Mastering the Custom Key Function for Sorting in Python 3

Learn how to create a `custom key function` for more complex sorting in Python 3, with practical examples to clarify the process.
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Mastering the Custom Key Function for Sorting in Python 3

Sorting in Python is usually straightforward, but sometimes you may encounter scenarios where the built-in methods just don’t cut it. Suppose you are required to sort a collection of numbers based on their absolute values and, in cases of equal absolute values, by their signs. This is where the custom key function technique comes into play.

In this post, we will explore how to implement a custom sorting function, breaking down the solution into digestible sections to ensure clarity.

Understanding the Problem

Imagine you have a list of integers, and you want to sort them based on two criteria:

Absolute Value: Numbers should be sorted by their absolute magnitude.

Sign: If two numbers have the same absolute value, the negative number should come before the positive one.

Example

Given an array:

[[See Video to Reveal this Text or Code Snippet]]

The desired output after sorting should be:

[[See Video to Reveal this Text or Code Snippet]]

Writing the Custom Key Function

The key to sorting with custom criteria in Python is through a function that returns a tuple. This tuple can then guide the sorting process based on your desired order.

Steps to Create a Custom Key Function

Define the Key Function: The key function should return a tuple containing the absolute value of the number and a boolean determining its sign.

Use abs() for Absolute Value: The function must use the abs() function to get the absolute value of the number.

Sort by Sign: For the sign, a simple comparison can be used. Non-negative values will be treated as True, and negative values as False, allowing negative numbers to be sorted before positive ones.

Implementation

Let’s implement this using a lambda function for clarity and conciseness.

[[See Video to Reveal this Text or Code Snippet]]

Breakdown of the Lambda Function

The sorting logic encapsulated in the lambda function can be understood as follows:

The first element, abs(v), provides the absolute value of the number.

The second element, v >= 0, evaluates to False for negative numbers and True for non-negative numbers.

When Python sorts the tuples produced by the lambda function, it compares them element-wise:

It first compares the absolute values.

If two absolute values are the same, it then compares the boolean indicators for the sign.

Final Output

When the above lambda function processes the input list:

[[See Video to Reveal this Text or Code Snippet]]

This yields sorted output based on our desired criteria.

Conclusion

Creating a custom key function in Python provides a powerful way to sort complex data types with multiple criteria. By leveraging tuples and the built-in abs() function, you can easily achieve your desired sorting order.

Next time you're faced with a complex sorting requirement, remember these principles, and you’ll be well-equipped to tackle the task with ease. Happy coding!