Python | Sort list of dates given as strings

Last Updated : 14 Dec, 2018

Given a list of dates in string format, write a Python program to sort the list of dates in ascending order.

Examples:

Input : dates = [“24 Jul 2017”, “25 Jul 2017”, “11 Jun 1996”, “01 Jan 2019”, “12 Aug 2005”, “01 Jan 1997”]
Output : 01 Jan 2007
10 Jul 2016
2 Dec 2017
11 Jun 2018
23 Jun 2018
01 Jan 2019

Approach:
In Python, we can use sort() (for in-place sorting) and sorted() (returns a new sorted list) functions for sorting lists. But by default, these in-built sorting functions will sort the list of strings in alphabetical order which would result in a wrong order in our case. Hence, we need to pass a key argument to tell the sorting function that we need to compare the list items in a particular way and sort them accordingly.

In Python, we have the datetime module which makes date based comparison easier. The datetime.strptime() function is used to convert a given string into datetime object. It accepts two arguments: date (string) and format (used to specify the format. for eg: %Y is used for specifying year) and returns a datetime object.

Syntax:

datetime.strptime(date, format)

The formatting that we require for this problem is as follows:

%d ---> for Day
%b ---> for Month
%Y ---> for Year

Hence, we need to pass the datetime object as the key argument in the sorting function to tell the sorting function that it needs to compare the strings by converting them into dates and sort them in the increasing order.

Below is the implementation of the above approach:

# Python3 program to sort the list of dates
# given in string format
  
# Import the datetime module
from datetime import datetime
    
# Function to print the data stored in the list 
def printDates(dates): 
   
    for i in range(len(dates)):  
        print(dates[i]) 
       
       
if __name__ == "__main__":  
  
    dates =  ["23 Jun 2018", "2 Dec 2017", "11 Jun 2018", 
              "01 Jan 2019", "10 Jul 2016", "01 Jan 2007"]  
      
    # Sort the list in ascending order of dates 
    dates.sort(key = lambda date: datetime.strptime(date, '%d %b %Y'))
    
    # Print the dates in a sorted order 
    printDates(dates) 

Output:

01 Jan 2007
10 Jul 2016
2 Dec 2017
11 Jun 2018
23 Jun 2018
01 Jan 2019

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