Object-Oriented Programming

A productive approach to defining new classes is to determine what instance attributes each object should have and what class attributes each class should have. First, describe the type of each attribute and how it will be used, then try to implement the class's methods in terms of those attributes.

Q1: Keyboard

Overview: A keyboard has a button for every letter of the alphabet. When a button is pressed, it outputs its letter by calling an output function (such as print). Whether that letter is uppercase or lowercase depends on how many times the caps lock key has been pressed.

First, implement the Button class, which takes a lowercase letter (a string) and a one-argument output function, such as Button('c', print).

The press method of a Button calls its output attribute (a function) on its letter attribute: either uppercase if caps_lock has been pressed an odd number of times or lowercase otherwise. The press method also increments pressed and returns the key that was pressed. Hint: 'hi'.upper() evaluates to 'HI'.

Second, implement the Keyboard class. A Keyboard has a dictionary called keys containing a Button (with its letter as its key) for each letter in LOWERCASE_LETTERS. It also has a list of the letters typed, which may be a mix of uppercase and lowercase letters.

The type method takes a string word containing only lowercase letters. It invokes the press method of the Button in keys for each letter in word, which adds a letter (either lowercase or uppercase depending on caps_lock) to the Keyboard's typed list. Important: Do not use upper or letter in your implementation of type; just call press instead.

Read the doctests and talk about:

• Why it's possible to press a button repeatedly with .press().press().press().
• Why pressing a button repeatedly sometimes prints on only one line and sometimes prints multiple lines.
• Why bored.typed has 10 elements at the end.

LOWERCASE_LETTERS = 'abcdefghijklmnopqrstuvwxyz'

class CapsLock:
    def __init__(self):
        self.pressed = 0

    def press(self):
        self.pressed += 1

class Button:
    """A button on a keyboard.

    >>> f = lambda c: print(c, end='')  # The end='' argument avoids going to a new line
    >>> k, e, y = Button('k', f), Button('e', f), Button('y', f)
    >>> s = e.press().press().press()
    eee
    >>> caps = Button.caps_lock
    >>> t = [x.press() for x in [k, e, y, caps, e, e, k, caps, e, y, e, caps, y, e, e]]
    keyEEKeyeYEE
    >>> u = Button('a', print).press().press().press()
    A
    A
    A
    """
    caps_lock = CapsLock()

    def __init__(self, letter, output):
        assert letter in LOWERCASE_LETTERS
        self.letter = letter
        self.output = output
        self.pressed = 0

    def press(self):
        """Call output on letter (maybe uppercased), then return the button that was pressed."""
        self.pressed += 1
        if self.caps_lock.pressed % 2 == 1:
            self.output(self.letter.upper())
        else:
            self.output(self.letter)
        return self

class Keyboard:
    """A keyboard.

    >>> Button.caps_lock.pressed = 0  # Reset the caps_lock key
    >>> bored = Keyboard()
    >>> bored.type('hello')
    >>> bored.typed
    ['h', 'e', 'l', 'l', 'o']
    >>> bored.keys['l'].pressed
    2

    >>> Button.caps_lock.press()
    >>> bored.type('hello')
    >>> bored.typed
    ['h', 'e', 'l', 'l', 'o', 'H', 'E', 'L', 'L', 'O']
    >>> bored.keys['l'].pressed
    4
    """
    def __init__(self):
        self.typed = []
        self.keys = {c: Button(c, self.typed.append) for c in LOWERCASE_LETTERS}

    def type(self, word):
        """Press the button for each letter in word."""
        assert all([w in LOWERCASE_LETTERS for w in word]), 'word must be all lowercase'
        for w in word:
            self.keys[w].press()

Description Time: Describe how new letters are added to typed each time a Button in keys is pressed. Instead of just reading your code, say what it does (e.g., "When the button of a keyboard is pressed ..."). One short sentence is enough to describe how new letters are added to typed.