def move(disks, src, dest, intermediate):
    if not disks:
        return
    move(disks[1:], src, intermediate, dest)
    movedisk(disk[0], src, dest)
    move(disks[1:], intermediate, dest, src)

def movedisk(disk, src, dest):
    print "move %d from %s to %s" % (disk, src, dest)

if __name__ == "__main__":
    move([3, 2, 1], "A", "B", "C")

def exponent(x, n):
    # ...
    pass

def exponent(x, n):
    if n == 0:
        return 1
    else:
        return x * exponent(x, n-1)

def fast_exponent(x, n):
    if n == 0:
        return 1
    elif n % 2 == 0:
        return exponent(x*x, n/2)
    else:
        return x * exponent(x, n-1)

def count_change(amount, coins):
    """Find the number of ways to change amount using the specified
    coin denominations.

    Assume that the denominations are specified in the descending order.
    """
    # ...

def count_change(amount, coins):
    if amount == 0:
        return 1
    elif amount < 0:
        return 0
    elif len(coins) == 0:
        return 0
    else:
       return count_change(amount-coins[0], coins) + \
              count_change(amount, coins[1:])

def yrange(n):
    i = 0
    while i < n:
        yield i
        i += 1

>>> y = yrange(3)
>>> y.next()
0
>>> y.next()
1
>>> y.next()
2
>>> y.next()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 14, in next
StopIteration

>>> list(yrange(3))
[0, 1, 2]

def integers():
    i = 1
    while True:
        yield i
        i = i + 1

def squares():
    for i in integers():
        yield i * i

def take(n, seq):
    """Returns first n values from the given sequence."""
    seq = iter(seq)
    result = []
    try:
        for i in range(n):
            result.append(seq.next())
    except StopIteration:
        pass
    return result

>>> take(5, squares())
[1, 4, 9, 16, 25]

def integers():
    i = 1
    while True:
        yield i
        i = i + 1

def squares():
    return (i*i for i in integers())

def take(n, seq):
    seq = iter(seq)
    return list(seq.next() for i in range(n))

>>> take(5, squares())
[1, 4, 9, 16, 25]

def permutations(xs):
    if not xs:
        return [[]]
    else:
        return ([x] + xs1 for x in xs
                          for xs1 in permutations(remove(x, xs)))

def solve(grid):
    pos = find_empty_position(grid) 
    if not pos:
        return grid

    ri, ci = pos
    for n in find_possible_values(grid, pos):
        grid[ri][ci] = n
        soln = solve(grid)
        if soln:
            return soln

    # failed to find a solution
    # put the original value back
    grid[ri][ci] = '.'

# sample puzzle are at http://magictour.free.fr/msk_009

def find_empty_position(grid):
    for row in range(9):
        for col in range(9):
            if grid[row, col] == '.':
                return (row, col)

def find_possible_values(grid, pos):
    return set("123456789") - \
            set(get_row(grid, pos)) - \
            set(get_col(grid, pos)) - \
            set(get_block(grid, pos))