5 Best Ways to Sort a List of Dictionaries by Date in Python

💡 Problem Formulation: When dealing with lists of dictionaries in Python, we often need to sort them by dates stored within. Suppose you have a list of dictionaries where each dictionary contains a ‘date’ key with a date string value. The goal is to sort this list in ascending or descending order based on these date values. For instance, an unsorted list like [{‘date’: ‘2023-01-01’}, {‘date’: ‘2022-12-31’}] should be sorted to [{‘date’: ‘2022-12-31’}, {‘date’: ‘2023-01-01’}] for ascending order.

Method 1: Using sorted() Function with Custom Key Function

Sorting a list of dictionaries by date in Python can be efficiently achieved by using the sorted() function along with a custom key function. The custom key function extracts the date as a datetime object from each dictionary, which the sorted() function then uses to arrange the dictionaries.

Here’s an example:

from datetime import datetime

def parse_date(d_item):
    return datetime.strptime(d_item['date'], '%Y-%m-%d')

data = [
    {'date': '2023-01-01'},
    {'date': '2022-12-31'}
]

sorted_data = sorted(data, key=parse_date)

The output of this code snippet:

[
    {'date': '2022-12-31'},
    {'date': '2023-01-01'}
]

This method defines a helper function parse_date that parses the ‘date’ key in each dictionary to a datetime object. The list is then sorted according to the datetime objects, ensuring the dictionaries are ordered by date.

Method 2: Lambda Function Within sorted()

The use of a lambda function within the sorted() function provides a concise and inline approach to sorting a list of dictionaries by date. The lambda function directly converts the date strings to datetime objects for sorting without the need for a separate function.

Here’s an example:

from datetime import datetime

data = [
    {'date': '2023-01-03'},
    {'date': '2023-01-01'}
]

sorted_data = sorted(data, key=lambda x: datetime.strptime(x['date'], '%Y-%m-%d'))

The output of this code snippet:

[
    {'date': '2023-01-01'},
    {'date': '2023-01-03'}
]

In this snippet, we define an anonymous inline function, known as a lambda, that performs the transformation of date strings to datetime objects for sorting. This method is clean and eliminates the need for an auxiliary named function.

Method 3: Using itemgetter with sorted()

Employing the itemgetter function from the operator module along with sorted() can sort a list of dictionaries by dates in string format. However, it’s important to note that this method assumes date strings are in a sortable format, like YYYY-MM-DD.

Here’s an example:

from operator import itemgetter

data = [
    {'date': '2023-01-15'},
    {'date': '2023-01-10'}
]

sorted_data = sorted(data, key=itemgetter('date'))

The output of this code snippet:

[
    {'date': '2023-01-10'},
    {'date': '2023-01-15'}
]

This code uses the itemgetter function to retrieve the ‘date’ value for sorting without additional transformations. It’s a concise and efficient way to sort dictionaries by date as long as the dates are in ISO format, which is inherently sortable.

Method 4: In-Place Sorting using list.sort()

The list’s own sort() method can perform in-place sorting of a list of dictionaries by date. This method is similar to using sorted() with a key function, but modifies the original list rather than creating a new one.

Here’s an example:

from datetime import datetime

data = [
    {'date': '2023-03-01'},
    {'date': '2023-02-28'}
]

data.sort(key=lambda x: datetime.strptime(x['date'], '%Y-%m-%d'))

The output of this code snippet:

[
    {'date': '2023-02-28'},
    {'date': '2023-03-01'}
]

This approach makes use of a lambda function to parse dates for the sorting criteria. The sort() method changes the order of the list items in place, which is more memory-efficient if the original sorted order is no longer required.

Bonus One-Liner Method 5: Using sorted() with Complex Lambda Function

For a quick one-liner, one can use a complex lambda function within sorted() to extract parts of the date string and sort the dictionaries by date when they are not in a standard format.

Here’s an example:

data = [
    {'date': '01-03-2023'},
    {'date': '01-01-2023'}
]

sorted_data = sorted(data, key=lambda x: tuple(map(int, x['date'].split('-')[::-1])))

The output of this code snippet:

[
    {'date': '01-01-2023'},
    {'date': '01-03-2023'}
]

This one-liner reverses the date string and converts it into a tuple of integers, which can then be used for sorting. This method is quite powerful but should be used with caution as it is less readable and may be difficult to understand and maintain.

Summary/Discussion

• Method 1: Using sorted() with a Custom Key Function. Requires auxiliary function. Provides clarity and flexibility.
• Method 2: Lambda with sorted(). Concise and requires no extra named function. Less clear for complex sorting logic.
• Method 3: itemgetter with sorted(). Very concise for ISO date formats. Not suitable for all date formats.
• Method 4: In-Place Sorting using list.sort(). Memory-efficient as it modifies the list in place. Alters the original list.
• Method 5: sorted() with a Complex Lambda Function. Compact one-liner for non-standard date formats. Potentially confusing and less maintainable.