cpdef

cpdef is a keyword in Cython used to declare a function that is accessible from both Python and C code. When you write a cpdef function, Cython generates a C-level implementation (like a cdef function) and a Python-callable wrapper that allows the function to be invoked from Python code. The wrapper translates Python arguments to C types and forwards the call to the C implementation.

From C or from Cython code, you can call the underlying C-level function directly, bypassing the Python wrapper. This means cpdef provides a bridge: Python calls go through the wrapper, while C calls can use the efficient C implementation. As a result, cpdef aims to offer Python accessibility without sacrificing the performance benefits of the C-level routine when invoked from Cython or C.

cpdef differs from def and cdef in scope and visibility. def defines a Python function object only, with Python-level calling semantics and no guaranteed C-level signature. cdef defines a C-level function that is not exposed to Python. cpdef exposes a function to Python while maintaining a C-level implementation for fast calls from C or from performance-critical paths.

In extension types (cdef classes), cpdef methods are exposed as Python methods and can also be called from C code through the type’s method table, offering flexibility for mixed Python/C usage. Overall, cpdef is useful when a function needs to be accessible to Python users and also to be efficiently callable from C or Cython code.