Challenge 3: Fix the Shared-List Bug — Possible Solution
====================================================================

class Dog:
    def __init__(self, name):
        self.name = name
        self.tricks = []   # instance attribute now, not a class attribute

    def add_trick(self, trick):
        self.tricks.append(trick)

rex = Dog("Rex")
fido = Dog("Fido")
rex.add_trick("sit")

print(fido.tricks)   # [] — Fido is unaffected

WHY THIS WORKS AS AN ANSWER
------------------------------
The buggy version's problem, per the chapter's warn-box, is that
tricks = [] sits in the CLASS body, making it one single list object
shared by every Dog instance — appending to it through any one dog
mutates the same list every other dog also sees.

The fix moves self.tricks = [] into __init__, turning it into an
INSTANCE attribute instead — the exact instance-vs-class distinction
this chapter opened with. Because __init__ runs separately for every
new Dog object, each call creates its OWN brand-new empty list, the
same "create a fresh instance's own copy inside __init__" principle
this chapter's Bank Account challenge also relies on.

add_trick still works the same way syntactically (self.tricks.append
(trick)), but now self.tricks refers to that one dog's own private
list rather than a class-wide shared one. So rex.add_trick("sit")
only ever touches rex's own tricks list, and fido.tricks correctly
stays the empty list it was created with — no leakage between
instances, which is exactly what the buggy version failed to
guarantee.