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Miscellaneous

General notes regarding convenience macros

pybind11 provides a few convenience macros such as :func:`PYBIND11_DECLARE_HOLDER_TYPE` and PYBIND11_OVERLOAD_*. Since these are "just" macros that are evaluated in the preprocessor (which has no concept of types), they will get confused by commas in a template argument; for example, consider:

PYBIND11_OVERLOAD(MyReturnType<T1, T2>, Class<T3, T4>, func)

The limitation of the C preprocessor interprets this as five arguments (with new arguments beginning after each comma) rather than three. To get around this, there are two alternatives: you can use a type alias, or you can wrap the type using the PYBIND11_TYPE macro:

// Version 1: using a type alias
using ReturnType = MyReturnType<T1, T2>;
using ClassType = Class<T3, T4>;
PYBIND11_OVERLOAD(ReturnType, ClassType, func);

// Version 2: using the PYBIND11_TYPE macro:
PYBIND11_OVERLOAD(PYBIND11_TYPE(MyReturnType<T1, T2>),
                  PYBIND11_TYPE(Class<T3, T4>), func)

The PYBIND11_MAKE_OPAQUE macro does not require the above workarounds.

Global Interpreter Lock (GIL)

When calling a C++ function from Python, the GIL is always held. The classes :class:`gil_scoped_release` and :class:`gil_scoped_acquire` can be used to acquire and release the global interpreter lock in the body of a C++ function call. In this way, long-running C++ code can be parallelized using multiple Python threads. Taking :ref:`overriding_virtuals` as an example, this could be realized as follows (important changes highlighted):

The call_go wrapper can also be simplified using the call_guard policy (see :ref:`call_policies`) which yields the same result:

m.def("call_go", &call_go, py::call_guard<py::gil_scoped_release>());

Binding sequence data types, iterators, the slicing protocol, etc.

Please refer to the supplemental example for details.

.. seealso::

    The file :file:`tests/test_sequences_and_iterators.cpp` contains a
    complete example that shows how to bind a sequence data type, including
    length queries (``__len__``), iterators (``__iter__``), the slicing
    protocol and other kinds of useful operations.


Partitioning code over multiple extension modules

It's straightforward to split binding code over multiple extension modules, while referencing types that are declared elsewhere. Everything "just" works without any special precautions. One exception to this rule occurs when extending a type declared in another extension module. Recall the basic example from Section :ref:`inheritance`.

py::class_<Pet> pet(m, "Pet");
pet.def(py::init<const std::string &>())
   .def_readwrite("name", &Pet::name);

py::class_<Dog>(m, "Dog", pet /* <- specify parent */)
    .def(py::init<const std::string &>())
    .def("bark", &Dog::bark);

Suppose now that Pet bindings are defined in a module named basic, whereas the Dog bindings are defined somewhere else. The challenge is of course that the variable pet is not available anymore though it is needed to indicate the inheritance relationship to the constructor of class_<Dog>. However, it can be acquired as follows:

py::object pet = (py::object) py::module::import("basic").attr("Pet");

py::class_<Dog>(m, "Dog", pet)
    .def(py::init<const std::string &>())
    .def("bark", &Dog::bark);

Alternatively, you can specify the base class as a template parameter option to class_, which performs an automated lookup of the corresponding Python type. Like the above code, however, this also requires invoking the import function once to ensure that the pybind11 binding code of the module basic has been executed:

py::module::import("basic");

py::class_<Dog, Pet>(m, "Dog")
    .def(py::init<const std::string &>())
    .def("bark", &Dog::bark);

Naturally, both methods will fail when there are cyclic dependencies.

Note that pybind11 code compiled with hidden-by-default symbol visibility (e.g. via the command line flag -fvisibility=hidden on GCC/Clang), which is required for proper pybind11 functionality, can interfere with the ability to access types defined in another extension module. Working around this requires manually exporting types that are accessed by multiple extension modules; pybind11 provides a macro to do just this:

class PYBIND11_EXPORT Dog : public Animal {
    ...
};

Note also that it is possible (although would rarely be required) to share arbitrary C++ objects between extension modules at runtime. Internal library data is shared between modules using capsule machinery [1] which can be also utilized for storing, modifying and accessing user-defined data. Note that an extension module will "see" other extensions' data if and only if they were built with the same pybind11 version. Consider the following example:

auto data = (MyData *) py::get_shared_data("mydata");
if (!data)
    data = (MyData *) py::set_shared_data("mydata", new MyData(42));

If the above snippet was used in several separately compiled extension modules, the first one to be imported would create a MyData instance and associate a "mydata" key with a pointer to it. Extensions that are imported later would be then able to access the data behind the same pointer.

[1]https://docs.python.org/3/extending/extending.html#using-capsules

Module Destructors

pybind11 does not provide an explicit mechanism to invoke cleanup code at module destruction time. In rare cases where such functionality is required, it is possible to emulate it using Python capsules or weak references with a destruction callback.

auto cleanup_callback = []() {
    // perform cleanup here -- this function is called with the GIL held
};

m.add_object("_cleanup", py::capsule(cleanup_callback));

This approach has the potential downside that instances of classes exposed within the module may still be alive when the cleanup callback is invoked (whether this is acceptable will generally depend on the application).

Alternatively, the capsule may also be stashed within a type object, which ensures that it not called before all instances of that type have been collected:

auto cleanup_callback = []() { /* ... */ };
m.attr("BaseClass").attr("_cleanup") = py::capsule(cleanup_callback);

Both approaches also expose a potentially dangerous _cleanup attribute in Python, which may be undesirable from an API standpoint (a premature explicit call from Python might lead to undefined behavior). Yet another approach that avoids this issue involves weak reference with a cleanup callback:

// Register a callback function that is invoked when the BaseClass object is colelcted
py::cpp_function cleanup_callback(
    [](py::handle weakref) {
        // perform cleanup here -- this function is called with the GIL held

        weakref.dec_ref(); // release weak reference
    }
);

// Create a weak reference with a cleanup callback and initially leak it
(void) py::weakref(m.attr("BaseClass"), cleanup_callback).release();

Note

PyPy (at least version 5.9) does not garbage collect objects when the interpreter exits. An alternative approach (which also works on CPython) is to use the :py:mod:`atexit` module [2], for example:

auto atexit = py::module::import("atexit");
atexit.attr("register")(py::cpp_function([]() {
    // perform cleanup here -- this function is called with the GIL held
}));
[2]https://docs.python.org/3/library/atexit.html

Generating documentation using Sphinx

Sphinx [3] has the ability to inspect the signatures and documentation strings in pybind11-based extension modules to automatically generate beautiful documentation in a variety formats. The python_example repository [4] contains a simple example repository which uses this approach.

There are two potential gotchas when using this approach: first, make sure that the resulting strings do not contain any TAB characters, which break the docstring parsing routines. You may want to use C++11 raw string literals, which are convenient for multi-line comments. Conveniently, any excess indentation will be automatically be removed by Sphinx. However, for this to work, it is important that all lines are indented consistently, i.e.:

// ok
m.def("foo", &foo, R"mydelimiter(
    The foo function

    Parameters
    ----------
)mydelimiter");

// *not ok*
m.def("foo", &foo, R"mydelimiter(The foo function

    Parameters
    ----------
)mydelimiter");

By default, pybind11 automatically generates and prepends a signature to the docstring of a function registered with module::def() and class_::def(). Sometimes this behavior is not desirable, because you want to provide your own signature or remove the docstring completely to exclude the function from the Sphinx documentation. The class options allows you to selectively suppress auto-generated signatures:

PYBIND11_MODULE(example, m) {
    py::options options;
    options.disable_function_signatures();

    m.def("add", [](int a, int b) { return a + b; }, "A function which adds two numbers");
}

Note that changes to the settings affect only function bindings created during the lifetime of the options instance. When it goes out of scope at the end of the module's init function, the default settings are restored to prevent unwanted side effects.

[3]http://www.sphinx-doc.org
[4]http://github.com/pybind/python_example
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