How to Sort List by Date in Python

In this tutorial, we will guide you through the process of sorting a list of date strings in Python. Specifically, we'll demonstrate how to sort a list by date and time, providing you with a clear example.

This Python program focuses on arranging a list of date strings in ascending order.

To begin, the program imports the datetime module from the datetime library, which is essential for handling date and time-related operations.

Next, it defines a list called inputDateList, containing four date strings, each in the 'dd-mm-yyyy' format.

The program proceeds to utilize the sort() function on the inputDateList. During the sorting process, a lambda function is employed as the key parameter to specify the comparison rule. The lambda function takes each date from the list and transforms it into a datetime object using the strptime() function.

The strptime() function parses the provided date string according to the specified format ("%d-%m-%Y") and converts it into a datetime object, facilitating proper sorting based on date and time.

Through this example, you will gain a comprehensive understanding of how to effectively sort a list of date and time values in Python.

main.py

# importing datetime
from datetime import datetime
  
# input list of date strings
inputDateList = ['23-07-2023', '05-06-2023', '04-06-2023', '02-06-2023']
  
# sorting the input list by formatting each date using the strptime() function
inputDateList.sort(key=lambda date: datetime.strptime(date, "%d-%m-%Y"))
  
# Printing the input list after sorting
print("List Sorting by Date:\n", inputDateList)

Output:

List Sorting by Date: ['02-06-2023', '04-06-2023', '05-06-2023', '23-06-2023']

Description: 

1. We import the datetime module from the datetime library.
2. We define a list called inputDateList containing four date strings in the 'dd-mm-yyyy' format.
3. We use the sort() method on inputDateList, and we provide a lambda function as the key parameter. This lambda function converts each date string into a datetime object using the strptime() function.
4. The strptime() function parses each date string according to the specified format ("%d-%m-%Y") and returns it as a datetime object, allowing proper date sorting.
5. Finally, we print the sorted list, which will display the dates in ascending order

Here's another Python code example that demonstrates how to sort a list of date strings in ascending order using the datetime module. In this example, we'll use a list of dates with various formats.

from datetime import datetime

# Define a list of date strings with different formats
inputDateList = ['10-08-2023', '22/05/21', '2022-12-05', 'Mar 15, 2020']

# Define a function to convert date strings into datetime objects
def convert_to_datetime(date_str):
    formats = ["%d-%m-%Y", "%d/%m/%y", "%Y-%m-%d", "%b %d, %Y"]
    for fmt in formats:
        try:
            return datetime.strptime(date_str, fmt)
        except ValueError:
            pass

# Sort the list by date
inputDateList.sort(key=convert_to_datetime)

# Print the sorted list
for date in inputDateList:
    print(date)

In this example:

1. We define a list called inputDateList containing date strings with different formats.
2. We create a function called convert_to_datetime(date_str) to convert date strings into datetime objects. This function iterates through a list of possible date formats until it successfully converts the string. This allows us to handle dates in various formats.
3. We use the sort() method on inputDateList and provide the convert_to_datetime function as the key parameter. This function is responsible for converting each date string into a datetime object for proper sorting.
4. The strptime() function is used within the convert_to_datetime function to parse each date string according to the specified formats.
5. Finally, we print the sorted list, which will display the dates in ascending order, regardless of their original format.

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