[Python-ideas] Fwd: Boolean behavior of None

Thu Jan 17 13:58:11 CET 2013

On 17 January 2013 10:44, Ilkka Pelkonen <ica at iki.fi> wrote:
> Hi all,
> I ran into an issue in expression evaluation with Python for Windows 2.7.3.
> Consider the following code:
>> expected_result = (expected_string != 'TRUE') # Boolean
> element = find_element() # Can return None or an instance of Element
> flag = (element and element.is_visible())
> if flag == expected_result:
> ..# Ok
> ..return
> # Otherwise perform some failure related stuff.
>> This code does not work. What happens on the 'flag' assignment row, is that
> if 'element' is None, the expression returns None, not False. This makes the
> if comparison to fail if expected_result is False, since boolean False is
> not None.
>> To me as a primarily C++ programmer it seems there could be two different
> changes here, either change the behavior of the 'and' expression, forcing it
> to return Boolean even if the latter part is not evaluated,
> and/or make the
> comparison "False == None" return True.

Hi Ikka..
My personal suggestion - rewrite your code to read:
flag = bool(element and element.is_visible())
instead.
That way you don't have to mention trying to change a 20 year old
behavior in a language with billions of lines of code in the wild
which should be kept compatible, at expense of thinking your
expressions.
Nor wait for the next major "4.0" release of Python for being able to
write your code.
 js
 -><-