A bit of seasonal fun: how well do you know Python?

[peterhinch]

Can you predict what this will do? Will it throw, and if so on which line? Otherwise, what output will it produce?

class Foo:
    bar = 0
    def __init__(self):
        self.bar += 1

foo_instance = Foo()
print(Foo.bar)
print(foo_instance.bar)

This is not a MicroPython bug. Other simple Python gotchas welcome.

[Josverl]

I think it will throw a HollidayException("The self.bar is not open yet")

[GitHubsSilverBullet]

This is not a MicroPython bug. Other simple Python gotchas welcome.

Interesting. I would have said that it throws the moment the instance is created.
Apparently as there's no instance variable (self.bar) to increment, init() seems to use the class variable instead.
That's nasty. ;-)

[rkompass]

I had the same thought and was surprised as well.
Thank you Peter for an interesting insight.
Of course the final Foo.bar value is now ???
print(Foo.bar)

[GitHubsSilverBullet]

Looks like

        self.bar += 1
        self.bar = Foo.bar + 1
        self.bar = __class__.bar + 1

are all the same. Though for clarity, I prefer the second one

[sosi-deadeye]

The code assigns 1 to the new instance variable self.bar, instead to the class variable bar.
If you do not assign self.bar, you've read access to the class variable through the instance.

Here an example:

import time


class Foo:
    _instances = set()

    @property
    def instances(self) -> int:
        """
        Return the number of instances
        """
        return len(self._instances)

    def __init__(self, name):
        # the static method __new__ could be
        # used to modify the class before returning it

        self._instances.add(self)
        # alternative, if assignment is required
        # cls = self.__class__
        # cls._instances = 42

        print(f"Foo has now {self.instances} instances")
        self.name = name
        self.timestamp = time.time()

    # __del__ is not called in Micropython,
    # but this works with CPython
    def remove(self):
        self._instances.remove(self)
        print(f"Foo has now {self.instances} instances")
        print(
            f"Removed {self.name} from instances of Foo after {time.time() - self.timestamp:.2f} s"
        )


print()
print("Instances of Foo:", Foo._instances)
f1 = Foo("First")
print("Instances of Foo:", Foo._instances)
print()
f2 = Foo("Second")
print("Instances of Foo:", Foo._instances)
print()
time.sleep(1)
f1.remove()
print("Instances of Foo:", Foo._instances)
print()
time.sleep(1)
f2.remove()
print("Instances of Foo:", Foo._instances)

[sosi-deadeye]

Additionally: This Code works because _instances is mutated and not assigned.

[peterhinch]

I think the moral of the story is always to reference a class variable by classname, and not by self:

class Foo:
    bar = 0
    def __init__(self):
       Foo.bar += 1

Season's greetings to all!

[rkompass]

O.K., another one:

nlist = [[]] * 4
nlist[0].append('hello')
nlist[1].append('world!')
print(nlist)

will print what?

[peterhinch]

Oh yes, I've encountered this before. Clue: what will this print?

>>> nlist[0] is nlist[1]

It's a very odd Python feature.

[shariltumin]

class K: pass

def a(o):
    o.n **= 2
    return o.n
def b(o,n):
    o.n = int(n**(0.5))
    return o.n

K.n = 2023; K.a = a; K.b = b

k0 = K()
print(k0.n == k0.b(k0.a()))

K.n = 2024

k1 = K()
print(k1.b(k1.a()) == k1.n)

print(k0.n == k1.n)
print(k0.n, '-->', k1.n)

Merry Christmas and Happy New Year.

[peterhinch]

I think this is another manifestation of the behaviour highlighted in the OP.