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314 lines
13 KiB
ReStructuredText
314 lines
13 KiB
ReStructuredText
.. _basics:
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First steps
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###########
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This sections demonstrates the basic features of pybind11. Before getting
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started, make sure that development environment is set up to compile the
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included set of test cases.
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Compiling the test cases
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========================
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Linux/MacOS
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-----------
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On Linux you'll need to install the **python-dev** or **python3-dev** packages as
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well as **cmake**. On Mac OS, the included python version works out of the box,
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but **cmake** must still be installed.
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After installing the prerequisites, run
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.. code-block:: bash
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mkdir build
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cd build
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cmake ..
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make pytest -j 4
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The last line will both compile and run the tests.
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Windows
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-------
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On Windows, only **Visual Studio 2015** and newer are supported since pybind11 relies
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on various C++11 language features that break older versions of Visual Studio.
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To compile and run the tests:
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.. code-block:: batch
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mkdir build
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cd build
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cmake ..
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cmake --build . --config Release --target pytest
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This will create a Visual Studio project, compile and run the target, all from the
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command line.
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.. Note::
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If all tests fail, make sure that the Python binary and the testcases are compiled
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for the same processor type and bitness (i.e. either **i386** or **x86_64**). You
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can specify **x86_64** as the target architecture for the generated Visual Studio
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project using ``cmake -A x64 ..``.
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.. seealso::
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Advanced users who are already familiar with Boost.Python may want to skip
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the tutorial and look at the test cases in the :file:`tests` directory,
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which exercise all features of pybind11.
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Creating bindings for a simple function
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=======================================
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Let's start by creating Python bindings for an extremely simple function, which
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adds two numbers and returns their result:
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.. code-block:: cpp
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int add(int i, int j) {
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return i + j;
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}
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For simplicity [#f1]_, we'll put both this function and the binding code into
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a file named :file:`example.cpp` with the following contents:
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.. code-block:: cpp
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#include <pybind11/pybind11.h>
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int add(int i, int j) {
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return i + j;
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}
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namespace py = pybind11;
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PYBIND11_PLUGIN(example) {
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py::module m("example", "pybind11 example plugin");
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m.def("add", &add, "A function which adds two numbers");
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return m.ptr();
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}
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The :func:`PYBIND11_PLUGIN` macro creates a function that will be called when an
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``import`` statement is issued from within Python. The next line creates a
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module named ``example`` (with the supplied docstring). The method
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:func:`module::def` generates binding code that exposes the
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``add()`` function to Python. The last line returns the internal Python object
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associated with ``m`` to the Python interpreter.
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.. note::
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Notice how little code was needed to expose our function to Python: all
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details regarding the function's parameters and return value were
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automatically inferred using template metaprogramming. This overall
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approach and the used syntax are borrowed from Boost.Python, though the
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underlying implementation is very different.
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pybind11 is a header-only-library, hence it is not necessary to link against
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any special libraries (other than Python itself). On Windows, use the CMake
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build file discussed in section :ref:`cmake`. On Linux and Mac OS, the above
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example can be compiled using the following command
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.. code-block:: bash
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$ c++ -O3 -shared -std=c++11 -I <path-to-pybind11>/include `python-config --cflags --ldflags` example.cpp -o example.so
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In general, it is advisable to include several additional build parameters
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that can considerably reduce the size of the created binary. Refer to section
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:ref:`cmake` for a detailed example of a suitable cross-platform CMake-based
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build system.
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Assuming that the created file :file:`example.so` (:file:`example.pyd` on Windows)
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is located in the current directory, the following interactive Python session
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shows how to load and execute the example.
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.. code-block:: pycon
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$ python
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Python 2.7.10 (default, Aug 22 2015, 20:33:39)
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[GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.0.59.1)] on darwin
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Type "help", "copyright", "credits" or "license" for more information.
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>>> import example
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>>> example.add(1, 2)
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3L
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>>>
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.. _keyword_args:
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Keyword arguments
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=================
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With a simple modification code, it is possible to inform Python about the
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names of the arguments ("i" and "j" in this case).
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.. code-block:: cpp
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m.def("add", &add, "A function which adds two numbers",
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py::arg("i"), py::arg("j"));
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:class:`arg` is one of several special tag classes which can be used to pass
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metadata into :func:`module::def`. With this modified binding code, we can now
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call the function using keyword arguments, which is a more readable alternative
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particularly for functions taking many parameters:
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.. code-block:: pycon
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>>> import example
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>>> example.add(i=1, j=2)
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3L
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The keyword names also appear in the function signatures within the documentation.
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.. code-block:: pycon
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>>> help(example)
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....
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FUNCTIONS
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add(...)
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Signature : (i: int, j: int) -> int
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A function which adds two numbers
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A shorter notation for named arguments is also available:
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.. code-block:: cpp
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// regular notation
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m.def("add1", &add, py::arg("i"), py::arg("j"));
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// shorthand
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using namespace pybind11::literals;
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m.def("add2", &add, "i"_a, "j"_a);
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The :var:`_a` suffix forms a C++11 literal which is equivalent to :class:`arg`.
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Note that the literal operator must first be made visible with the directive
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``using namespace pybind11::literals``. This does not bring in anything else
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from the ``pybind11`` namespace except for literals.
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.. _default_args:
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Default arguments
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=================
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Suppose now that the function to be bound has default arguments, e.g.:
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.. code-block:: cpp
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int add(int i = 1, int j = 2) {
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return i + j;
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}
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Unfortunately, pybind11 cannot automatically extract these parameters, since they
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are not part of the function's type information. However, they are simple to specify
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using an extension of :class:`arg`:
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.. code-block:: cpp
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m.def("add", &add, "A function which adds two numbers",
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py::arg("i") = 1, py::arg("j") = 2);
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The default values also appear within the documentation.
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.. code-block:: pycon
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>>> help(example)
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....
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FUNCTIONS
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add(...)
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Signature : (i: int = 1, j: int = 2) -> int
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A function which adds two numbers
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The shorthand notation is also available for default arguments:
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.. code-block:: cpp
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// regular notation
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m.def("add1", &add, py::arg("i") = 1, py::arg("j") = 2);
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// shorthand
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m.def("add2", &add, "i"_a=1, "j"_a=2);
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.. _supported_types:
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Supported data types
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====================
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The following basic data types are supported out of the box (some may require
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an additional extension header to be included). To pass other data structures
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as arguments and return values, refer to the section on binding :ref:`classes`.
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+---------------------------------+--------------------------+-------------------------------+
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| Data type | Description | Header file |
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+=================================+==========================+===============================+
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| ``int8_t``, ``uint8_t`` | 8-bit integers | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``int16_t``, ``uint16_t`` | 16-bit integers | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``int32_t``, ``uint32_t`` | 32-bit integers | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``int64_t``, ``uint64_t`` | 64-bit integers | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``ssize_t``, ``size_t`` | Platform-dependent size | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``float``, ``double`` | Floating point types | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``bool`` | Two-state Boolean type | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``char`` | Character literal | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``wchar_t`` | Wide character literal | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``const char *`` | UTF-8 string literal | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``const wchar_t *`` | Wide string literal | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::string`` | STL dynamic UTF-8 string | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::wstring`` | STL dynamic wide string | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::pair<T1, T2>`` | Pair of two custom types | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::tuple<...>`` | Arbitrary tuple of types | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::reference_wrapper<...>`` | Reference type wrapper | :file:`pybind11/pybind11.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::complex<T>`` | Complex numbers | :file:`pybind11/complex.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::array<T, Size>`` | STL static array | :file:`pybind11/stl.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::vector<T>`` | STL dynamic array | :file:`pybind11/stl.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::list<T>`` | STL linked list | :file:`pybind11/stl.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::map<T1, T2>`` | STL ordered map | :file:`pybind11/stl.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::unordered_map<T1, T2>`` | STL unordered map | :file:`pybind11/stl.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::set<T>`` | STL ordered set | :file:`pybind11/stl.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::unordered_set<T>`` | STL unordered set | :file:`pybind11/stl.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::function<...>`` | STL polymorphic function | :file:`pybind11/functional.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::chrono::duration<...>`` | STL time duration | :file:`pybind11/chrono.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``std::chrono::time_point<...>``| STL date/time | :file:`pybind11/chrono.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``Eigen::Matrix<...>`` | Eigen: dense matrix | :file:`pybind11/eigen.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``Eigen::Map<...>`` | Eigen: mapped memory | :file:`pybind11/eigen.h` |
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+---------------------------------+--------------------------+-------------------------------+
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| ``Eigen::SparseMatrix<...>`` | Eigen: sparse matrix | :file:`pybind11/eigen.h` |
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+---------------------------------+--------------------------+-------------------------------+
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.. [#f1] In practice, implementation and binding code will generally be located
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in separate files.
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