Implementing the buffer protocol¶
Cython objects can expose memory buffers to Python code by implementing the “buffer protocol”. This chapter shows how to implement the protocol and make use of the memory managed by an extension type from NumPy.
A matrix class¶
The following Cython/C++ code implements a matrix of floats, where the number of columns is fixed at construction time but rows can be added dynamically.
# matrix.pyx from libcpp.vector cimport vector cdef class Matrix: cdef unsigned ncols cdef vector[float] v def __cinit__(self, unsigned ncols): self.ncols = ncols def add_row(self): """Adds a row, initially zero-filled.""" self.v.extend(self.ncols)
There are no methods to do anything productive with the matrices’ contents.
We could implement custom
__setitem__, etc. for this,
but instead we’ll use the buffer protocol to expose the matrix’s data to Python
so we can use NumPy to do useful work.
Implementing the buffer protocol requires adding two methods,
which Cython handles specially.
from cpython cimport Py_buffer from libcpp.vector cimport vector cdef class Matrix: cdef Py_ssize_t ncols cdef Py_ssize_t shape cdef Py_ssize_t strides cdef vector[float] v def __cinit__(self, Py_ssize_t ncols): self.ncols = ncols def add_row(self): """Adds a row, initially zero-filled.""" self.v.extend(self.ncols) def __getbuffer__(self, Py_buffer *buffer, int flags): cdef Py_ssize_t itemsize = sizeof(self.v) self.shape = self.v.size() / self.ncols self.shape = self.ncols # Stride 1 is the distance, in bytes, between two items in a row; # this is the distance between two adjacent items in the vector. # Stride 0 is the distance between the first elements of adjacent rows. self.strides = <Py_ssize_t>( <char *>&(self.v) - <char *>&(self.v)) self.strides = self.ncols * self.strides buffer.buf = <char *>&(self.v) buffer.format = 'f' # float buffer.internal = NULL # see References buffer.itemsize = itemsize buffer.len = self.v.size() * itemsize # product(shape) * itemsize buffer.ndim = 2 buffer.obj = self buffer.readonly = 0 buffer.shape = self.shape buffer.strides = self.strides buffer.suboffsets = NULL # for pointer arrays only def __releasebuffer__(self, Py_buffer *buffer): pass
Matrix.__getbuffer__ fills a descriptor structure,
Py_buffer, that is defined by the Python C-API.
It contains a pointer to the actual buffer in memory,
as well as metadata about the shape of the array and the strides
(step sizes to get from one element or row to the next).
strides members are pointers
that must point to arrays of type and size
These arrays have to stay alive as long as any buffer views the data,
so we store them on the
Matrix object as members.
The code is not yet complete, but we can already compile it and test the basic functionality.
>>> from matrix import Matrix >>> import numpy as np >>> m = Matrix(10) >>> np.asarray(m) array(, shape=(0, 10), dtype=float32) >>> m.add_row() >>> a = np.asarray(m) >>> a[:] = 1 >>> m.add_row() >>> a = np.asarray(m) >>> a array([[ 1., 1., 1., 1., 1., 1., 1., 1., 1., 1.], [ 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.]], dtype=float32)
Now we can view the
Matrix as a NumPy
and modify its contents using standard NumPy operations.
Memory safety and reference counting¶
Matrix class as implemented so far is unsafe.
add_row operation can move the underlying buffer,
which invalidates any NumPy (or other) view on the data.
If you try to access values after an
you’ll get outdated values or a segfault.
This is where
__releasebuffer__ comes in.
We can add a reference count to each matrix,
and lock it for mutation whenever a view exists.
cdef class Matrix: # ... cdef int view_count def __cinit__(self, Py_ssize_t ncols): self.ncols = ncols self.view_count = 0 def add_row(self): if self.view_count > 0: raise ValueError("can't add row while being viewed") self.v.resize(self.v.size() + self.ncols) def __getbuffer__(self, Py_buffer *buffer, int flags): # ... as before self.view_count += 1 def __releasebuffer__(self, Py_buffer *buffer): self.view_count -= 1
We skipped some input validation in the code.
flags argument to
__getbuffer__ comes from
(and other clients) and is an OR of boolean flags
that describe the kind of array that is requested.
Strictly speaking, if the flags contain
__getbuffer__ must raise a
These macros can be
(The matrix-in-vector structure actually conforms to
but that would prohibit
__getbuffer__ from filling in the strides.
A single-row matrix is F-contiguous, but a larger matrix is not.)
The buffer interface used here is set out in PEP 3118, Revising the buffer protocol.
A tutorial for using this API from C is on Jake Vanderplas’s blog, An Introduction to the Python Buffer Protocol.
Reference documentation is available for Python 3 and Python 2. The Py2 documentation also describes an older buffer protocol that is no longer in use; since Python 2.6, the PEP 3118 protocol has been implemented, and the older protocol is only relevant for legacy code.