Скачать или смотреть Check if an Item is an Instance but not a Subclass in Python

Learn how to check if an object is an instance of a class without it being a subclass in Python. Discover the solution using simple code examples and explanations.
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This video is based on the question https://stackoverflow.com/q/64447317/ asked by the user 'Linden' ( https://stackoverflow.com/u/12641805/ ) and on the answer https://stackoverflow.com/a/64459232/ provided by the user 'Xtrem532' ( https://stackoverflow.com/u/5278549/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Checking for Exact Class Instances in Python

In Python programming, it's common to deal with class hierarchies where subclasses inherit from one another. A situation can arise where you need to determine if a specific object is an instance of a certain class, but not any of its subclasses. This can be tricky because the built-in isinstance() function returns True for both instances of the class and its subclasses. So how do we tackle this problem?

The Problem Statement

Given a set of class definitions where one class is the parent of several subclasses, the goal is to determine if a particular object is an instance of the parent class and not of any subclasses. Our input would look something like this:

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

The Simple Solution

To achieve this, we utilize the _class_ attribute of the object. This attribute directly points to the class of the object itself. By comparing this attribute with the class passed as an argument, we can ascertain if the object is an exact match.

Implementation

Here’s how to implement the function:

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

Explanation

obj.__class__: This retrieves the exact class of the object obj.

is klass: This checks if the class of obj is exactly the same as klass, which refers to the class we want to check against.

By using this comparison, we get a reliable way to check for the exact class instance.

A More Pythonic Option

While the previous solution works effectively, there’s also a more “pythonic” way to achieve the same outcome without needing to access any dunder methods. Here’s an alternative implementation:

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

Why Choose This Method?

Avoiding Dunder Attributes: Accessing .__class__ involves a dunder attribute, while using type(obj) does not.

Code Clarity: This method might be more readable for those who are newer to Python.

Important Tips

Avoid Keywords: Don't use class as a variable name, since it's a reserved keyword in Python. Instead, use alternative names like klass or typ.

Variable Shadows: Avoid naming your object variable as object, as it shadows the built-in object type in Python. Use names like obj for clarity.

Conclusion

If you ever find yourself needing to check if an object is an instance of a specific class without it being a subclass, remember the techniques outlined above. By leveraging the _class_ attribute or the type() function, you can effectively write robust functions that serve your needs.

Now you can confidently tell whether an item is strictly an instance of a class or a subclass thereof!