Python | Sort list of dates given as strings

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

# 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

Using sorted():

One approach that could be used to sort a list of dates in Python is to use the built-in sorted function in combination with a custom key function that converts each date string to a datetime object using the datetime.strptime function. The key function can then be passed to the sorted function as the key argument, causing the dates to be sorted according to their datetime representation rather than their string representation.

For example:

Python3

from datetime import datetime

def sort_dates(dates):
    # Define a key function that converts a date string to a datetime object
    def date_key(date_string):
        return datetime.strptime(date_string, '%d %b %Y')

    # Use the sorted function to sort the list of dates, using the date_key function as the key
    return sorted(dates, key=date_key)

# Example usage
dates = ["24 Jul 2017", "25 Jul 2017", "11 Jun 1996", "01 Jan 2019", "12 Aug 2005", "01 Jan 1997"]
sorted_dates = sort_dates(dates)
print(sorted_dates)  # Output: ["11 Jun 1996", "01 Jan 1997", "12 Aug 2005", "24 Jul 2017", "25 Jul 2017", "01 Jan 2019"]
#This code is contributed by Edula Vinay Kumar Reddy

Output

['11 Jun 1996', '01 Jan 1997', '12 Aug 2005', '24 Jul 2017', '25 Jul 2017', '01 Jan 2019']

This approach has a time complexity of O(nlogn) due to the use of the sorted function, which uses a sorting algorithm with a logarithmic time complexity. The space complexity is O(n) since the sorted function creates a new list with the same size as the original list to hold the sorted dates.