[Python-ideas] Fwd: Boolean behavior of None

[Joao S. O. Bueno]

On 17 January 2013 11:10, Ilkka Pelkonen <ilkka.pelkonen at iki.fi> wrote:
> Hi Oleg, others,
> It's not that it can't be done, just that it does something you don't
> expect. I've been professionally working with C++ for nine years in
> large-scale Windows systems, and I do expect a boolean expression return a
> boolean value.
>
> Or, can you show me an example how the developer would benefit of the
> current behavior? Any operator traditionally considered as boolean will do.


Ikka,

Python is a dynamic typed language. As such, there is no strict type checking
for most operations.

The behavior of boolean operations for Python 2.x is well defiend and described
here:

http://docs.python.org/2/reference/expressions.html#boolean-operations

If you are testing for "truthfullness" of a given object, using the
"==" value for that,
as in your "if flag == expected_result:"  is definetelly a
non-recomended pratice.
Objects that have False or True value have always been well defined in Python,
and that definition follows common sense closely, on what should be False.

No one would expect "None" to be True. The behavior of yielding the
first part of the expression in
a failed "and" operation is not unique to Python, and AFAIK, has been
inspired from C -
and there are tons of code which rely directly on this behavior. (Even
though I'd agree a lot of this
code, emulating the ternary operator in a time before it was available
in Python 2.5
is very poorly written)


> Regards,
> Ilkka
>