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Comment by ff0066mote | Hacker News Reader

Forgive me if I am wrong, but I believe that section 1 contains mistakes. In the implementations of evens_only, shouldn't both of the places where modulus is used be preceded by "not"?

This isn't meant to detract from the truth that "Beautiful [code] is better than ugly," I merely thought the implementation should be correct.

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or better yet i % 2 == 0

I would go for more pythonic "not i % 2".

I would argue that that is not very pythonic, since it breaks the "explicit is better than implicit" guideline.

Most Python people are okay with relying on the behaviour of what is truthful and what is not. They would often prefer to see

    if foo:
        do_stuff()

over

    if len(foo) > 0:
        do_stuff()

I agree. Using the truthiness of non-boolean values has always seemed questionable in any language (Javascript is possibly the worst offender). For many statically typed languages this is caught as a syntax error, which is nice for those of use prone to occasionally typos.

The boolean equivalents of non-boolean values are well-defined and fairly simple in Python, and I believe it's encouraged (or at least extremely common) to use them as such.

In Haskell this is caught as a type error. But you can make your own version:

    class ExtendedBool t where
        toBool :: t -> Bool

    instance ExtendedBool Int where
        toBool = (0/=)
    instance ExtendedBool [a] where
        toBool = not . null

    if' v a b =
        if (toBool v)
        then  a
        else b

    main = if' "String"
           (print "True")
           (print "False")

Yeah, I don't think you can use i % 2 as an equivalent of i % 2 == 0. Maybe it's a python 3 thing, but on my 2.6.1 the mod operator just returns the remainder, it doesn't change its output if it's a boolean context (which would be perlish and highly unpythonic).

I think i % 2 is just straight up incorrect.

If we're talking about style, I think i % 2 == 0 is much more clear than (not i % 2). It says explicitly that the remainder of i divided by 2 is 0.

> Yeah, I don't think you can use i % 2 as an equivalent of i % 2 == 0. Maybe it's a python 3 thing, but on my 2.6.1 the mod operator just returns the remainder, it doesn't change its output if it's a boolean context (which would be perlish and highly unpythonic).

    $ python3
    >>> not 0
    True

Python doesn't have Perl-like contexts, but it does treat lots of values as True or False. I do agree that

    i % 2 == 0

is more explicit here.

It seems to work in older Python versions (tested with 2.5). I think the new implementation is more natural: even%2 = 0, and 0 is false, while uneven%2 = 1, and 1 is true.

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