Nested lambda statements when sorting lists

Question

I wish to sort the below list first by the number, then by the text.

lst = [\'b-3\', \'a-2\', \'c-4\', \'d-2\']

# result:
# [\'a-2\', \'d-2\', \'b-3\', \'c-         


        

Answer 1

There are 2 points to note:

• One-line answers are not necessarily better. Using a named function is likely to make your code easier to read.
• You are likely not looking for a nested lambda statement, as function composition is not part of the standard library (see Note #1). What you can do easily is have one lambda function return the result of another lambda function.

Therefore, the correct answer can found in Lambda inside lambda.

For your specific problem, you can use:

res = sorted(lst, key=lambda x: (lambda y: (int(y[1]), y[0]))(x.split('-')))

Remember that lambda is just a function. You can call it immediately after defining it, even on the same line.

Note #1: The 3rd party toolz library does allow composition:

from toolz import compose

res = sorted(lst, key=compose(lambda x: (int(x[1]), x[0]), lambda x: x.split('-')))

Note #2: As @chepner points out, the deficiency of this solution (repeated function calls) is one of the reasons why PEP-572 is considered implemented in Python 3.8.