Скачать или смотреть Making __getattr__ and Properties Work Seamlessly in Python

Learn how to effectively manage properties and prevent custom attributes in your Python classes using `__getattr__` and `__setattr__`.
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Understanding Properties and _getattr_ in Python

When working with Python classes, especially when you start using properties, custom attributes, and defining behavior for them, it can sometimes get tricky. In this guide, we will explore a common problem involving properties and the _getattr_ method, focusing on how to manage object attributes effectively while preventing users from adding custom attributes to your object.

The Problem

You may want to create a class with properties and allow the inclusion of extra attributes when an instance is created. However, you also want to prevent users from freely defining their custom attributes, raising an error when an attempt is made to do so.

Let's consider the following class implementation:

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

In this implementation, person.custom_attribute = None would work, but we want to throw an error if someone tries to add a custom attribute.

The Solution

To resolve this issue, you can customize the _setattr_ method. This method can be overridden to control how attributes are set on an object. Here’s how you can implement it:

Check for Existing Attributes: Use hasattr() to check if the attribute already exists.

Use __slots__: This allows you to explicitly declare which attributes can exist on the instance, preventing arbitrary additions.

Here’s an updated version of the Person class:

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

Key Points in the Solution

Use of __setattr__:

It checks if an attribute already exists or if it’s declared in __slots__. If not, it raises an error.

This prevents users from defining unauthorized attributes.

Maintaining Properties:

Since properties work with the standard attribute access methods, your implementation won’t interfere with properties like name.

Conclusion

With this approach, you’ve successfully managed to create a Python class that allows properties and extra defined attributes while preventing arbitrary custom attributes. The combination of using _getattr_ for attribute access and overriding _setattr_ to control modifications creates a well-structured class that adheres to your design requirements.

By following these guidelines, you can enhance your class design in Python while maintaining clear boundaries for attribute management.