---
id: extensionclass
version: "6.3"
license: ZPL-2.1
license_treatment: unclear
maintenance: active
---

# ExtensionClass — Metaclass for subclassable extension types

License: unclear · Maintenance: active · Downloads: 169.6K/mo

## What it is and what it does

ExtensionClass is a metaclass that allows classes defined in C extension modules to be subclassed from Python code—a capability not normally available with standard extension types. It also provides ComputedAttribute (similar to property but works on instances and preserves Acquisition semantics) and MethodObject (for attaching callable methods to extension classes). The package is primarily maintained for legacy Zope applications; modern Python code should use standard new-style classes and descriptors instead.

The package has no runtime dependencies and ships as compiled wheels for Python 3.10–3.14 across macOS (Intel and ARM64), Linux (i686, x86_64, aarch64), and Windows (x86_64 and ARM64). It is actively maintained and has been updated to support free-threaded Python 3.14t, though it is explicitly recommended only for applications that require it for backward compatibility.

Use it for:

- Maintaining legacy Zope applications that depend on ExtensionClass-based object models.
- Subclassing C extension types when standard Python descriptors or properties are insufficient.
- Preserving Acquisition wrapper semantics in extension-based object hierarchies.
- Attaching computed attributes to extension class instances that behave like properties.

## Worth the install?

AI-flagged interpretation of the facts above — verify before relying.

Provides a metaclass that enables extension module classes to be subclassed in Python, along with utilities for computed attributes and method objects on ExtensionClass instances.

Yes, but only if you are maintaining a legacy Zope application or have a specific need to subclass C extension types. The package is actively maintained and has no known vulnerabilities. For new projects, use Python's built-in descriptor protocol and properties instead. Install friction is moderate due to compiled wheels, but prebuilt binaries are available for common platforms.

## Install

pip install extensionclass
uv add extensionclass
poetry add extensionclass

## Installing ExtensionClass

Before you install: Medium install friction due to compiled wheels for multiple platforms and Python versions (3.10–3.14). Actively maintained with recent release (106 days ago) and ongoing support for new Python versions including free-threaded 3.14t.

License in practice: Licensed under ZPL-2.1 (Zope Public License 2.1), a permissive open-source license. License treatment is marked unclear in the fact sheet, so verify compatibility with your project's licensing requirements before use.

Quickstart:

import extensionclass

class MyExtension(extensionclass.ExtensionClass):
    pass

obj = MyExtension()

Requires Python 3.10 or later; compiled C extension wheels are provided for common platforms but may require a C compiler on unsupported architectures.

Verify before relying:

- Whether ZPL-2.1 is compatible with your project's license (marked unclear in metadata).
- Performance characteristics compared to modern Python descriptors and properties.
- Whether Acquisition wrapper semantics are still needed for your use case versus standard Python patterns.

## Package facts

- License: ZPL-2.1 (unclear)
- Python support: supports_current
- Install friction: medium
- Maintenance: active
- Downloads: 169.6K/month (top 15,000 on PyPI)
- Known vulnerabilities: none known

## Tags

extension class metaclass, subclass C extension types, extensionclass python, computed attributes extension, method objects extension classes, zope extension classes, legacy extension class support, legacy-zope, c-extension, metaclass

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