cpyext

cpyext is a CPython C API compatibility layer used by the PyPy project to run CPython-style C extensions. It exposes a substantial portion of the CPython C API on top of PyPy’s runtime, allowing extension modules compiled against the CPython headers to be loaded and used within PyPy with minimal modification.

Implementation and operation: The cpyext component provides emulations of CPython’s C API, including types, functions, and interfaces that extensions expect. When a CPython extension calls into the C API, cpyext translates those calls into PyPy’s internal object model and execution environment. It also handles reference counting and object lifetime in a way that aims to preserve CPython semantics while operating within PyPy’s memory management model. Extensions are built against CPython headers and then executed inside PyPy, which links to the cpyext runtime to provide the CPython-compatible surface.

Scope and limitations: cpyext implements a large, practical subset of the CPython C API, enough for many common extensions to work. However, it is not a complete drop-in replica of CPython’s API; some APIs, behaviors, or platform-specific assumptions may be missing or behave differently. Some extensions may require additional work or may not be fully compatible. There is typically some performance overhead from bridging CPython C calls to PyPy’s runtime, and extensions with highly CPython-specific optimizations may experience issues.

Relation and use: cpyext is a component of the PyPy project focused on CPython interoperability. It serves as one path to run existing CPython extension ecosystems on PyPy, alongside PyPy’s native Python implementations and alternative integration approaches. Documentation and current status are maintained within PyPy’s project resources. See also PyPy, CPython C API, and Python C extensions.