Class for sorting pool balls

Question

I have written this code for fun. I'd like to hear your suggestions about how to make it more compact and pythonic.

from random import shuffle
class Pool(object):
 """
 Simple object that allows to sort
 """
 def __init__(self):
 """
 During initalization the final grid containing
 ordered balls is created
 1 is placeholder for Solid balls
 0 is placeholder for Striped balls
 """
 self.grid = [[1],[0,0],[1,0,1],[0,1,0,0],[1,0,1,0,1]]
 def create_ball_set(self):
 """
 This function returns a list containing
 a shuffled pool balls set
 """
 balls = []
 [balls.append([number]) for number in range(1, 16)]
 for ball in balls:
 if ball[0] < 9:
 ball.append("Solid")
 else:
 ball.append("Striped")
 shuffle(balls)
 return balls
 def sort_ball_set(self, unsorted_balls):
 """
 This function returns a
 list of sorted balls from a list of
 shuffled balls
 """
 # Ball 1 always goes in 1st place
 self.grid[0][0] = unsorted_balls.pop(unsorted_balls.index([1, 'Solid']))
 # Ball 8 always goes in the 2nd row, in the middle
 self.grid[2][1] = unsorted_balls.pop(unsorted_balls.index([8, 'Solid']))
 # Creating an empty list for solid balls
 unsorted_solid_balls = []
 # Same thing but for striped balls
 unsorted_striped_balls = []
 # Now it is time to divide solid balls from striped ones
 for ball in unsorted_balls:
 if ball[1] == 'Solid':
 unsorted_solid_balls.append(ball)
 elif ball[1] == 'Striped':
 unsorted_striped_balls.append(ball)
 # Once the balls are divided it is time to put them in the grid
 for grid_row_index, grid_row in enumerate(self.grid):
 for grid_col_index, grid_col_value in enumerate(grid_row):
 # In their corresponding placeholder spot
 if grid_col_value == 1:
 # 1 is for solid balls
 self.grid[grid_row_index][grid_col_index] = unsorted_solid_balls.pop()
 elif grid_col_value == 0:
 # 0 is for striped balls
 self.grid[grid_row_index][grid_col_index] = unsorted_striped_balls.pop()
 return self.grid
pool_table = Pool()
print "Getting new set of balls from the pool table:"
unsorted_balls = pool_table.create_ball_set()
print unsorted_balls
print "Sorted balls:"
sorted_balls = pool_table.sort_ball_set(unsorted_balls)
# Pretty printing sorted balls
for row in sorted_balls:
 print row

Answer 1

There are several aspects of your self.grid that I don't like:

• It is constant, so it should be in ALL_CAPS.
• It can be shared by all instances of Pool, so it should be an attribute of the Pool class, not of each Pool instance.
• The convention of using 1 and 0 to represent solids and stripes is unnecessarily cryptic and necessitates comments like 1 is placeholder for Solid balls and # 1 is for solid balls. Everywhere else in your class, you are using strings 'Solid' and 'Striped', which is more obvious.
• It neglects to specify that the 1 ball always goes first and the 8 ball always goes in the middle. These two balls then have to be handled as exceptional cases in sort_ball_set().

It's hard to say what your Pool class represents. Each instance doesn't really have any object state. (As mentioned above, self.grid doesn't count.) You're basically just using it as a kind of namespace. I would repurpose it to represent a list of balls.

---

I don't think that it is a good idea to represent each ball as a list of length 2:

• Lists are most suited for storing homogeneous items of arbitrary length. An object with a fixed number of attributes, each of a different type, would be better represented using a tuple.
• The task already necessitates dealing with two-dimensional lists. Introducing more indexing makes the code more cumbersome and less readable.
• For even better readability, you should use a namedtuple, so that you can refer to ball.n and ball.coloration instead of magic indices like ball[0] and ball[1].

---

In create_ball_set(), this is an abuse of a list comprehension solely for its side-effects:

balls = []
[balls.append([number]) for number in range(1, 16)]

The weirdness is more apparent if you run it in the REPL:

>>> balls = []
>>> [balls.append([number]) for number in range(1, 16)]
[None, None, None, None, None, None, None, None, None, None, None, None, None, None, None]
>>> balls
[[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]]

The conventional expression should be balls = [[n] for n in range(1, 16)].

---

Suggested solution

I would put more work into defining a TEMPLATE that fully describes what the triangle has to look like.

from collections import namedtuple
import random
PoolBall = namedtuple('PoolBall', 'n coloration')
class PoolBalls(object):
 def filter(criterion):
 return (lambda ball: ball.n == criterion) if criterion in (1, 8) else \
 (lambda ball: ball.n not in (1, 8) and ball.coloration == criterion)
 TEMPLATE = [[filter(criterion) for criterion in row] for row in [
 [1],
 ['Striped', 'Striped'],
 ['Solid', 8, 'Solid'],
 ['Striped', 'Solid', 'Striped', 'Striped'],
 ['Solid', 'Striped', 'Solid', 'Striped', 'Solid'],
 ]
 ]
 def __init__(self):
 self.balls = [PoolBall(n, 'Solid') for n in range(1, 9)] + \
 [PoolBall(n, 'Striped') for n in range(9, 16)]
 def shuffle(self):
 random.shuffle(self.balls)
 def arrange(self):
 balls = self.balls[::-1]
 def pick(filter):
 for i in range(len(balls) - 1, -1, -1):
 if filter(balls[i]):
 return balls.pop(i)
 return [[pick(filter) for filter in row] for row in self.TEMPLATE]
if __name__ == '__main__':
 print "Getting new set of balls from the pool table:"
 pool_table = PoolBalls()
 pool_table.shuffle()
 print pool_table.balls
 print "Arranged balls:"
 for row in pool_table.arrange():
 print row

• Pitto: Simply wow. Another amazing input and fuel to learn and program for countless time. I deeply thank you!

Answer 2

I think it is usually better to take advantage of the built-in functions. Python lists already have a sort function, so you would expect a list of pool balls to be sortable by the start arrangement (a list of tuples/lists is already sortable, Python compares them element wise).

One way to achieve that is with a custom key function:

order = [1, 13, 11, 7, 8, 6, 9, 5, 14, 15, 2, 12, 3, 10, 4]
# Simplified balls list, as the fill style is not really needed for this
balls = [[n] for n in range(1, 16)]
balls.sort(key=lambda ball: order.index(ball[0]))

This is very succinct, but you could also promote the lambda key function to a proper function:

def pool_start_order(ball):
 order = [1, 13, 11, 7, 8, 6, 9, 5, 14, 15, 2, 12, 3, 10, 4]
 return order.index(ball[0])
balls = [[n] for n in range(1, 16)]
balls.sort(key=pool_start_order)

Another alternative is to make a PoolBall class, that is sortable, something like this:

class PoolBall(object):
 order = [1, 13, 11, 7, 8, 6, 9, 5, 14, 15, 2, 12, 3, 10, 4]
 def __init__(self, number):
 assert 1 <= number <= 15
 self.number = number
 self.fill = 'Solid' if number <= 8 else 'Striped'
 self.order = PoolBall.order.index(number)
 def __lt__(self, other):
 return self.order < other.order
 def __eq__(self, other):
 return self.number == other.number
 def __str__(self):
 return "{self.number} {self.fill}".format(self=self)
 def __repr__(self):
 return "{self.__class__.__name__}({self.number})".format(self=self)
 balls = [PoolBall(i) for i in range(1, 16)]
 balls.sort()

Here, the __str__ and __repr__ are just there for pretty printing.

Whichever way you go, once you have a correctly sorted list of balls, you can use a function that successively takes one more ball, something like:

def pyramid_arrangement(balls):
 """
 Given a list of balls, arranges them in a pyramid shape.
 Yields each row.
 Leaves the balls list empty.
 """
 take = 1
 while balls:
 yield [balls.pop() for _ in range(take)]
 take += 1

Which you can use like this:

if __name__ == "__main__":
 balls = [PoolBall(i) for i in range(1, 16)]
 balls.sort(reverse=True)
 for row in pyramid_arrangement(balls):
 print row

• Pitto: This is simply great. I've quickly read it twice and I had the chance to stare ad the abyss of my ignorance. Once recovered I'll put serious time into understanding your code. Thanks for your time, it is highly appreciated.
• 200_success: I don't understand what you are doing with order = [1, 13, 11, ..., 4]. Won't that result in exactly the same specific arrangement every time?