A good class protects its own rules. Rather than letting any code change balance directly, a bank account offers methods that check things first:

class Account:
    def __init__(self, owner):
        self.owner = owner
        self.balance = 0

    def deposit(self, amount):
        if amount <= 0:
            raise ValueError("deposit must be positive")
        self.balance += amount

acc = Account("Ada")
acc.deposit(50)
print(acc.balance)
try:
    acc.deposit(-10)
except ValueError as e:
    print("Refused:", e)

Python doesn't strictly stop anyone writing acc.balance = 1000000, but by convention code outside the class uses the methods. (Attributes meant to be private are often named with a leading underscore, like _balance, as a sign: hands off.)

When designing a class, ask two questions: what does it know? (attributes) and what can it do? (methods). Nouns in a description often become classes and attributes; verbs often become methods.