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Comment by cma | Hacker News Reader

No one mentioned Python's strong typing hidden feature:

    (True + True) == 2

(sorry, one of my few pet-peeves after it bit me one time)

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This is just a side-effect of bool being a subclass of int and True and False having the values 1 and 0, respectively. It might seem weird or surprising, but it's not a problem with Python's type system.

    >>> issubclass(bool, int)
    True
    >>> isinstance(True, int)
    True
    >>> int(True)
    1
    >>> int(False)
    0
    >>> True == 1
    True
    >>> False == 0
    True

Seems fine just so long as (False + False) doesn't equal True.

When is it ever desirable? I'm open to it having a use, but I don't see one. The only thing I've gotten from it is mismatched types that made it past some unit tests but blew up in the real world.

I was making a glib joke, but to answer your question this is the tradeoff of using a not-so-strongly-typed language. Weakly-typed languages often save you some typing (if you'll forgive the pun) and implicitly convert in ways that can make it difficult to prevent those mistakes.

It's just like Duck Typing- you're trading the need to explicitly specify an interface for the potential dangers of letting your interpreter "know what you mean".

If this is frequently a problem, well... there are a lot of strongly-typed languages out there waiting for you to give them a try.

Python is fairly strongly typed in general. (At least compared to Perl and the like. Examples like True + True notwithstanding.) It is just also very dynamic.

> I was making a glib joke, but to answer your question this is the tradeoff of using a not-so-strongly-typed language.

Not in this case. In this case it results from:

1. Python historically not having booleans and using 0 and 1 instead

2. As a result, `bool` inherits from `int` for compatibility reasons

    >>> issubclass(bool, int)
    True

with False mapping to 0 and True to 1 for compatibility purposes

    >>> int(False), int(True)
    (0, 1)

3. Add isn't overridden on bool

    >>> bool.__add__
    <slot wrapper '__add__' of 'int' objects>

therefore, when adding two booleans you're really adding two integers.

Most non-strongly-typed languages will not have the following behavior

    >>> "foo" + 3
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: cannot concatenate 'str' and 'int' objects

In fact, to expand on the latter part of my comment, "weak typing" is generally understood as implying implicit type coercions or conversions at runtime. PHP and Javascript famously convert between numbers and strings depending on a bunch of factors (and usually the wrong way leading to unreliable results).

In that sense, Python is a very strongly typed language: from the top of my head, the only example of implicit type conversion in Python is the promotion of `int` to `long`, which has been removed from 3.x (all integer types were unified). All other conversions have to be explicit as far as I remember. That makes Python more strongly (but less statically) typed than Java (see the very strange overloading of the `+` operator when any operand is a string, as well as autoboxing since 1.5) and C# (C# allows for implicit type conversions, including in userland, and the cast operator doubles up as an explicit type conversion operator which I find terrible)

I used it in Code Golf to select a string from a list based on the truth of some conditions.

So no, it does not have a use :)

It's useful because it allows you to add together booleans to get a count of how many are true.

You can check if two out of three are true with

(bool1 + bool2 + bool3) >= 2

You can do

count_negatives = sum(number < 0 for number in numbers)

This is only tangentially related, but try looking up "Iverson brackets" and Knuth's "Two notes on notation". One argument there is that False - True = -1 makes sign(x) = [x > 0] - [x < 0], which is convenient. I don't know of a case where True + True = 2 helps, but maybe something involving dual sums would be relevant.

The Iverson Bracket would already exist in a python that didn't subclass bool from int; '[boolean_expression]' would be, in python notation: 'int(boolean_expression)'.

Sounds kinda like I'm making the same argument as Knuth in forcing '[]' over '()'.

In mathematics

  True + True == False

might be a common interpretation.

When? Because the + symbol is often used to represent boolean OR, so T + T == T.

Maybe with a one-bit half-adder?

Two element finite field.

Indeed. There + becomes isomorphic to XOR and * to AND.

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