Enforce items at beginning and end of list

Question

How can I modify this list so that all p\'s appear at the beginning, the q\'s at the end, and the values in between are sorted alphabetically?

Answer 1

You can use the following lambda function as the key in sorted():

l1 = sorted(l, key=lambda x: ((x == 'q') - (x == 'p'), x))

print(l1)
# ['p', 'p', 'a', 'b', 'c', 'd', 'f', 'g', 'n', 't', 'z', 'q', 'q']

The function generates the following comparison keys:

func = lambda x: ((x == 'q') - (x == 'p'), x)

for i in l1:
    print(func(i))

Output:

(-1, 'p')
(-1, 'p')
(0, 'a')
(0, 'b')
...
(0, 't')
(0, 'z')
(1, 'q')
(1, 'q')