input: Write a function to create a BankAccount class that can deposit, withdraw and check balance.
  assert account.get_balance() == 0
  account.deposit(100)
  assert account.get_balance() == 100
  account.withdraw(50)
  assert account.get_balance() == 50
  account.deposit(200)
  assert account.get_balance() == 250
output: def BankAccount():
      class BankAccount:
          def __init__(self):
              self.balance = 0
          
          def deposit(self, amount):
              self.balance += amount
          
          def withdraw(self, amount):
              if amount <= self.balance:
                  self.balance -= amount
          
          def get_balance(self):
              return self.balance
      
      return BankAccount()
  
  account = BankAccount()