Problem 1.1 Weights

Implement the planet data abstraction by completing the planet constructor and the size selector so that a planet is represented using a two-element list where the first element is the string 'planet' and the second element is its size. The total_weight example is provided to demonstrate use of the mobile, arm, and planet abstractions.

# The constructor and selectors of the planet

def planet(size):
    """Construct a planet of some size.
    
    >>> planet(5)
    ['planet', 5]
    """
    assert size > 0
    "*** YOUR CODE HERE ***"

def size(w):
    """Select the size of a planet.
    
    >>> p = planet(5)
    >>> size(p)
    5
    """
    assert is_planet(w), 'must call size on a planet'
    "*** YOUR CODE HERE ***"

def is_planet(w):
    """Whether w is a planet."""
    return type(w) == list and len(w) == 2 and w[0] == 'planet'

# examples and usage

def examples():
    t = mobile(arm(1, planet(2)),
               arm(2, planet(1)))
    u = mobile(arm(5, planet(1)),
               arm(1, mobile(arm(2, planet(3)),
                              arm(3, planet(2)))))
    v = mobile(arm(4, t), arm(2, u))
    return (t, u, v)

def total_weight(m):
    """Return the total weight of m, a planet or mobile.

    >>> t, u, v = examples()
    >>> total_weight(t)
    3
    >>> total_weight(u)
    6
    >>> total_weight(v)
    9
    """
    if is_planet(m):
        return size(m)
    else:
        assert is_mobile(m), "must get total weight of a mobile or a planet"
        return total_weight(end(left(m))) + total_weight(end(right(m)))

Note: the code listed above is the part you should fill in for the lab, see lab04.py for the complete codes.