feat: add copy interface for dataclass

[guan404ming]

Related Issue

#356

Why

Dataclasses need copy support for creating shallow copies, deep copies, and modified instances.

How

• Add __copy__ for copy.copy() support
• Add __deepcopy__ for copy.deepcopy() support
• Add __replace__ for creating modified copies (Python 3.13+ dataclasses.replace() compatible)
• Bypass custom __init__ by calling FFI constructor directly")

[gemini-code-assist]

Summary of Changes

Hello @guan404ming, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the usability of dataclasses by integrating standard Python copying mechanisms. It provides robust support for creating shallow copies, deep copies, and modified instances, ensuring that dataclasses behave as expected with Python's copy module and offer flexible instance modification. The implementation carefully handles FFI-backed constructors and field initialization rules.

Highlights

• Copy Interface Implementation: Introduced __copy__ for shallow copying, __deepcopy__ for deep copying, and __replace__ for creating modified instances of dataclasses. These methods are automatically generated for dataclasses unless explicitly defined by the user.
• FFI Constructor Bypass: The newly added copy methods (__copy__, __deepcopy__, __replace__) bypass custom Python __init__ methods by directly calling the FFI constructor (__c_ffi_init__), ensuring correct initialization for FFI-backed dataclasses.
• Field Classification Utility: A new utility function, _classify_fields_for_copy, was added to categorize dataclass fields into those passed to the FFI constructor, those with init=True (replaceable), and those with init=False (non-replaceable), streamlining the logic for copy operations.
• Python 3.13+ dataclasses.replace() Compatibility: The __replace__ method is designed to be compatible with the behavior of dataclasses.replace() in Python 3.13+, allowing only fields with init=True to be modified and raising a TypeError for invalid or non-initializable fields.
• Comprehensive Testing: New test cases have been added to validate the functionality of __copy__, __deepcopy__, and __replace__ for various scenarios, including handling of init=False fields and invalid field replacements.

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[gemini-code-assist]

Code Review

This pull request introduces support for copy.copy(), copy.deepcopy(), and a __replace__ method for FFI dataclasses. This is a valuable addition for creating shallow, deep, and modified copies of dataclass instances. The implementation correctly generates these methods dynamically and integrates them into the c_class decorator. The accompanying tests cover the new functionality well. I've identified a bug in the field classification logic that could lead to redundant assignments during copy operations, and I've provided a suggestion to fix it. Overall, this is a great enhancement.