Miscellaneous#

General notes regarding convenience macros#

pybind11 provides a few convenience macros such as PYBIND11_DECLARE_HOLDER_TYPE() and PYBIND11_OVERRIDE_*. 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_OVERRIDE(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_OVERRIDE(ReturnType, ClassType, func);

// Version 2: using the PYBIND11_TYPE macro:
PYBIND11_OVERRIDE(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)#

The Python C API dictates that the Global Interpreter Lock (GIL) must always be held by the current thread to safely access Python objects. As a result, when Python calls into C++ via pybind11 the GIL must be held, and pybind11 will never implicitly release the GIL.

void my_function() {
    /* GIL is held when this function is called from Python */
}

PYBIND11_MODULE(example, m) {
    m.def("my_function", &my_function);
}

pybind11 will ensure that the GIL is held when it knows that it is calling Python code. For example, if a Python callback is passed to C++ code via std::function, when C++ code calls the function the built-in wrapper will acquire the GIL before calling the Python callback. Similarly, the PYBIND11_OVERRIDE family of macros will acquire the GIL before calling back into Python.

When writing C++ code that is called from other C++ code, if that code accesses Python state, it must explicitly acquire and release the GIL.

The classes gil_scoped_release and 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, but great care must be taken when any gil_scoped_release appear: if there is any way that the C++ code can access Python objects, gil_scoped_acquire should be used to reacquire the GIL. Taking Overriding virtual functions in Python as an example, this could be realized as follows (important changes highlighted):

class PyAnimal : public Animal {
public:
    /* Inherit the constructors */
    using Animal::Animal;

    /* Trampoline (need one for each virtual function) */
    std::string go(int n_times) {
        /* PYBIND11_OVERRIDE_PURE will acquire the GIL before accessing Python state */
        PYBIND11_OVERRIDE_PURE(
            std::string, /* Return type */
            Animal,      /* Parent class */
            go,          /* Name of function */
            n_times      /* Argument(s) */
        );
    }
};

PYBIND11_MODULE(example, m) {
    py::class_<Animal, PyAnimal> animal(m, "Animal");
    animal
        .def(py::init<>())
        .def("go", &Animal::go);

    py::class_<Dog>(m, "Dog", animal)
        .def(py::init<>());

    m.def("call_go", [](Animal *animal) -> std::string {
        // GIL is held when called from Python code. Release GIL before
        // calling into (potentially long-running) C++ code
        py::gil_scoped_release release;
        return call_go(animal);
    });
}

The call_go wrapper can also be simplified using the call_guard policy (see Additional call policies) which yields the same result:

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

Common Sources Of Global Interpreter Lock Errors#

Failing to properly hold the Global Interpreter Lock (GIL) is one of the more common sources of bugs within code that uses pybind11. If you are running into GIL related errors, we highly recommend you consult the following checklist.

Do you have any global variables that are pybind11 objects or invoke pybind11 functions in either their constructor or destructor? You are generally not allowed to invoke any Python function in a global static context. We recommend using lazy initialization and then intentionally leaking at the end of the program.
Do you have any pybind11 objects that are members of other C++ structures? One commonly overlooked requirement is that pybind11 objects have to increase their reference count whenever their copy constructor is called. Thus, you need to be holding the GIL to invoke the copy constructor of any C++ class that has a pybind11 member. This can sometimes be very tricky to track for complicated programs Think carefully when you make a pybind11 object a member in another struct.
C++ destructors that invoke Python functions can be particularly troublesome as destructors can sometimes get invoked in weird and unexpected circumstances as a result of exceptions.
You should try running your code in a debug build. That will enable additional assertions within pybind11 that will throw exceptions on certain GIL handling errors (reference counting operations).

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

Please refer to the supplemental example for details.

参见

The 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 Inheritance and automatic downcasting.

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 = reinterpret_cast<MyData *>(py::get_shared_data("mydata"));
if (!data)
    data = static_cast<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 collected
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();

备注

PyPy does not garbage collect objects when the interpreter exits. An alternative approach (which also works on CPython) is to use the 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");
}

pybind11 also appends all members of an enum to the resulting enum docstring. This default behavior can be disabled by using the disable_enum_members_docstring() function of the options class.

With disable_user_defined_docstrings() all user defined docstrings of module_::def(), class_::def() and enum_() are disabled, but the function signatures and enum members are included in the docstring, unless they are disabled separately.

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

Avoiding C++ types in docstrings#

Docstrings are generated at the time of the declaration, e.g. when .def(...) is called. At this point parameter and return types should be known to pybind11. If a custom type is not exposed yet through a py::class_ constructor or a custom type caster, its C++ type name will be used instead to generate the signature in the docstring:

|  __init__(...)
|      __init__(self: example.Foo, arg0: ns::Bar) -> None
                                         ^^^^^^^

This limitation can be circumvented by ensuring that C++ classes are registered with pybind11 before they are used as a parameter or return type of a function:

PYBIND11_MODULE(example, m) {

    auto pyFoo = py::class_<ns::Foo>(m, "Foo");
    auto pyBar = py::class_<ns::Bar>(m, "Bar");

    pyFoo.def(py::init<const ns::Bar&>());
    pyBar.def(py::init<const ns::Foo&>());
}

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