From c79706ff4ce591df2151db5504d3c224f3c9965f Mon Sep 17 00:00:00 2001 From: Andreas Sandberg Date: Mon, 27 Feb 2017 13:17:51 +0000 Subject: ext: Add pybind rev f4b81b3 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Change-Id: I52e4fc9ebf2f59da57d8cf8f3e37cc79598c2f5f Signed-off-by: Andreas Sandberg Reviewed-by: Andreas Hansson Reviewed-by: Curtis Dunham Reviewed-on: https://gem5-review.googlesource.com/2229 Reviewed-by: Tony Gutierrez Reviewed-by: Jason Lowe-Power Reviewed-by: Pierre-Yves Péneau --- ext/pybind11/docs/classes.rst | 439 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 439 insertions(+) create mode 100644 ext/pybind11/docs/classes.rst (limited to 'ext/pybind11/docs/classes.rst') diff --git a/ext/pybind11/docs/classes.rst b/ext/pybind11/docs/classes.rst new file mode 100644 index 000000000..872977684 --- /dev/null +++ b/ext/pybind11/docs/classes.rst @@ -0,0 +1,439 @@ +.. _classes: + +Object-oriented code +#################### + +Creating bindings for a custom type +=================================== + +Let's now look at a more complex example where we'll create bindings for a +custom C++ data structure named ``Pet``. Its definition is given below: + +.. code-block:: cpp + + struct Pet { + Pet(const std::string &name) : name(name) { } + void setName(const std::string &name_) { name = name_; } + const std::string &getName() const { return name; } + + std::string name; + }; + +The binding code for ``Pet`` looks as follows: + +.. code-block:: cpp + + #include + + namespace py = pybind11; + + PYBIND11_PLUGIN(example) { + py::module m("example", "pybind11 example plugin"); + + py::class_(m, "Pet") + .def(py::init()) + .def("setName", &Pet::setName) + .def("getName", &Pet::getName); + + return m.ptr(); + } + +:class:`class_` creates bindings for a C++ `class` or `struct`-style data +structure. :func:`init` is a convenience function that takes the types of a +constructor's parameters as template arguments and wraps the corresponding +constructor (see the :ref:`custom_constructors` section for details). An +interactive Python session demonstrating this example is shown below: + +.. code-block:: pycon + + % python + >>> import example + >>> p = example.Pet('Molly') + >>> print(p) + + >>> p.getName() + u'Molly' + >>> p.setName('Charly') + >>> p.getName() + u'Charly' + +.. seealso:: + + Static member functions can be bound in the same way using + :func:`class_::def_static`. + +Keyword and default arguments +============================= +It is possible to specify keyword and default arguments using the syntax +discussed in the previous chapter. Refer to the sections :ref:`keyword_args` +and :ref:`default_args` for details. + +Binding lambda functions +======================== + +Note how ``print(p)`` produced a rather useless summary of our data structure in the example above: + +.. code-block:: pycon + + >>> print(p) + + +To address this, we could bind an utility function that returns a human-readable +summary to the special method slot named ``__repr__``. Unfortunately, there is no +suitable functionality in the ``Pet`` data structure, and it would be nice if +we did not have to change it. This can easily be accomplished by binding a +Lambda function instead: + +.. code-block:: cpp + + py::class_(m, "Pet") + .def(py::init()) + .def("setName", &Pet::setName) + .def("getName", &Pet::getName) + .def("__repr__", + [](const Pet &a) { + return ""; + } + ); + +Both stateless [#f1]_ and stateful lambda closures are supported by pybind11. +With the above change, the same Python code now produces the following output: + +.. code-block:: pycon + + >>> print(p) + + +.. [#f1] Stateless closures are those with an empty pair of brackets ``[]`` as the capture object. + +.. _properties: + +Instance and static fields +========================== + +We can also directly expose the ``name`` field using the +:func:`class_::def_readwrite` method. A similar :func:`class_::def_readonly` +method also exists for ``const`` fields. + +.. code-block:: cpp + + py::class_(m, "Pet") + .def(py::init()) + .def_readwrite("name", &Pet::name) + // ... remainder ... + +This makes it possible to write + +.. code-block:: pycon + + >>> p = example.Pet('Molly') + >>> p.name + u'Molly' + >>> p.name = 'Charly' + >>> p.name + u'Charly' + +Now suppose that ``Pet::name`` was a private internal variable +that can only be accessed via setters and getters. + +.. code-block:: cpp + + class Pet { + public: + Pet(const std::string &name) : name(name) { } + void setName(const std::string &name_) { name = name_; } + const std::string &getName() const { return name; } + private: + std::string name; + }; + +In this case, the method :func:`class_::def_property` +(:func:`class_::def_property_readonly` for read-only data) can be used to +provide a field-like interface within Python that will transparently call +the setter and getter functions: + +.. code-block:: cpp + + py::class_(m, "Pet") + .def(py::init()) + .def_property("name", &Pet::getName, &Pet::setName) + // ... remainder ... + +.. seealso:: + + Similar functions :func:`class_::def_readwrite_static`, + :func:`class_::def_readonly_static` :func:`class_::def_property_static`, + and :func:`class_::def_property_readonly_static` are provided for binding + static variables and properties. Please also see the section on + :ref:`static_properties` in the advanced part of the documentation. + +Dynamic attributes +================== + +Native Python classes can pick up new attributes dynamically: + +.. code-block:: pycon + + >>> class Pet: + ... name = 'Molly' + ... + >>> p = Pet() + >>> p.name = 'Charly' # overwrite existing + >>> p.age = 2 # dynamically add a new attribute + +By default, classes exported from C++ do not support this and the only writable +attributes are the ones explicitly defined using :func:`class_::def_readwrite` +or :func:`class_::def_property`. + +.. code-block:: cpp + + py::class_(m, "Pet") + .def(py::init<>()) + .def_readwrite("name", &Pet::name); + +Trying to set any other attribute results in an error: + +.. code-block:: pycon + + >>> p = example.Pet() + >>> p.name = 'Charly' # OK, attribute defined in C++ + >>> p.age = 2 # fail + AttributeError: 'Pet' object has no attribute 'age' + +To enable dynamic attributes for C++ classes, the :class:`py::dynamic_attr` tag +must be added to the :class:`py::class_` constructor: + +.. code-block:: cpp + + py::class_(m, "Pet", py::dynamic_attr()) + .def(py::init<>()) + .def_readwrite("name", &Pet::name); + +Now everything works as expected: + +.. code-block:: pycon + + >>> p = example.Pet() + >>> p.name = 'Charly' # OK, overwrite value in C++ + >>> p.age = 2 # OK, dynamically add a new attribute + >>> p.__dict__ # just like a native Python class + {'age': 2} + +Note that there is a small runtime cost for a class with dynamic attributes. +Not only because of the addition of a ``__dict__``, but also because of more +expensive garbage collection tracking which must be activated to resolve +possible circular references. Native Python classes incur this same cost by +default, so this is not anything to worry about. By default, pybind11 classes +are more efficient than native Python classes. Enabling dynamic attributes +just brings them on par. + +.. _inheritance: + +Inheritance +=========== + +Suppose now that the example consists of two data structures with an +inheritance relationship: + +.. code-block:: cpp + + struct Pet { + Pet(const std::string &name) : name(name) { } + std::string name; + }; + + struct Dog : Pet { + Dog(const std::string &name) : Pet(name) { } + std::string bark() const { return "woof!"; } + }; + +There are two different ways of indicating a hierarchical relationship to +pybind11: the first specifies the C++ base class as an extra template +parameter of the :class:`class_`: + +.. code-block:: cpp + + py::class_(m, "Pet") + .def(py::init()) + .def_readwrite("name", &Pet::name); + + // Method 1: template parameter: + py::class_(m, "Dog") + .def(py::init()) + .def("bark", &Dog::bark); + +Alternatively, we can also assign a name to the previously bound ``Pet`` +:class:`class_` object and reference it when binding the ``Dog`` class: + +.. code-block:: cpp + + py::class_ pet(m, "Pet"); + pet.def(py::init()) + .def_readwrite("name", &Pet::name); + + // Method 2: pass parent class_ object: + py::class_(m, "Dog", pet /* <- specify Python parent type */) + .def(py::init()) + .def("bark", &Dog::bark); + +Functionality-wise, both approaches are equivalent. Afterwards, instances will +expose fields and methods of both types: + +.. code-block:: pycon + + >>> p = example.Dog('Molly') + >>> p.name + u'Molly' + >>> p.bark() + u'woof!' + +Overloaded methods +================== + +Sometimes there are several overloaded C++ methods with the same name taking +different kinds of input arguments: + +.. code-block:: cpp + + struct Pet { + Pet(const std::string &name, int age) : name(name), age(age) { } + + void set(int age) { age = age; } + void set(const std::string &name) { name = name; } + + std::string name; + int age; + }; + +Attempting to bind ``Pet::set`` will cause an error since the compiler does not +know which method the user intended to select. We can disambiguate by casting +them to function pointers. Binding multiple functions to the same Python name +automatically creates a chain of function overloads that will be tried in +sequence. + +.. code-block:: cpp + + py::class_(m, "Pet") + .def(py::init()) + .def("set", (void (Pet::*)(int)) &Pet::set, "Set the pet's age") + .def("set", (void (Pet::*)(const std::string &)) &Pet::set, "Set the pet's name"); + +The overload signatures are also visible in the method's docstring: + +.. code-block:: pycon + + >>> help(example.Pet) + + class Pet(__builtin__.object) + | Methods defined here: + | + | __init__(...) + | Signature : (Pet, str, int) -> NoneType + | + | set(...) + | 1. Signature : (Pet, int) -> NoneType + | + | Set the pet's age + | + | 2. Signature : (Pet, str) -> NoneType + | + | Set the pet's name + +If you have a C++14 compatible compiler [#cpp14]_, you can use an alternative +syntax to cast the overloaded function: + +.. code-block:: cpp + + py::class_(m, "Pet") + .def("set", py::overload_cast(&Pet::set), "Set the pet's age") + .def("set", py::overload_cast(&Pet::set), "Set the pet's name"); + +Here, ``py::overload_cast`` only requires the parameter types to be specified. +The return type and class are deduced. This avoids the additional noise of +``void (Pet::*)()`` as seen in the raw cast. If a function is overloaded based +on constness, the ``py::const_`` tag should be used: + +.. code-block:: cpp + + struct Widget { + int foo(int x, float y); + int foo(int x, float y) const; + }; + + py::class_(m, "Widget") + .def("foo_mutable", py::overload_cast(&Widget::foo)) + .def("foo_const", py::overload_cast(&Widget::foo, py::const_)); + + +.. [#cpp14] A compiler which supports the ``-std=c++14`` flag + or Visual Studio 2015 Update 2 and newer. + +.. note:: + + To define multiple overloaded constructors, simply declare one after the + other using the ``.def(py::init<...>())`` syntax. The existing machinery + for specifying keyword and default arguments also works. + +Enumerations and internal types +=============================== + +Let's now suppose that the example class contains an internal enumeration type, +e.g.: + +.. code-block:: cpp + + struct Pet { + enum Kind { + Dog = 0, + Cat + }; + + Pet(const std::string &name, Kind type) : name(name), type(type) { } + + std::string name; + Kind type; + }; + +The binding code for this example looks as follows: + +.. code-block:: cpp + + py::class_ pet(m, "Pet"); + + pet.def(py::init()) + .def_readwrite("name", &Pet::name) + .def_readwrite("type", &Pet::type); + + py::enum_(pet, "Kind") + .value("Dog", Pet::Kind::Dog) + .value("Cat", Pet::Kind::Cat) + .export_values(); + +To ensure that the ``Kind`` type is created within the scope of ``Pet``, the +``pet`` :class:`class_` instance must be supplied to the :class:`enum_`. +constructor. The :func:`enum_::export_values` function exports the enum entries +into the parent scope, which should be skipped for newer C++11-style strongly +typed enums. + +.. code-block:: pycon + + >>> p = Pet('Lucy', Pet.Cat) + >>> p.type + Kind.Cat + >>> int(p.type) + 1L + + +.. note:: + + When the special tag ``py::arithmetic()`` is specified to the ``enum_`` + constructor, pybind11 creates an enumeration that also supports rudimentary + arithmetic and bit-level operations like comparisons, and, or, xor, negation, + etc. + + .. code-block:: cpp + + py::enum_(pet, "Kind", py::arithmetic()) + ... + + By default, these are omitted to conserve space. -- cgit v1.2.3