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. Regards, Ilkka On Thu, Jan 17, 2013 at 2:51 PM, Oleg Broytman wrote:

...

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

On Thu, Jan 17, 2013 at 02:44:03PM +0200, Ilkka Pelkonen wrote: 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.

No need to change the language. Just do

flag = bool(element and element.is_visible())

Oleg. -- Oleg Broytman http://phdru.name/ phd@phdru.name Programmers don't die, they just GOSUB without RETURN. _______________________________________________ Python-ideas mailing list Python-ideas@python.org http://mail.python.org/mailman/listinfo/python-ideas

On 17 January 2013 11:10, Ilkka Pelkonen 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

On 1/17/2013 8:10 AM, Ilkka Pelkonen 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.

Regards, Ilkka

Ilkka, welcome to the Python community. Python is a wonderfully expressive language once you learn its subtleties. Python and C++ are different. If they weren't, we'd only have one language, not two. The short-circuiting operations "and" and "or" behave as they do for a reason. As an example, a common way to deal with default values: def accumulate(value, to=None): to = to or [] to.append(value) # Forget whether this is a good function or not.. return to If "or" always returned a boolean, as I'm assuming you'd prefer, then we'd have a much clumsier time defaulting values like this. --Ned.

To sum this up, I also thought of the None vs False issue some more, and found a use case which can bring a benefit for the separation: a function could normally return True or False, or None in a special case. Because of the separation, the developer can handle all the cases appropriately. Too little thinking, too much action. I wonder why I didn't google it this time, the issue is all over the net like any question you can imagine to ask, at least in case of Python. :) Sorry for the trouble. I'll stay around with a little lower profile. :) Regards again, Ilkka On Thu, Jan 17, 2013 at 7:03 PM, Ilkka Pelkonen wrote:

Thank you all. It was just that when I started with Python, everything worked right like I expected, and I found the ways to do anything I've needed all the way until today, so when I came across with this, it appeared to me a clear bug in the language/interpreter. Casting to bool is indeed a good solution and practice, and I do now agree that there's no point in changing the language - like Antoine said, we're talking control flow operators here, not exactly boolean. (This might be a good addition to the documentation.)

Thank you Ned for the warm welcome and everyone for your input. I hope to be able to contribute in the future. :)

Best Regards, Ilkka

On Thu, Jan 17, 2013 at 6:07 PM, Mathias Panzenböck < grosser.meister.morti@gmx.net> wrote:

...

This change would break a lot of existing code and would make Python awkwardly stand out from all other modern dynamically typed languages (e.g. Ruby and JavaScript). You often write things like this:

def foo(bar=None): bar = bar or [] ...

Or:

obj = obj and obj.property

The proposed change would needlessly complicate these things and break existing code. Forcing a bool type really doesn't require that much code (bool(expr)) and is good practise anyway.

On 01/17/2013 01:44 PM, Ilkka Pelkonen 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. Although potentially complex, I'd myself go for the first approach. It seems to me more logical that False != None than an 'and' expression returning non-boolean. Also the latter change might require people change their code, while the former should not require any modifications.

This behavior probably results in lots of errors when people like me, used to more traditional languages, take on Python in a serious manner. I like the concept 'pythonic', and am trying to apply it to practice like above.

Hoping to hear your thoughts, Regards,

Ilkka Pelkonen

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