Скачать или смотреть How to Create a None Returning Callable in Python

Learn how to implement a simple solution for using a callable that discards its argument and returns `None`, optimized for better clarity and debugging.
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This video is based on the question https://stackoverflow.com/q/77334235/ asked by the user 'Ross Bencina' ( https://stackoverflow.com/u/2013747/ ) and on the answer https://stackoverflow.com/a/77334333/ provided by the user 'wjandrea' ( https://stackoverflow.com/u/4518341/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction

Python is a versatile programming language, but sometimes its flexibility can lead to challenges when integrating with third-party libraries. A common issue arises when you need to provide a callable that returns None, but your existing function returns a different type—like bool. This guide addresses how to discard unwanted arguments and ensure your function returns None, making it compatible with library requirements.

The Problem

The specific situation we face is the need for a callable structure that matches the following form:

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However, suppose you have a function that returns a boolean and you want to adapt it for use with some_third_party_function. For example:

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The challenge is how to supply this function such that its return value is effectively ignored, and it meets the requirement of returning None without changing its original implementation.

When attempting to use a simple lambda, you might encounter type checking issues:

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This fails because the lambda still attempts to return the boolean result of func_to_use_as_callback(), rather than returning None.

Solution Overview

While there isn't a built-in function in Python's standard library to achieve this directly, you can easily create a custom solution. Below are a few approaches to implement a None returning callable:

Basic Implementation

The simplest approach is to define a helper function that discards its argument and returns None:

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This method does work, but it can become verbose, especially when dealing with multiple functions.

Using a Wrapper Function

To improve clarity when debugging your code, consider wrapping the original function in another function that ensures None is returned:

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This approach keeps the original function intact yet fulfills the return requirement.

A More Reusable Class-Based Solution

To enhance reusability and clarity, you can encapsulate this functionality within a class. This allows for more sophisticated setups, such as retaining the function’s signature and providing better debugging information:

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Example Usage

When you create a ResultDiscarder instance, it will look like this:

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The output will clearly indicate what function is being wrapped, which is beneficial for debugging:

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Comparison of Approaches

Using the ResultDiscarder class provides several advantages:

Clarity: It’s easier to track what function is being ignored.

Versatility: You can pass any number of arguments to the wrapped function.

Simplicity: Having this class makes your intention clear with less boilerplate code.

Conclusion

Creating a callable that returns None while encapsulating another function's behavior is straightforward, whether using a simple argument-discarding function or a more structured class-based approach. By choosing the best-suited method, you enhance your code's readability and maintainability—key aspects of Python programming.

With this understanding, you should feel equipped to adapt any function to fit the specific requirements of third-party libraries in Python.