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Truthiness

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  Truthiness Simon Kennedy <sffjunkie@gmail.com> - 2014-10-23 07:30 -0700
    Re: Truthiness Ian Kelly <ian.g.kelly@gmail.com> - 2014-10-23 08:45 -0600
      Re: Truthiness Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-10-24 11:42 +1100
    Re: Truthiness Alain Ketterlin <alain@dpt-info.u-strasbg.fr> - 2014-10-23 16:47 +0200
      Re: Truthiness Wolfgang Maier <wolfgang.maier@biologie.uni-freiburg.de> - 2014-10-23 17:01 +0200
      Re: Truthiness Denis McMahon <denismfmcmahon@gmail.com> - 2014-10-23 16:07 +0000
    Re: Truthiness Wolfgang Maier <wolfgang.maier@biologie.uni-freiburg.de> - 2014-10-23 16:49 +0200
    Re: Truthiness Skip Montanaro <skip.montanaro@gmail.com> - 2014-10-23 09:51 -0500
    Re: Truthiness Simon Kennedy <sffjunkie@gmail.com> - 2014-10-23 07:56 -0700
      Re: Truthiness random832@fastmail.us - 2014-10-23 11:24 -0400

#79523 — Truthiness

Just out of academic interest, is there somewhere in the Python docs where the following is explained?

>>> 3 == True
False
>>> if 3:
	print("It's Twue")
	
It's Twue

i.e. in the if statement 3 is True but not in the first

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#79527

On Thu, Oct 23, 2014 at 8:30 AM, Simon Kennedy <sffjunkie@gmail.com> wrote:
> Just out of academic interest, is there somewhere in the Python docs where the following is explained?

https://docs.python.org/3/reference/expressions.html#booleans

>>>> 3 == True
> False
>>>> if 3:
>         print("It's Twue")
>
> It's Twue
>
> i.e. in the if statement 3 is True but not in the first

No, 3 is merely true, not True.  True is just the name of a particular
singleton object that is also true.

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#79553

Ian Kelly wrote:

> No, 3 is merely true, not True.  True is just the name of a particular
> singleton object that is also true.

Sometimes I distinguish between "true" and "True", where True is the
canonical boolean true object, but I prefer to refer
to "true-like", "true-ish", or "truthy" objects as a shorthand
for "evaluates the same as True in a boolean context"

Likewise for "false-like, false-ish, falsey" for objects which evaluate the
same as False in a boolean context.

It should also be pointed out that, for built-ins and the standard library
at least, a good distinction to make is that representations of Nothing
(e.g. None, empty string, empty list, zero, empty set, etc.) are falsey,
while representations of Something (e.g. non-empty strings, non-empty
lists, numbers other than zero, non-empty sets, etc.) are truthy.


-- 
Steven

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#79528

Simon Kennedy <sffjunkie@gmail.com> writes:

> Just out of academic interest, is there somewhere in the Python docs where the following is explained?
>
>>>> 3 == True
> False
>>>> if 3:
> 	print("It's Twue")
> 	
> It's Twue
>
> i.e. in the if statement 3 is True but not in the first

https://docs.python.org/2/reference/compound_stmts.html#the-if-statement

says: "The if statement [...] selects exactly one of the suites by
evaluating the expressions one by one until one is found to be true (see
section Boolean operations for the definition of true and false)"

and then:

https://docs.python.org/2/reference/expressions.html#booleans

says: "In the context of Boolean operations, and also when expressions
are used by control flow statements, the following values are
interpreted as false: False, None, numeric zero of all types, and empty
strings and containers (including strings, tuples, lists, dictionaries,
sets and frozensets). All other values are interpreted as true."

(links are to the 2.7 version of the reference manual, I think not much
has changed in 3.* versions.)

-- Alain.

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#79534

On 10/23/2014 04:47 PM, Alain Ketterlin wrote:
> Simon Kennedy <sffjunkie@gmail.com> writes:
>
>> Just out of academic interest, is there somewhere in the Python docs where the following is explained?
>>
>>>>> 3 == True
>> False
>>>>> if 3:
>> 	print("It's Twue")
>> 	
>> It's Twue
>>
>> i.e. in the if statement 3 is True but not in the first
>
> https://docs.python.org/2/reference/compound_stmts.html#the-if-statement
>
> says: "The if statement [...] selects exactly one of the suites by
> evaluating the expressions one by one until one is found to be true (see
> section Boolean operations for the definition of true and false)"
>

Exactly, but in

if 3 == True:

the expression is 3==True , in which 3 and True compare unequal, thus, 
the expression is false.
On the other hand, in

if 3:

the expression to evaluate is just the int object and the rules below apply.

> and then:
>
> https://docs.python.org/2/reference/expressions.html#booleans
>
> says: "In the context of Boolean operations, and also when expressions
> are used by control flow statements, the following values are
> interpreted as false: False, None, numeric zero of all types, and empty
> strings and containers (including strings, tuples, lists, dictionaries,
> sets and frozensets). All other values are interpreted as true."
>
> (links are to the 2.7 version of the reference manual, I think not much
> has changed in 3.* versions.)
>
> -- Alain.
>

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#79539

On Thu, 23 Oct 2014 16:47:27 +0200, Alain Ketterlin wrote:

> says: "In the context of Boolean operations, and also when expressions
> are used by control flow statements, the following values are
> interpreted as false: False, None, numeric zero of all types, and empty
> strings and containers (including strings, tuples, lists, dictionaries,
> sets and frozensets). All other values are interpreted as true."

Yep, and I expect this to bite S4H shortly, when he can't understand why 
the following are not all of the same truthiness:

0 (falsey - numeric 0)
[] (falsey - empty set)
"" (falsey - empty string)
[""] (truthy - non empty set)
[0] (truthy - non empty set)

-- 
Denis McMahon, denismfmcmahon@gmail.com

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#79530

On 10/23/2014 04:30 PM, Simon Kennedy wrote:
> Just out of academic interest, is there somewhere in the Python docs where the following is explained?
>

https://docs.python.org/3/library/stdtypes.html#truth-value-testing

>>>> 3 == True
> False

as opposed to:

https://docs.python.org/3/library/stdtypes.html#comparisons

>>>> if 3:
> 	print("It's Twue")
> 	
> It's Twue
>
> i.e. in the if statement 3 is True but not in the first
>

Here is the misunderstanding: it is not true that 3 is True in the if 
example, but it's evaluated as True in a boolean context. Illustration:

 >>> 3 == True
False

 >>> bool(3) == True
True

 >>> bool(3) is True
True

If you combine if and a comparison, it behaves like in your first example:

 >>>
if 3 == True:
     print('True')
else:
     print('False')

False

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#79532

[Multipart message — attachments visible in raw view] — view raw

On Thu, Oct 23, 2014 at 9:30 AM, Simon Kennedy <sffjunkie@gmail.com> wrote:

> Just out of academic interest, is there somewhere in the Python docs where
> the following is explained?


Yes:

https://docs.python.org/3/library/stdtypes.html#truth-value-testing
https://docs.python.org/2.7/library/stdtypes.html#truth-value-testing

Skip

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#79533

Thanks everyone. That's a thorough enough explanation for me.

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#79535

On Thu, Oct 23, 2014, at 10:56, Simon Kennedy wrote:
> Thanks everyone. That's a thorough enough explanation for me.

You should know, though, that numeric values equal to 1 (and 0 for
False) _are_ == True. This works for dictionary keys, array indexes,
etc. The bool type is actually a subclass of int. Mathematical
operations on it will simply return an int, or float for division -
logical operations and bitwise and/or/xor return bool (bitwise not
returns an int because it returns the values -1 and -2)

So checking if something is == True is the same as checking if it's ==
1, and checking if it's == False is the same as checking if it's == 0.

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