Neutraldict Reloaded creative solution for Sum by Type by veky - python coding challenges

Enable Javascript in your browser and then refresh this page, for a much enhanced experience.

Neutraldict Reloaded solution in Creative category for Sum by Type by veky

def sum_by_types(a):
    acc = type('', (dict,), dict(__missing__=staticmethod(type.__call__)))()
    for elem in a: acc[type(elem)] += elem
    return acc[str], acc[int]

Sept. 20, 2021

Comments:

robertabegg on Sept. 20, 2021, 6:14 p.m.
What! you can define a class with a function? I am not sure I could use such knowledge for good purposes :)

veky on Sept. 21, 2021, 1:05 a.m.
I'm sure you can, you just don't dare. :-D But really, the point is: classes are objects. Just as 4, -5 and 325658534 are instances of class `int`, so int, str, and collections.Counter are instances of class `type`. You just invoke a class "constructor", give necessary data (here: name, base classes and vars) and you get your object. No miracle. :-)

tom-tom_f993b3abc4814d4cb5476e on Oct. 3, 2021, 1:47 p.m.
Very interesting solution, thank you! I had to delve into docs. But... This solutions (like many others) ignores <a href="https://en.wikipedia.org/wiki/Liskov_substitution_principle">Liskov substitution principle</a>, and fails, for example, at <pre class='brush: python'>assert sum_by_types([True]) == ('', 1) </pre> However, for this simple training mission, this may not be a problem.

veky on Oct. 4, 2021, 3:02 a.m.
You're completely correct, and your example shows that every time we use `type` instead of `isinstance`, we lose the power to treat instances of subclasses of A as instances of A (which they are). However, Liskov is not really strongly adhered to in Python's philosophy (because interfaces aren't rigidly defined, among other reasons), and not many people regard `[True]` as "An array of strings and integers" (though it certainly is). There's another consideration, and it's the power of extensibility. The way it's written, this solution can be used without modification for lists containing tuples, and other things supporting + (fractions, for example). However, that feature is impossible to satisfy together with Liskov, of course.

List, list and list again

0 %

You ClassRoom Demo is Ready With the new tool CheckiO ClassRoom, you will be able to analyze your students' progress, have your own leaderboard, change missions order on the map, and so on.