How to sort the list based on the string?

Question

Here I am try to sort the list of strings based on the one string, Here my test cases,

First Case:

par_str = "abcd" list_str = ['bb','ba','ab'] sorted(list_str, key=par_str)

Output: ['ab','ba','bb']

Second Case:

par_str = "bacd" list_str = ['bb','ba','ab'] sorted(list_str, key=par_str)

Output: ['bb','ba','ab']

From the First case list order changed based on par_str string. And Second test case the list order no change because it has same priority. So How to write above logic. I have tried but I am getting an error TypeError: 'str' object is not callable

The key argument in sorted must be a callable, as in not a string. — Joel Cornett
– Joel Cornett, Commented Feb 25, 2014 at 5:48
@dhana in sorted(list_str, key=par_str), key=par_str is wrong — Grijesh Chauhan
– Grijesh Chauhan, Commented Feb 25, 2014 at 6:06

falsetru · Accepted Answer · 2014-02-25 05:50:17Z

Make a mapping between characters and ranks, and use that information in key function.

>>> par_str = "abcd" >>> order = {c:i for i, c in enumerate(par_str)} # <------------ >>> order {'a': 1, 'c': 2, 'b': 0, 'd': 3} >>> list_str = ['bb','ba','ab'] >>> sorted(list_str, key=lambda s: [order[c] for c in s]) ['ab', 'ba', 'bb'] >>> par_str = "bacd" >>> order = {c:i for i, c in enumerate(par_str)} # <----------- >>> order {'a': 1, 'c': 2, 'b': 0, 'd': 3} >>> list_str = ['bb','ba','ab'] >>> sorted(list_str, key=lambda s: [order[c] for c in s]) ['bb', 'ba', 'ab']

@dhana which line you don't understand. it is sorted on the basis of list of numbers. Very char is mapped to a number using order as list.
@dhana, The return value of the key function is used as comparison keys.
@dhana try >>> [1, 2, 3] < [1, 3, 3] on your system. Also try: >>> [order[c] for c in s]

John La Rooy · Accepted Answer · 2014-02-25 06:39:59Z

>>> par_str = "abcd" >>> list_str = ['bb','ba','ab'] >>> >>> def key_func(s): ... return [par_str.index(i) for i in s] ... >>> sorted(list_str, key=key_func) ['ab', 'ba', 'bb'] >>> par_str = "bacd" >>> sorted(list_str, key=key_func) ['bb', 'ba', 'ab']

If par_str is long enough it will be worthwhile using the helper dict in @falsetru's answer.

It's preferable to use a full function definition rather than a lambda if you would like to be able to test your key function

Here is a little trick to make the key function really fast. (The dict is subtly different to @falsetru's)

>>> par_str = "abcd" >>> list_str = ['bb','ba','ab'] >>> sorted(list_str, key=lambda s, trans={ord(c): i for i, c in enumerate(par_str)}:s.translate(trans)) ['ab', 'ba', 'bb'] >>> par_str = "bacd" >>> sorted(list_str, key=lambda s, trans={ord(c): i for i, c in enumerate(par_str)}:s.translate(trans)) ['bb', 'ba', 'ab']

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