True/False value testing

[Chris Angelico]

On Thu, Jan 7, 2016 at 9:36 PM, ast <nomail at invalid.com> wrote:
> Hello
>
> For integer, 0 is considered False and any other value True
>
>>>> A=0
>>>> A==False
>
> True
>>>>
>>>> A==True
>
> False
>>>>
>>>> A=1
>>>> A==False
>
> False
>>>>
>>>> A==True
>
> True
>
> It works fine
>
> For string, "" is considered False and any other value True,
> but it doesn't work
>
>>>> A = ""
>>>> A==False
>
> False
>>>>
>>>> A==True
>
> False
>>>>
>>>> A = 'Z'
>>>> A==False
>
> False
>>>>
>>>> A==True
>
> False
>
>
> What happens ???
>
> thx

There's a difference between "is considered false" and "is equal to
False". In Python, most collections are considered false if they are
empty:

def truthiness(x):
    if x:
        print(repr(x) + " is true")
    else:
        print(repr(x) + " is false")

truthiness([])
truthiness([1,2,3])
truthiness("")
truthiness("hello")
truthiness(0)
truthiness(73)
truthiness({})
truthiness({1:2, 3:4})

However, none of these will compare *equal* to the Boolean values True
and False, save for the integers 1 and 0. In fact, True is a special
form of the integer 1, and False is a special form of the integer 0;
they are considered to represent those numbers just as much as the
floating point values 1.0 and 0.0 do:

from decimal import Decimal
from fractions import Fraction
# Caveat, see footnote [1]
if Fraction(0, 1) == 0.0 == 0 == False == Decimal("0"):
    print("See? All representations of zero are equal")

Intuition tells you quite obviously that the integers 1 and 73 are not
equal, but both of them are very definitely truthy values. In the same
way, a non-empty list is truthy, but it's not equal to True.

Does that answer your question?

ChrisA

[1] In Python 2.7, Decimal("0") != Fraction(0, 1). I suspect this is a
bug, as it's been fixed in Python 3; both of them are equal to the
integer 0, but they're not equal to each other. So the order of checks
in that demo is actually significant.