.. _basics:

First steps

###########

This sections demonstrates the basic features of pybind11. Before getting

started, make sure that development environment is set up to compile the

included set of test cases.

Compiling the test cases

========================

Linux/MacOS

-----------

On Linux  you'll need to install the **python-dev** or **python3-dev** packages as

well as **cmake**. On Mac OS, the included python version works out of the box,

but **cmake** must still be installed.

After installing the prerequisites, run

.. code-block:: bash

   mkdir build

   cd build

   cmake ..

   make check -j 4

The last line will both compile and run the tests.

Windows

-------

On Windows, only **Visual Studio 2015** and newer are supported since pybind11 relies

on various C++11 language features that break older versions of Visual Studio.

To compile and run the tests:

.. code-block:: batch

   mkdir build

   cd build

   cmake ..

   cmake --build . --config Release --target check

This will create a Visual Studio project, compile and run the target, all from the

command line.

.. Note::

    If all tests fail, make sure that the Python binary and the testcases are compiled

    for the same processor type and bitness (i.e. either **i386** or **x86_64**). You

    can specify **x86_64** as the target architecture for the generated Visual Studio

    project using ``cmake -A x64 ..``.

.. seealso::

    Advanced users who are already familiar with Boost.Python may want to skip

    the tutorial and look at the test cases in the :file:`tests` directory,

    which exercise all features of pybind11.

Header and namespace conventions

================================

For brevity, all code examples assume that the following two lines are present:

.. code-block:: cpp

    #include <pybind11/pybind11.h>

    namespace py = pybind11;

Some features may require additional headers, but those will be specified as needed.

.. _simple_example:

Creating bindings for a simple function

=======================================

Let's start by creating Python bindings for an extremely simple function, which

adds two numbers and returns their result:

.. code-block:: cpp

    int add(int i, int j) {

        return i + j;

    }

For simplicity [#f1]_, we'll put both this function and the binding code into

a file named :file:`example.cpp` with the following contents:

.. code-block:: cpp

    #include <pybind11/pybind11.h>

    int add(int i, int j) {

        return i + j;

    }

    PYBIND11_MODULE(example, m) {

        m.doc() = "pybind11 example plugin"; // optional module docstring

        m.def("add", &add, "A function which adds two numbers");

    }

.. [#f1] In practice, implementation and binding code will generally be located

         in separate files.

The :func:`PYBIND11_MODULE` macro creates a function that will be called when an

``import`` statement is issued from within Python. The module name (``example``)

is given as the first macro argument (it should not be in quotes). The second

argument (``m``) defines a variable of type :class:`py::module <module>` which

is the main interface for creating bindings. The method :func:`module::def`

generates binding code that exposes the ``add()`` function to Python.

.. note::

    Notice how little code was needed to expose our function to Python: all

    details regarding the function's parameters and return value were

    automatically inferred using template metaprogramming. This overall

    approach and the used syntax are borrowed from Boost.Python, though the

    underlying implementation is very different.

pybind11 is a header-only library, hence it is not necessary to link against

any special libraries and there are no intermediate (magic) translation steps.

On Linux, the above example can be compiled using the following command:

.. code-block:: bash

    $ c++ -O3 -Wall -shared -std=c++11 -fPIC `python3 -m pybind11 --includes` example.cpp -o example`python3-config --extension-suffix`

For more details on the required compiler flags on Linux and MacOS, see

:ref:`building_manually`. For complete cross-platform compilation instructions,

refer to the :ref:`compiling` page.

The `python_example`_ and `cmake_example`_ repositories are also a good place

to start. They are both complete project examples with cross-platform build

systems. The only difference between the two is that `python_example`_ uses

Python's ``setuptools`` to build the module, while `cmake_example`_ uses CMake

(which may be preferable for existing C++ projects).

.. _python_example: https://github.com/pybind/python_example

.. _cmake_example: https://github.com/pybind/cmake_example

Building the above C++ code will produce a binary module file that can be

imported to Python. Assuming that the compiled module is located in the

current directory, the following interactive Python session shows how to

load and execute the example:

.. code-block:: pycon

    $ python

    Python 2.7.10 (default, Aug 22 2015, 20:33:39)

    [GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.0.59.1)] on darwin

    Type "help", "copyright", "credits" or "license" for more information.

    >>> import example

    >>> example.add(1, 2)

    3L

    >>>

.. _keyword_args:

Keyword arguments

=================

With a simple modification code, it is possible to inform Python about the

names of the arguments ("i" and "j" in this case).

.. code-block:: cpp

    m.def("add", &add, "A function which adds two numbers",

          py::arg("i"), py::arg("j"));

:class:`arg` is one of several special tag classes which can be used to pass

metadata into :func:`module::def`. With this modified binding code, we can now

call the function using keyword arguments, which is a more readable alternative

particularly for functions taking many parameters:

.. code-block:: pycon

    >>> import example

    >>> example.add(i=1, j=2)

    3L

The keyword names also appear in the function signatures within the documentation.

.. code-block:: pycon

    >>> help(example)

    ....

    FUNCTIONS

        add(...)

            Signature : (i: int, j: int) -> int

            A function which adds two numbers

A shorter notation for named arguments is also available:

.. code-block:: cpp

    // regular notation

    m.def("add1", &add, py::arg("i"), py::arg("j"));

    // shorthand

    using namespace pybind11::literals;

    m.def("add2", &add, "i"_a, "j"_a);

The :var:`_a` suffix forms a C++11 literal which is equivalent to :class:`arg`.

Note that the literal operator must first be made visible with the directive

``using namespace pybind11::literals``. This does not bring in anything else

from the ``pybind11`` namespace except for literals.

.. _default_args:

Default arguments

=================

Suppose now that the function to be bound has default arguments, e.g.:

.. code-block:: cpp

    int add(int i = 1, int j = 2) {

        return i + j;

    }

Unfortunately, pybind11 cannot automatically extract these parameters, since they

are not part of the function's type information. However, they are simple to specify

using an extension of :class:`arg`:

.. code-block:: cpp

    m.def("add", &add, "A function which adds two numbers",

          py::arg("i") = 1, py::arg("j") = 2);

The default values also appear within the documentation.

.. code-block:: pycon

    >>> help(example)

    ....

    FUNCTIONS

        add(...)

            Signature : (i: int = 1, j: int = 2) -> int

            A function which adds two numbers

The shorthand notation is also available for default arguments:

.. code-block:: cpp

    // regular notation

    m.def("add1", &add, py::arg("i") = 1, py::arg("j") = 2);

    // shorthand

    m.def("add2", &add, "i"_a=1, "j"_a=2);

Exporting variables

===================

To expose a value from C++, use the ``attr`` function to register it in a

module as shown below. Built-in types and general objects (more on that later)

are automatically converted when assigned as attributes, and can be explicitly

converted using the function ``py::cast``.

.. code-block:: cpp

    PYBIND11_MODULE(example, m) {

        m.attr("the_answer") = 42;

        py::object world = py::cast("World");

        m.attr("what") = world;

    }

These are then accessible from Python:

.. code-block:: pycon

    >>> import example

    >>> example.the_answer

    42

    >>> example.what

    'World'

.. _supported_types:

Supported data types

====================

A large number of data types are supported out of the box and can be used

seamlessly as functions arguments, return values or with ``py::cast`` in general.

For a full overview, see the :doc:`advanced/cast/index` section.