Скачать или смотреть How to Properly Use __init__ in ctypes Constructors for C Structs in Python

Learn how to correctly implement a Python constructor for a C struct using ctypes, ensuring the proper initialization of variables and functionality.
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How to Properly Use _init_ in ctypes Constructors for C Structs in Python

When working with Python and C, particularly with the ctypes library, you might encounter scenarios where you want to instantiate a C struct within a Python class. However, problems can arise, especially when trying to use standard constructors like __init__. This guide dives into a specific example to help you understand how to correctly implement a ctypes constructor for a C struct.

The Problem

Imagine you have the following C struct:

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

This struct Position has a corresponding implementation in Python using ctypes. Initially, you may manually set the properties like so:

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This works perfectly and outputs:

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

But what if you want to encapsulate this behavior within the Python class constructor? That’s where things can get tricky.

The Common Mistake

Many will attempt to create the Python Position class with a constructor like this:

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

While it seems reasonable, Python does not allow reassigning self in this manner. When you run:

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You might get an output of:

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Clearly, this is not what you want.

The Solution

Instead of using _init__, the key is to use the __new_ method. The _new_ method is responsible for creating a new instance of the class, which allows you to return an instance generated from your C function.

Step-by-Step Implementation

Define Your C Function and Struct:
Ensure your C struct and the function that initializes it are properly defined.

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

Create Python Class with __new__:
Update your Python class as follows:

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Expected Output

When you run the updated code, you should get the output:

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

This correctly reflects the values set in your C function.

Conclusion

By utilizing the _new_ method instead of __init__, you can properly instantiate a class in Python with functionality provided by a C struct through ctypes. This approach is crucial for ensuring that the initialized values reflect those defined in your C code.

Using _new_ allows you to control the instantiation of your class, while managing the underlying C interoperability cleanly and efficiently.

Now, go ahead and implement this in your projects! If you have any questions or need further assistance, feel free to ask.