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366 lines
14 KiB
366 lines
14 KiB
.. _numpy:
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NumPy
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#####
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Buffer protocol
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===============
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Python supports an extremely general and convenient approach for exchanging
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data between plugin libraries. Types can expose a buffer view [#f2]_, which
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provides fast direct access to the raw internal data representation. Suppose we
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want to bind the following simplistic Matrix class:
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.. code-block:: cpp
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class Matrix {
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public:
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Matrix(size_t rows, size_t cols) : m_rows(rows), m_cols(cols) {
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m_data = new float[rows*cols];
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}
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float *data() { return m_data; }
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size_t rows() const { return m_rows; }
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size_t cols() const { return m_cols; }
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private:
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size_t m_rows, m_cols;
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float *m_data;
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};
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The following binding code exposes the ``Matrix`` contents as a buffer object,
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making it possible to cast Matrices into NumPy arrays. It is even possible to
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completely avoid copy operations with Python expressions like
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``np.array(matrix_instance, copy = False)``.
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.. code-block:: cpp
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py::class_<Matrix>(m, "Matrix", py::buffer_protocol())
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.def_buffer([](Matrix &m) -> py::buffer_info {
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return py::buffer_info(
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m.data(), /* Pointer to buffer */
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sizeof(float), /* Size of one scalar */
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py::format_descriptor<float>::format(), /* Python struct-style format descriptor */
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2, /* Number of dimensions */
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{ m.rows(), m.cols() }, /* Buffer dimensions */
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{ sizeof(float) * m.rows(), /* Strides (in bytes) for each index */
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sizeof(float) }
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);
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});
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Supporting the buffer protocol in a new type involves specifying the special
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``py::buffer_protocol()`` tag in the ``py::class_`` constructor and calling the
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``def_buffer()`` method with a lambda function that creates a
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``py::buffer_info`` description record on demand describing a given matrix
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instance. The contents of ``py::buffer_info`` mirror the Python buffer protocol
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specification.
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.. code-block:: cpp
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struct buffer_info {
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void *ptr;
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ssize_t itemsize;
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std::string format;
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ssize_t ndim;
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std::vector<ssize_t> shape;
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std::vector<ssize_t> strides;
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};
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To create a C++ function that can take a Python buffer object as an argument,
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simply use the type ``py::buffer`` as one of its arguments. Buffers can exist
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in a great variety of configurations, hence some safety checks are usually
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necessary in the function body. Below, you can see an basic example on how to
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define a custom constructor for the Eigen double precision matrix
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(``Eigen::MatrixXd``) type, which supports initialization from compatible
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buffer objects (e.g. a NumPy matrix).
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.. code-block:: cpp
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/* Bind MatrixXd (or some other Eigen type) to Python */
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typedef Eigen::MatrixXd Matrix;
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typedef Matrix::Scalar Scalar;
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constexpr bool rowMajor = Matrix::Flags & Eigen::RowMajorBit;
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py::class_<Matrix>(m, "Matrix", py::buffer_protocol())
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.def("__init__", [](Matrix &m, py::buffer b) {
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typedef Eigen::Stride<Eigen::Dynamic, Eigen::Dynamic> Strides;
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/* Request a buffer descriptor from Python */
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py::buffer_info info = b.request();
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/* Some sanity checks ... */
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if (info.format != py::format_descriptor<Scalar>::format())
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throw std::runtime_error("Incompatible format: expected a double array!");
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if (info.ndim != 2)
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throw std::runtime_error("Incompatible buffer dimension!");
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auto strides = Strides(
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info.strides[rowMajor ? 0 : 1] / (py::ssize_t)sizeof(Scalar),
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info.strides[rowMajor ? 1 : 0] / (py::ssize_t)sizeof(Scalar));
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auto map = Eigen::Map<Matrix, 0, Strides>(
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static_cast<Scalar *>(info.ptr), info.shape[0], info.shape[1], strides);
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new (&m) Matrix(map);
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});
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For reference, the ``def_buffer()`` call for this Eigen data type should look
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as follows:
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.. code-block:: cpp
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.def_buffer([](Matrix &m) -> py::buffer_info {
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return py::buffer_info(
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m.data(), /* Pointer to buffer */
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sizeof(Scalar), /* Size of one scalar */
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py::format_descriptor<Scalar>::format(), /* Python struct-style format descriptor */
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2, /* Number of dimensions */
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{ m.rows(), m.cols() }, /* Buffer dimensions */
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{ sizeof(Scalar) * (rowMajor ? m.cols() : 1),
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sizeof(Scalar) * (rowMajor ? 1 : m.rows()) }
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/* Strides (in bytes) for each index */
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);
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})
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For a much easier approach of binding Eigen types (although with some
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limitations), refer to the section on :doc:`/advanced/cast/eigen`.
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.. seealso::
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The file :file:`tests/test_buffers.cpp` contains a complete example
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that demonstrates using the buffer protocol with pybind11 in more detail.
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.. [#f2] http://docs.python.org/3/c-api/buffer.html
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Arrays
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======
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By exchanging ``py::buffer`` with ``py::array`` in the above snippet, we can
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restrict the function so that it only accepts NumPy arrays (rather than any
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type of Python object satisfying the buffer protocol).
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In many situations, we want to define a function which only accepts a NumPy
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array of a certain data type. This is possible via the ``py::array_t<T>``
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template. For instance, the following function requires the argument to be a
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NumPy array containing double precision values.
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.. code-block:: cpp
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void f(py::array_t<double> array);
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When it is invoked with a different type (e.g. an integer or a list of
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integers), the binding code will attempt to cast the input into a NumPy array
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of the requested type. Note that this feature requires the
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:file:`pybind11/numpy.h` header to be included.
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Data in NumPy arrays is not guaranteed to packed in a dense manner;
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furthermore, entries can be separated by arbitrary column and row strides.
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Sometimes, it can be useful to require a function to only accept dense arrays
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using either the C (row-major) or Fortran (column-major) ordering. This can be
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accomplished via a second template argument with values ``py::array::c_style``
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or ``py::array::f_style``.
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.. code-block:: cpp
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void f(py::array_t<double, py::array::c_style | py::array::forcecast> array);
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The ``py::array::forcecast`` argument is the default value of the second
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template parameter, and it ensures that non-conforming arguments are converted
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into an array satisfying the specified requirements instead of trying the next
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function overload.
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Structured types
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================
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In order for ``py::array_t`` to work with structured (record) types, we first
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need to register the memory layout of the type. This can be done via
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``PYBIND11_NUMPY_DTYPE`` macro, called in the plugin definition code, which
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expects the type followed by field names:
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.. code-block:: cpp
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struct A {
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int x;
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double y;
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};
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struct B {
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int z;
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A a;
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};
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// ...
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PYBIND11_MODULE(test, m) {
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// ...
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PYBIND11_NUMPY_DTYPE(A, x, y);
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PYBIND11_NUMPY_DTYPE(B, z, a);
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/* now both A and B can be used as template arguments to py::array_t */
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}
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The structure should consist of fundamental arithmetic types, ``std::complex``,
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previously registered substructures, and arrays of any of the above. Both C++
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arrays and ``std::array`` are supported. While there is a static assertion to
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prevent many types of unsupported structures, it is still the user's
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responsibility to use only "plain" structures that can be safely manipulated as
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raw memory without violating invariants.
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Vectorizing functions
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=====================
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Suppose we want to bind a function with the following signature to Python so
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that it can process arbitrary NumPy array arguments (vectors, matrices, general
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N-D arrays) in addition to its normal arguments:
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.. code-block:: cpp
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double my_func(int x, float y, double z);
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After including the ``pybind11/numpy.h`` header, this is extremely simple:
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.. code-block:: cpp
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m.def("vectorized_func", py::vectorize(my_func));
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Invoking the function like below causes 4 calls to be made to ``my_func`` with
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each of the array elements. The significant advantage of this compared to
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solutions like ``numpy.vectorize()`` is that the loop over the elements runs
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entirely on the C++ side and can be crunched down into a tight, optimized loop
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by the compiler. The result is returned as a NumPy array of type
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``numpy.dtype.float64``.
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.. code-block:: pycon
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>>> x = np.array([[1, 3],[5, 7]])
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>>> y = np.array([[2, 4],[6, 8]])
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>>> z = 3
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>>> result = vectorized_func(x, y, z)
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The scalar argument ``z`` is transparently replicated 4 times. The input
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arrays ``x`` and ``y`` are automatically converted into the right types (they
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are of type ``numpy.dtype.int64`` but need to be ``numpy.dtype.int32`` and
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``numpy.dtype.float32``, respectively).
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.. note::
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Only arithmetic, complex, and POD types passed by value or by ``const &``
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reference are vectorized; all other arguments are passed through as-is.
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Functions taking rvalue reference arguments cannot be vectorized.
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In cases where the computation is too complicated to be reduced to
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``vectorize``, it will be necessary to create and access the buffer contents
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manually. The following snippet contains a complete example that shows how this
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works (the code is somewhat contrived, since it could have been done more
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simply using ``vectorize``).
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.. code-block:: cpp
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#include <pybind11/pybind11.h>
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#include <pybind11/numpy.h>
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namespace py = pybind11;
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py::array_t<double> add_arrays(py::array_t<double> input1, py::array_t<double> input2) {
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auto buf1 = input1.request(), buf2 = input2.request();
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if (buf1.ndim != 1 || buf2.ndim != 1)
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throw std::runtime_error("Number of dimensions must be one");
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if (buf1.size != buf2.size)
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throw std::runtime_error("Input shapes must match");
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/* No pointer is passed, so NumPy will allocate the buffer */
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auto result = py::array_t<double>(buf1.size);
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auto buf3 = result.request();
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double *ptr1 = (double *) buf1.ptr,
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*ptr2 = (double *) buf2.ptr,
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*ptr3 = (double *) buf3.ptr;
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for (size_t idx = 0; idx < buf1.shape[0]; idx++)
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ptr3[idx] = ptr1[idx] + ptr2[idx];
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return result;
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}
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PYBIND11_MODULE(test, m) {
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m.def("add_arrays", &add_arrays, "Add two NumPy arrays");
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}
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.. seealso::
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The file :file:`tests/test_numpy_vectorize.cpp` contains a complete
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example that demonstrates using :func:`vectorize` in more detail.
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Direct access
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=============
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For performance reasons, particularly when dealing with very large arrays, it
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is often desirable to directly access array elements without internal checking
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of dimensions and bounds on every access when indices are known to be already
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valid. To avoid such checks, the ``array`` class and ``array_t<T>`` template
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class offer an unchecked proxy object that can be used for this unchecked
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access through the ``unchecked<N>`` and ``mutable_unchecked<N>`` methods,
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where ``N`` gives the required dimensionality of the array:
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.. code-block:: cpp
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m.def("sum_3d", [](py::array_t<double> x) {
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auto r = x.unchecked<3>(); // x must have ndim = 3; can be non-writeable
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double sum = 0;
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for (ssize_t i = 0; i < r.shape(0); i++)
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for (ssize_t j = 0; j < r.shape(1); j++)
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for (ssize_t k = 0; k < r.shape(2); k++)
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sum += r(i, j, k);
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return sum;
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});
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m.def("increment_3d", [](py::array_t<double> x) {
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auto r = x.mutable_unchecked<3>(); // Will throw if ndim != 3 or flags.writeable is false
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for (ssize_t i = 0; i < r.shape(0); i++)
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for (ssize_t j = 0; j < r.shape(1); j++)
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for (ssize_t k = 0; k < r.shape(2); k++)
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r(i, j, k) += 1.0;
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}, py::arg().noconvert());
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To obtain the proxy from an ``array`` object, you must specify both the data
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type and number of dimensions as template arguments, such as ``auto r =
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myarray.mutable_unchecked<float, 2>()``.
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If the number of dimensions is not known at compile time, you can omit the
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dimensions template parameter (i.e. calling ``arr_t.unchecked()`` or
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``arr.unchecked<T>()``. This will give you a proxy object that works in the
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same way, but results in less optimizable code and thus a small efficiency
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loss in tight loops.
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Note that the returned proxy object directly references the array's data, and
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only reads its shape, strides, and writeable flag when constructed. You must
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take care to ensure that the referenced array is not destroyed or reshaped for
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the duration of the returned object, typically by limiting the scope of the
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returned instance.
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The returned proxy object supports some of the same methods as ``py::array`` so
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that it can be used as a drop-in replacement for some existing, index-checked
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uses of ``py::array``:
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- ``r.ndim()`` returns the number of dimensions
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- ``r.data(1, 2, ...)`` and ``r.mutable_data(1, 2, ...)``` returns a pointer to
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the ``const T`` or ``T`` data, respectively, at the given indices. The
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latter is only available to proxies obtained via ``a.mutable_unchecked()``.
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- ``itemsize()`` returns the size of an item in bytes, i.e. ``sizeof(T)``.
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- ``ndim()`` returns the number of dimensions.
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- ``shape(n)`` returns the size of dimension ``n``
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- ``size()`` returns the total number of elements (i.e. the product of the shapes).
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- ``nbytes()`` returns the number of bytes used by the referenced elements
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(i.e. ``itemsize()`` times ``size()``).
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.. seealso::
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The file :file:`tests/test_numpy_array.cpp` contains additional examples
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demonstrating the use of this feature.
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