In this assignment, you will use the object-oriented programming paradigm to model a bank account in Python. Like in the previous assignments, we are not building a user interface, but underlying data structures and functionality which will be thoroughly tested.

We will provide a polished API so that other developers can use our code to build their applications. That means, special attention will be given to implement data model methods such as __str__ and __repr__.

The account class represents a bank account. It is constructed without any arguments.

my_account = Account()

Here's an example of how the end product usage could look:

>>> account = Account()
>>> account.deposit(200)
>>> account.get_balance()
200.0
>>> account.withdraw(10)
>>> account.get_balance()
190.0

A transaction class represents a monetary transaction event. It has two properties: an amount, and a timestamp.

A Transaction class must be instantiated with an amount argument, and has an optional timestamp which defaults to None. If the timestamp is not provided, it is automatically set to the current date & time.

>>> from banking import Transaction
>>> transaction = Transaction(100.0)
>>> transaction.timestamp
datetime.datetime(2018, 12, 31, 8, 3, 1)
>>> transaction.amount
100.0

>>> import datetime as dt
>>> transaction = Transaction(10, dt.datetime(2002, 1, 10))
>>> transaction.timestamp
datetime.datetime(2002, 1, 10, 0, 0, 0)

• (/1) Write you classes in a module called banking.py

• (/1) Write your tests in a module called test_banking.py

• Write a class called Transaction as defined above:

  • (/2) Class definition
  • (/4) __init__ method & tests ensuring that:
    • The amount parameter is bound to self
    • The provided optional timestamp parameter is bound to self. If no value is provided, use the current datetime.
  • (/2) __repr__ which returns an appropriate string + test. Remember, good practice for a repr is to produce a string which can be copy & pasted into the interpreter to reproduce a similar instance.
  • (/2) __str__ which returns a string containing both the timestamp and amount. E.g.
    • 2019-01-01: +$1,234.56
    • 2020-01-01: -$17.25

• Write a class called Account as defined above:

  • (/2) Class definition
  • (/6) A deposit method which:
    • (/1) Is named deposit
    • (/1) Accepts a single parameter called amount
    • (/2) The amount is converted to a positive value + tests
    • (/2) A new instance of Transaction is created using the amount. The new transaction instance is then appended to the account's transactions. Provide test(s).
  • (/6) A withdraw method which:
    • (/1) Is named withdraw
    • (/1) Accepts a single parameter called amount
    • (/2) The amount is converted to a negative value + tests
    • (/2) A new instance of Transaction is created with the converted amount. The instance is appended to the list of transactions. Provide test(s).
  • (/5) A balance retrieval method which:
    • (/1) Is named get_balance
    • (/4) Returns the sum of all transaction amounts stored on the object.
      • If no transactions exist, this should return 0
      • If the transactions consist of a deposit of 100, a withdrawl of 90, and deposit of 10, the balance should be 20.
      • Challenge: see if you can figure out how to use the sum() function on the transactions list

The final commit must have a green checkmark beside it as a result of a successful Travis CI run.

-1% will be deducted from the assignment for each code violation.

Note: Beware of mutable default values as class properties. Read the following article on a common "gotcha" surrounding the defining default values using mutable types like lists and dicts.

https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments

The same situation described applies to classes. Example:

# Danger!
class MyClass:
    my_list = []


# Safe
class MyClass:
    def __init__(self):
        self.my_list = []