Sorting list of lists based on a given string in Python

Question

I have a list of lists like the following:

list_of_lists = [
   ('test_ss', 'Test 1'),
   ('test_2_ss', 'Test 2'),
   ('test_3_ss', 'Test 3'),
   ('test_ss', 'Test 4')
]

I need to sort this list of lists by the first item in each list based on a given variable string.

As an example, I want to sort by 'test_ss' to the resulting list of lists would be:

sorted_list_of_lists = [
   ('test_ss', 'Test 1'),
   ('test_ss', 'Test 4'),
   ('test_2_ss', 'Test 2'),
   ('test_3_ss', 'Test 3'),
]

I've tried a number of examples off SO and others (Sorting a list of lists based on a list of strings, Sorting lists based on a particular element - Python, sorting multiple lists based on a single list in python, etc) but haven't found the right approach (or I've just not been following those examples correctly.

Any pointers?

@arman - thanks for the edit...I thought I did it right but apparently I did not :) — Eric D. Brown D.Sc.
– Eric D. Brown D.Sc., Commented Jan 5, 2016 at 21:24
What if the first string of two elements is equal, is the sorting arbitrary, or do you want to introduce a tie breaker? — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Jan 5, 2016 at 21:26
apparently you want some kind of a natural sort, since test_2_ss should come before test_ss (since 2's code is lower than s'). Do sorted(list_of_lists) and see it for yourself — Pynchia
– Pynchia, Commented Jan 5, 2016 at 21:27
What does it mean "sort by 'test_ss'", sort by the first position? — karakfa
– karakfa, Commented Jan 5, 2016 at 21:28

Sede · Accepted Answer · 2016-01-05 21:54:46Z

2

You can use a simple key function like this:

In [59]: def compare(element):
   ....:     return element[0] == 'test_ss' or 99999
   ....: 

In [60]: sorted(list_of_lists, key=compare)
Out[60]: 
[('test_ss', 'Test 1'),
 ('test_ss', 'Test 4'),
 ('test_2_ss', 'Test 2'),
 ('test_3_ss', 'Test 3')]

answered Jan 5, 2016 at 21:54

Sede

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1 Comment

Eric D. Brown D.Sc. Over a year ago

This does exactly what I need in a way that is perfect for my implementation. Thank you.

Open AI - Opting Out · Accepted Answer · 2016-01-05 21:32:21Z

1

If you want to partition, just return False if the string matches:

>>> sorted(list_of_lists, key=lambda value: value[0] != 'test_ss')
[('test_ss', 'Test 1'),
 ('test_ss', 'Test 4'),
 ('test_2_ss', 'Test 2'),
 ('test_3_ss', 'Test 3')]

edited Jan 5, 2016 at 21:32

answered Jan 5, 2016 at 21:30

Open AI - Opting Out

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7 Comments

Pynchia Over a year ago

that may turn out to be a good wild guess on what the OP wants

Open AI - Opting Out Over a year ago

@Pynchia wild guess!

Pynchia Over a year ago

OK, maybe I am the only one who hasn't understood the question. I thought he wanted to sort by a specific position of the tuple, but it's unclear (see above my comment to the question)

Eric D. Brown D.Sc. Over a year ago

@PeterWood - I think this is actually it. Let me pay around with it.

Pynchia Over a year ago

@PeterWood upvoted. I think you'd nailed it from the beginning

|

karakfa · Accepted Answer · 2016-01-05 22:18:19Z

0

this will do

tuples = [
   ('test_ss', 'Test 1'),
   ('test_2_ss', 'Test 2'),
   ('test_3_ss', 'Test 3'),
   ('test_ss', 'Test 4')
]

sorted(tuples, key=lambda x: x[0].replace('test_ss','test_ _ss'))

[('test_ss', 'Test 1'),
 ('test_ss', 'Test 4'),
 ('test_2_ss', 'Test 2'),
 ('test_3_ss', 'Test 3')]

you want to treat 'test_ss' semantically as the smallest element in sorting order but it's not in textual representation. key.replace(..) handles it.

answered Jan 5, 2016 at 22:18

karakfa

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