>
> By the way, I searched in the documentation what "obj in list" meant and
> couldn't find a precise definition (does it test for equality or
> identity with one of the values in list ? equality, it seems) ; did I
> miss something ?[/color]

Where/how did you search? http://docs.python.org/lib/typesseq.html
states unambiguously that "x in s" returns "True if an item of s is
equal to x, else False"

HTH,

Carsten.

[Steven Bethard]

Re: &quot;0 in [True,False]&quot; returns True

Pierre Quentel wrote:[color=blue]
> In some program I was testing if a variable was a boolean, with this
> test : if v in [True,False]
>
> My script didn't work in some cases and I eventually found that for v =
> 0 the test returned True[/color]

This seems like a strange test. What is this code trying to do?

If you're sure you have to do this, I would do either:

if isinstance(v, bool):
...

or

if v is True or v is False:
...

But it really seems like this code is trying to code some other language
in Python...

STeVe

[Fredrik Lundh]

Re: &quot;0 in [True,False]&quot; returns True

bonono@gmail.co m wrote:
[color=blue][color=green]
> > but seriously, unless you're writing an introspection tool, testing for
> > bool is pretty silly. just use "if v" or "if not v", and leave the rest to
> > Python.
> >[/color]
> The OP's code(and his work around) doesn't look like he is testing for
> boolean[/color]

which of course explains why he wrote

In some program I was testing if a variable was a boolean

in the post I replied to...
[color=blue]
> but more like the data type of something. I thought there is some idiom
> in python which said something like "don't assume" ?[/color]

"think before you post" ?

</F>

[bonono@gmail.com]

Re: &quot;0 in [True,False]&quot; returns True


Fredrik Lundh wrote:[color=blue]
> bonono@gmail.co m wrote:
>[color=green][color=darkred]
> > > but seriously, unless you're writing an introspection tool, testing for
> > > bool is pretty silly. just use "if v" or "if not v", and leave the rest to
> > > Python.
> > >[/color]
> > The OP's code(and his work around) doesn't look like he is testing for
> > boolean[/color]
>
> which of course explains why he wrote
>
> In some program I was testing if a variable was a boolean
>
> in the post I replied to...
>[color=green]
> > but more like the data type of something. I thought there is some idiom
> > in python which said something like "don't assume" ?[/color]
>
> "think before you post" ?
>[/color]
Don't know what you mean.

He seems to be testing "boolean type", not whether it is true or false.

[Antoon Pardon]

Re: &quot;0 in [True,False]&quot; returns True

Op 2005-12-12, Fredrik Lundh schreef <fredrik@python ware.com>:[color=blue]
> Pierre Quentel wrote:
>[color=green]
>> In some program I was testing if a variable was a boolean, with this
>> test : if v in [True,False]
>>
>> My script didn't work in some cases and I eventually found that for v =
>> 0 the test returned True
>>
>> So I changed my test for the obvious "if type(v) is bool", but I still
>> find it confusing that "0 in [True,False]" returns True
>>
>> By the way, I searched in the documentation what "obj in list" meant and
>> couldn't find a precise definition (does it test for equality or
>> identity with one of the values in list ? equality, it seems) ; did I
>> miss something ?[/color]
>[color=green][color=darkred]
>>>> issubclass(bool , int)[/color][/color]
> True[color=green][color=darkred]
>>>> isinstance(Fals e, int)[/color][/color]
> True[color=green][color=darkred]
>>>> False == 0[/color][/color]
> True[color=green][color=darkred]
>>>> int(False)[/color][/color]
> 0
>
> but seriously, unless you're writing an introspection tool, testing for
> bool is pretty silly. just use "if v" or "if not v", and leave the rest to
> Python.[/color]

That depends on the circumstances. I have code where a particular
variable can be a boolean or an integer. I don't want that code
to behave the same on 0 and False nor on any other number and
True.

--
Antoon Pardon

[bonono@gmail.com]

Re: &quot;0 in [True,False]&quot; returns True


Antoon Pardon wrote:[color=blue]
> Op 2005-12-12, Fredrik Lundh schreef <fredrik@python ware.com>:[color=green]
> > Pierre Quentel wrote:
> >[color=darkred]
> >> In some program I was testing if a variable was a boolean, with this
> >> test : if v in [True,False]
> >>
> >> My script didn't work in some cases and I eventually found that for v =
> >> 0 the test returned True
> >>
> >> So I changed my test for the obvious "if type(v) is bool", but I still
> >> find it confusing that "0 in [True,False]" returns True
> >>
> >> By the way, I searched in the documentation what "obj in list" meant and
> >> couldn't find a precise definition (does it test for equality or
> >> identity with one of the values in list ? equality, it seems) ; did I
> >> miss something ?[/color]
> >[color=darkred]
> >>>> issubclass(bool , int)[/color]
> > True[color=darkred]
> >>>> isinstance(Fals e, int)[/color]
> > True[color=darkred]
> >>>> False == 0[/color]
> > True[color=darkred]
> >>>> int(False)[/color]
> > 0
> >
> > but seriously, unless you're writing an introspection tool, testing for
> > bool is pretty silly. just use "if v" or "if not v", and leave the rest to
> > Python.[/color]
>
> That depends on the circumstances. I have code where a particular
> variable can be a boolean or an integer. I don't want that code
> to behave the same on 0 and False nor on any other number and
> True.
>[/color]
Then your program/implementation/requirement is wrong because it
doesn't fit in the use case of "if v:" or "if not v:", refactor ;-)

[Steve Holden]

Re: &quot;0 in [True,False]&quot; returns True

Pierre Quentel wrote:[color=blue]
> Hi all,
>
> In some program I was testing if a variable was a boolean, with this
> test : if v in [True,False]
>
> My script didn't work in some cases and I eventually found that for v =
> 0 the test returned True
>
> So I changed my test for the obvious "if type(v) is bool", but I still
> find it confusing that "0 in [True,False]" returns True
>
> By the way, I searched in the documentation what "obj in list" meant and
> couldn't find a precise definition (does it test for equality or
> identity with one of the values in list ? equality, it seems) ; did I
> miss something ?
>[/color]
It actually uses the __contains__() method of the right-hand operand,
and in the case of a list that will test for equality of the left-hand
operand to one of the list elements. Since False == 0 that's why you see
what you do.

The really interesting question your post raises, though, is "Why do you
feel it's necessary to test to see whether a variable is a Boolean?".

regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC www.holdenweb.com
PyCon TX 2006 www.python.org/pycon/

[Paul Rubin]

Re: &quot;0 in [True,False]&quot; returns True

Steve Holden <steve@holdenwe b.com> writes:[color=blue]
> The really interesting question your post raises, though, is "Why do
> you feel it's necessary to test to see whether a variable is a
> Boolean?".[/color]

What's the point of having Booleans, if you can't tell them from integers?

[Antoon Pardon]

Re: &quot;0 in [True,False]&quot; returns True

Op 2005-12-13, Steve Holden schreef <steve@holdenwe b.com>:[color=blue]
> Pierre Quentel wrote:[color=green]
>> Hi all,
>>
>> In some program I was testing if a variable was a boolean, with this
>> test : if v in [True,False]
>>
>> My script didn't work in some cases and I eventually found that for v =
>> 0 the test returned True
>>
>> So I changed my test for the obvious "if type(v) is bool", but I still
>> find it confusing that "0 in [True,False]" returns True
>>
>> By the way, I searched in the documentation what "obj in list" meant and
>> couldn't find a precise definition (does it test for equality or
>> identity with one of the values in list ? equality, it seems) ; did I
>> miss something ?
>>[/color]
> It actually uses the __contains__() method of the right-hand operand,
> and in the case of a list that will test for equality of the left-hand
> operand to one of the list elements. Since False == 0 that's why you see
> what you do.
>
> The really interesting question your post raises, though, is "Why do you
> feel it's necessary to test to see whether a variable is a Boolean?".[/color]

I can give you one example. I have written a tube class. A tube behaves
like Queue but it has additional code so that it can be registed with
gtk in the same way as file descriptor can be registered with
io_add_watch. The way this is implemented is by registering an idle
handler when the tube is not empty and removing it when the tube is
empty. So I have a variable cb_src (for callback source) that can be
a boolean or an integer. The possible values are

False: Not registered by the user
True: Registered by the user but no nternal idle callback registerd
a number: gtk integer ID, from the registered idle callback handler.

--
Antoon Pardon

[Erik Max Francis]

Re: &quot;0 in [True,False]&quot; returns True

Paul Rubin wrote:
[color=blue]
> Steve Holden <steve@holdenwe b.com> writes:[color=green]
>> The really interesting question your post raises, though, is "Why do
>> you feel it's necessary to test to see whether a variable is a
>> Boolean?".[/color]
>
> What's the point of having Booleans, if you can't tell them from integers?[/color]

Because

return True

is clearer than

return 1

if the purpose of the return value is to indicate a Boolean rather than
an arbitrary integer.

--
Erik Max Francis && max@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 20 N 121 53 W && AIM erikmaxfrancis
The basis of optimism is sheer terror.
-- Oscar Wilde

[Steve Holden]

Re: &quot;0 in [True,False]&quot; returns True

Paul Rubin wrote:[color=blue]
> Steve Holden <steve@holdenwe b.com> writes:
>[color=green]
>>The really interesting question your post raises, though, is "Why do
>>you feel it's necessary to test to see whether a variable is a
>>Boolean?".[/color]
>
>
> What's the point of having Booleans, if you can't tell them from integers?[/color]

Booleans are specifically defined as a subtype of int at the C level.
One might also ask "what's the point of having floats if you can't tell
them from integers":
[color=blue][color=green][color=darkred]
>>> 0.0 in [1,2,0,4][/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

It just so happens that __contains__() uses an equality test (which it
should) and equality tests perform certain coercions (which they
arguably shouldn't, but in that case I wouldn't be the one doing the
arguing).

"""
The only thing that changes is the preferred values to represent
truth values when returned or assigned explicitly. Previously,
these preferred truth values were 0 and 1; the PEP changes the
preferred values to False and True, and changes built-in
operations to return these preferred values.
"""

PEP 285.

regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC www.holdenweb.com
PyCon TX 2006 www.python.org/pycon/

[Fredrik Lundh]

Re: &quot;0 in [True,False]&quot; returns True

Erik Max Francis wrote:
[color=blue][color=green]
> > What's the point of having Booleans, if you can't tell them from integers?[/color]
>
> Because
>
> return True
>
> is clearer than
>
> return 1
>
> if the purpose of the return value is to indicate a Boolean rather than
> an arbitrary integer.[/color]

the real reason booleans were added was that sillyness like

True = 1 == 1
False = not True

and

return 1 # true

and

class Boolean:

def __init__(self, value = 0):
self.value = operator.truth( value)

def __cmp__(self, other):
if isinstance(othe r, Boolean):
other = other.value
return cmp(self.value, other)

def __repr__(self):
if self.value:
return "<Boolean True at %x>" % id(self)
else:
return "<Boolean False at %x>" % id(self)

def __int__(self):
return self.value

def __nonzero__(sel f):
return self.value

True, False = Boolean(1), Boolean(0)

were all too common in the wild.

for the full story, see

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

and, to briefly return to the original topic, note that

"This PEP does *not* change the fact that almost all object types
can be used as truth values. For example, when used in an if
statement, an empty list is false and a non-empty one is true;
this does not change and there is no plan to ever change this.

The only thing that changes is the preferred values to represent
truth values when returned or assigned explicitly. Previously,
these preferred truth values were 0 and 1; the PEP changes the
preferred values to False and True, and changes built-in
operations to return these preferred values."

in general, returning True and False is pythonic, explicitly testing for
them is not.

</F>

[Robin Becker]

Re: &quot;0 in [True,False]&quot; returns True

Carsten Haese wrote:
........[color=blue]
>
> Where/how did you search? http://docs.python.org/lib/typesseq.html
> states unambiguously that "x in s" returns "True if an item of s is
> equal to x, else False"[/color]
......

exactly and I see
[color=blue][color=green][color=darkred]
>>> 0==False[/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

so I guess nothing is wrong :)
--
Robin Becker