I find this behavior surprising:

  >>> d = {0: "int"}
  >>> d[False] = "bool"
  >>> d
  {0: 'bool'}
  >>> nan = float('nan')
  >>> d[nan] = 0
  >>> d[nan] = 1
  >>> d
  {0: 'bool', nan: 0, nan: 1}

Yes, yes, it's because issubclass(bool, int) == True, and nan != nan, but weird.

And if we include Python 2 you get the truly ridiculous:

  >>> dict() < set()
  True
  >>> dict < set()
  False
  >>> 0 < dict
  True
  >>> 0 < 1j
  TypeError: no ordering relation is defined for complex numbers

To compare e.g. a dictionary and set we compare the _names_ of the types! Except complex numbers, because they don't have an ordering. I actually have come across this as a bug in production.

Personally I'd just make any comparison involving different types an error. That probably interacts poorly with subtyping, but it's a price I'm willing to pay.

The `nan` thing is actually because `float('nan')` can (and does) return a new object (well, with a different ID) each time. So it's like going `d[object()] = 'a'; d[object()] = 'b'` and not retaining the objects for lookup.

> Personally I'd just make any comparison involving different types an error

As you point out, this is what Python 3 did for `None`. But it's been idiomatic to have 0 = False when languages didn't have `False`; so `bool(0) == False` makes sense, but also `sum(boolean for boolean in sequence)` works. So you could argue that for `True` and `False`, meaningful coercion is possible, and in reverse, if zero is false-y and everything else is truth-y (like in C), then that also makes sense. Imagine if you had to cast every number to a boolean explicitly! So I guess having `d[0] == d[False]` is the lesser of evils.

My favourite of these is Haskell

> "" = [] True

Which makes perfect sense if you know any Haskell, but looks like a JavaScript wat if you don't.

I gave a talk on Python idiosyncrasies a while back that you might enjoy:

https://speakerdeck.com/alangpierce/python-puzzlers

Several of the examples from that talk have equivalents in JavaScript where JavaScript does better. I think pretty much any real-world language has lots of surprises like this that you need to get used to, although I certainly admit JavaScript (particularly JS type coercion) tends to have especially surprising behavior.

I'd say JSs biggest problem is that the surprising behaviours aren't mostly sitting in dark corners waiting to bite you. They're standing there in the open, where everyone is likely to encounter them.

> Find an equivalently surprising example in... Let's use the other poster's choice, python.

> I'll wait.

I don't write Python, but even so it took about 30 seconds, using the same area of the linked article as a starting point:

    >>> 0 > None
    True

At this point, I fully expect some attempt at a post-hoc rationalization for why Python's choice here is unimpeachable.

python 2 made a rational, well-considered choice, though I do like the python 3 replacement better. Despite its self-consistency, python 2 ended up causing hard-to-find errors (mostly when people accidentally compared "0" with 0 though; it mostly wasn't a problem with None).

In python 2, cross-type comparisons are strictly ordered. So:

    >>> None == 0
    False
    >>> None < 0
    True
    >>> None <= 0
    True
    >>> None > 0
    False

Completely consistent, no special cases, and if you sort() a list containing multiple types, it'll always come out in an unsurprising, consistent order.

Is this python 2 or python 3, afaik that was fixed in python 3.

This error has been fixed in Python 3.

Nope, that's pretty ridiculous!