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-4']

Attempt 1

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

I was not happy with this since it required splitting a string twice, to extract the relevant components.

Attempt 2

I came up with the below solution. But I am hoping there is a more succinct solution via Pythonic lambda statements.

def sorter_func(x):
    text, num = x.split('-')
    return int(num), text

res = sorted(lst, key=sorter_func)

I looked at Understanding nested lambda function behaviour in python but couldn't adapt this solution directly. Is there a more succinct way to rewrite the above code?

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.

Answer 2

We can wrap the list returned by split('-') under another list and then we can use a loop to handle it:

# Using list-comprehension
>>> sorted(lst, key=lambda x: [(int(num), text) for text, num in [x.split('-')]])
['a-2', 'd-2', 'b-3', 'c-4']
# Using next()
>>> sorted(lst, key=lambda x: next((int(num), text) for text, num in [x.split('-')]))
['a-2', 'd-2', 'b-3', 'c-4']