Custom sorting a list in python

Question

I want to sort the following list with some elements as dates and some are Quarters.

input_ls = ['Nov-2015', 'Quarter 1, 2016', 'Jan-2016', 'Dec-2015',
           'Feb-2016', 'Quarter 4, 2015']

Expected output is as follows. How can I do this?

output_ls = ['Quarter 1, 2016', 'Feb-2016', 'Jan-2016', 'Quarter 4, 2015'
             'Dec-2015', 'Nov-2015']

Answer 1

Here is a solution using datetutil and regex:

from dateutil import parser
import re
input_ls = ['Nov-2015', 'Quarter 1, 2016', 'Jan-2016', 'Dec-2015','Feb-2016', 'Quarter 4, 2015']
res = []
for x in input_ls:
    #Match x if it is having a digit followed by a comma followed by a space and followed by four digits
    qtr = re.findall('(\d),\s(\d{4})',x)
    #If match not found parse it as date by prefixing '1,' to the string
    if len(qtr) == 0:
        res.append(parse('1,' + x))
    #If matching then format it to a string format that is acceptable to the dateutil parser
    else:
        res.append(parse(str(int(qtr[0][0])*3)+'-30-'+qtr[0][1]))
#Sort the output list
out = zip(res,input_ls)
out.sort()
#Reverse it 
out.reverse()

And this is how the output will look like:

Here is another way of doing using sort key probably the most pythonic way - with help from Martijn Pieters

from dateutil import parser
import re
input_ls = ['Nov-2015', 'Quarter 1, 2016', 'Jan-2016', 'Dec-2015','Feb-2016', 'Quarter 4, 2015']

def sort_key(x):
    qtr = re.findall('(\d),\s(\d{4})',x)
    #If match not found parse it as date by prefixing '1,' to the string
    if len(qtr) == 0:
        return parse('1,' + x)
    #If matching then format it to a string format that is acceptable to the dateutil parser
    else:
        return parse(str(int(qtr[0][0])*3)+'-30-'+qtr[0][1])

input_ls.sort(key=sort_key)

And this is how the output will look like: