Warning

Please be advised that the reference documentation discussing pybind11 internals is currently incomplete. Please refer to the previous sections and the pybind11 header files for the nitty gritty details.

# Reference

## Macros

PYBIND11_MODULE(name, variable)

  This macro creates the entry point that will be invoked when the Python interpreter imports an extension module. The module name is given as the fist argument and it should not be in quotes. The second macro argument defines a variable of type `py::module_` which can be used to initialize the module.

  The entry point is marked as “maybe unused” to aid dead-code detection analysis: since the entry point is typically only looked up at runtime and not referenced during translation, it would otherwise appear as unused (“dead”) code.

      PYBIND11_MODULE(example, m) {
          m.doc() = "pybind11 example module";

          // Add bindings here
          m.def("foo", []() {
              return "Hello, World!";
          });
      }

## Convenience classes for arbitrary Python types

### Common member functions

template<typename Derived>
class object_api : public pyobject_tag

  A mixin class which adds common functions to `handle`, `object` and various accessors. The only requirement for `Derived` is to implement `PyObject *Derived::ptr() const`.

  Public Functions

  iterator begin() const

    Return an iterator equivalent to calling `iter()` in Python. The object must be a collection which supports the iteration protocol.

  iterator end() const

    Return a sentinel which ends iteration.

  item_accessor operator[](handle key) const

    Return an internal functor to invoke the object’s sequence protocol. Casting the returned `detail::item_accessor` instance to a `handle` or `object` subclass causes a corresponding call to `__getitem__`. Assigning a `handle` or `object` subclass causes a call to `__setitem__`.

  item_accessor operator[](const char *key) const

    See above (the only difference is that they key is provided as a string literal)

  obj_attr_accessor attr(handle key) const

    Return an internal functor to access the object’s attributes. Casting the returned `detail::obj_attr_accessor` instance to a `handle` or `object` subclass causes a corresponding call to `getattr`. Assigning a `handle` or `object` subclass causes a call to `setattr`.

  str_attr_accessor attr(const char *key) const

    See above (the only difference is that they key is provided as a string literal)

  args_proxy operator*() const

    Matches * unpacking in Python, e.g. to unpack arguments out of a `tuple` or `list` for a function call. Applying another * to the result yields ** unpacking, e.g. to unpack a dict as function keyword arguments. See Calling Python functions.

  template<typename T>
  bool contains(T &&item) const

    Check if the given item is contained within this object, i.e. `item in obj`.

  template<return_value_policy policy = return_value_policy::automatic_reference, typename ...Args>
  object operator()(Args&&... args) const

    Assuming the Python object is a function or implements the `__call__` protocol, `operator()` invokes the underlying function, passing an arbitrary set of parameters. The result is returned as a `object` and may need to be converted back into a Python object using `handle::cast()`.

    When some of the arguments cannot be converted to Python objects, the function will throw a `cast_error` exception. When the Python function call fails, a `error_already_set` exception is thrown.

  inline bool is(object_api const &other) const

    Equivalent to `obj is other` in Python.

  inline bool is_none() const

    Equivalent to `obj is None` in Python.

  inline bool equal(object_api const &other) const

    Equivalent to obj == other in Python.

  str_attr_accessor doc() const

    Get or set the object’s docstring, i.e. `obj.__doc__`.

  inline int ref_count() const

    Return the object’s current reference count.

### Without reference counting

class handle : public detail::object_api<handle>

  Holds a reference to a Python object (no reference counting)

  The `handle` class is a thin wrapper around an arbitrary Python object (i.e. a `PyObject *` in Python’s C API). It does not perform any automatic reference counting and merely provides a basic C++ interface to various Python API functions.

  See also

  The `object` class inherits from `handle` and adds automatic reference counting features.

  Subclassed by args_proxy, kwargs_proxy, object

  Public Functions

  handle() = default

    The default constructor creates a handle with a `nullptr`-valued pointer.

  inline handle(PyObject *ptr)

    Creates a `handle` from the given raw Python object pointer.

  inline PyObject *ptr() const

    Return the underlying `PyObject *` pointer.

  inline const handle &inc_ref() const &

    Manually increase the reference count of the Python object. Usually, it is preferable to use the `object` class which derives from `handle` and calls this function automatically. Returns a reference to itself.

  inline const handle &dec_ref() const &

    Manually decrease the reference count of the Python object. Usually, it is preferable to use the `object` class which derives from `handle` and calls this function automatically. Returns a reference to itself.

  template<typename T>
  T cast() const

    Attempt to cast the Python object into the given C++ type. A `cast_error` will be throw upon failure.

  inline explicit operator bool() const

    Return `true` when the `handle` wraps a valid Python object.

  inline bool operator==(const handle &h) const

    Deprecated: Check that the underlying pointers are the same. Equivalent to `obj1 is obj2` in Python.

### With reference counting

class object : public handle

  Holds a reference to a Python object (with reference counting)

  Like `handle`, the `object` class is a thin wrapper around an arbitrary Python object (i.e. a `PyObject *` in Python’s C API). In contrast to `handle`, it optionally increases the object’s reference count upon construction, and it always decreases the reference count when the `object` instance goes out of scope and is destructed. When using `object` instances consistently, it is much easier to get reference counting right at the first attempt.

  Subclassed by bool_, buffer, bytearray, bytes, capsule, dict, dtype, ellipsis, exception< type >, float_, function, generic_type, int_, iterable, iterator, list, memoryview, module_, none, sequence, set, slice, staticmethod, str, tuple, type, weakref

  Public Functions

  inline object(const object &o)

    Copy constructor; always increases the reference count.

  inline object(object &&other) noexcept

    Move constructor; steals the object from `other` and preserves its reference count.

  inline ~object()

    Destructor; automatically calls `handle::dec_ref()`

  inline handle release()

    Resets the internal pointer to `nullptr` without decreasing the object’s reference count. The function returns a raw handle to the original Python object.

template<typename T>
T reinterpret_borrow(handle h)

  Declare that a `handle` or `PyObject *` is a certain type and borrow the reference. The target type `T` must be `object` or one of its derived classes. The function doesn’t do any conversions or checks. It’s up to the user to make sure that the target type is correct.

      PyObject *p = PyList_GetItem(obj, index);
      py::object o = reinterpret_borrow<py::object>(p);
      // or
      py::tuple t = reinterpret_borrow<py::tuple>(p); // <-- `p` must be already be a `tuple`

template<typename T>
T reinterpret_steal(handle h)

  Like `reinterpret_borrow()`, but steals the reference.

  >     PyObject *p = PyObject_Str(obj);
  >     py::str s = reinterpret_steal<py::str>(p); // <-- `p` must be already be a `str`

## Convenience classes for specific Python types

class module_ : public object

  Wrapper for Python extension modules.

  Public Functions

  inline explicit module_(const char *name, const char *doc = nullptr)

    Create a new top-level Python module with the given name and docstring.

  template<typename Func, typename ...Extra>
  inline module_ &def(const char *name_, Func &&f, const Extra&... extra)

    Create Python binding for a new function within the module scope. `Func` can be a plain C++ function, a function pointer, or a lambda function. For details on the `Extra&& ... extra` argument, see section Passing extra arguments to def or class_.

  inline module_ def_submodule(const char *name, const char *doc = nullptr)

    Create and return a new Python submodule with the given name and docstring. This also works recursively, i.e.

        py::module_ m("example", "pybind11 example plugin");
        py::module_ m2 = m.def_submodule("sub", "A submodule of 'example'");
        py::module_ m3 = m2.def_submodule("subsub", "A submodule of 'example.sub'");

  inline void reload()

    Reload the module or throws `error_already_set`.

  inline void add_object(const char *name, handle obj, bool overwrite = false)

    Adds an object to the module using the given name. Throws if an object with the given name already exists.

    `overwrite` should almost always be false: attempting to overwrite objects that pybind11 has established will, in most cases, break things.

  Public Static Functions

  static inline module_ import(const char *name)

    Import and return a module or throws `error_already_set`.

  static inline module_ create_extension_module(const char *name, const char *doc, module_def *def)

    Create a new top-level module that can be used as the main module of a C extension.

    For Python 3, `def` should point to a statically allocated module_def. For Python 2, `def` can be a nullptr and is completely ignored.

## Convenience functions converting to Python types

template<return_value_policy policy = return_value_policy::automatic_reference, typename ...Args>
tuple make_tuple(Args&&... args_)

template<return_value_policy Policy = return_value_policy::reference_internal, typename Iterator, typename Sentinel, typename ValueType = typename detail::iterator_access<Iterator>::result_type, typename ...Extra>
iterator make_iterator(Iterator first, Sentinel last, Extra&&... extra)

  Makes a python iterator from a first and past-the-end C++ InputIterator.

template<return_value_policy Policy = return_value_policy::reference_internal, typename Type, typename ...Extra>
iterator make_iterator(Type &value, Extra&&... extra)

  Makes an iterator over values of an stl container or other container supporting `std::begin()`/`std::end()`

template<return_value_policy Policy = return_value_policy::reference_internal, typename Iterator, typename Sentinel, typename KeyType = typename detail::iterator_key_access<Iterator>::result_type, typename ...Extra>
iterator make_key_iterator(Iterator first, Sentinel last, Extra&&... extra)

  Makes a python iterator over the keys (`.first`) of a iterator over pairs from a first and past-the-end InputIterator.

template<return_value_policy Policy = return_value_policy::reference_internal, typename Type, typename ...Extra>
iterator make_key_iterator(Type &value, Extra&&... extra)

  Makes an iterator over the keys (`.first`) of a stl map-like container supporting `std::begin()`/`std::end()`

template<return_value_policy Policy = return_value_policy::reference_internal, typename Iterator, typename Sentinel, typename ValueType = typename detail::iterator_value_access<Iterator>::result_type, typename ...Extra>
iterator make_value_iterator(Iterator first, Sentinel last, Extra&&... extra)

  Makes a python iterator over the values (`.second`) of a iterator over pairs from a first and past-the-end InputIterator.

template<return_value_policy Policy = return_value_policy::reference_internal, typename Type, typename ...Extra>
iterator make_value_iterator(Type &value, Extra&&... extra)

  Makes an iterator over the values (`.second`) of a stl map-like container supporting `std::begin()`/`std::end()`

## Passing extra arguments to `def` or `class_`

group annotations
  struct is_method
    #include <attr.h>

    Annotation for methods.

  struct is_operator
    #include <attr.h>

    Annotation for operators.

  struct is_final
    #include <attr.h>

    Annotation for classes that cannot be subclassed.

  struct scope
    #include <attr.h>

    Annotation for parent scope.

  struct doc
    #include <attr.h>

    Annotation for documentation.

  struct name
    #include <attr.h>

    Annotation for function names.

  struct sibling
    #include <attr.h>

    Annotation indicating that a function is an overload associated with a given “sibling”.

  template<typename T>
  struct base
    #include <attr.h>

    Annotation indicating that a class derives from another given type.

  template<size_t Nurse, size_t Patient>
  struct keep_alive
    #include <attr.h>

    Keep patient alive while nurse lives.

  struct multiple_inheritance
    #include <attr.h>

    Annotation indicating that a class is involved in a multiple inheritance relationship.

  struct dynamic_attr
    #include <attr.h>

    Annotation which enables dynamic attributes, i.e. adds `__dict__` to a class.

  struct buffer_protocol
    #include <attr.h>

    Annotation which enables the buffer protocol for a type.

  struct metaclass
    #include <attr.h>

    Annotation which requests that a special metaclass is created for a type.

    Public Functions

    inline explicit metaclass(handle value)

      Override pybind11’s default metaclass.

  struct custom_type_setup
    #include <attr.h>

    Specifies a custom callback with signature `void (PyHeapTypeObject*)` that may be used to customize the Python type.

    The callback is invoked immediately before `PyType_Ready`.

    Note: This is an advanced interface, and uses of it may require changes to work with later versions of pybind11. You may wish to consult the implementation of `make_new_python_type` in `detail/classes.h` to understand the context in which the callback will be run.

  struct module_local
    #include <attr.h>

    Annotation that marks a class as local to the module:

  struct arithmetic
    #include <attr.h>

    Annotation to mark enums as an arithmetic type.

  struct prepend
    #include <attr.h>

    Mark a function for addition at the beginning of the existing overload chain instead of the end.

  template<typename ...Ts>
  struct call_guard

    A call policy which places one or more guard variables (`Ts...`) around the function call.

    For example, this definition:

        m.def("foo", foo, py::call_guard<T>());

    is equivalent to the following pseudocode:

        m.def("foo", [](args...) {
            T scope_guard;
            return foo(args...); // forwarded arguments
        });

  template<>
  struct call_guard<>
  template<typename T>
  struct call_guard<T>
  template<typename T, typename ...Ts>
  struct call_guard<T, Ts...>
    struct type
  struct arg
    #include <cast.h>

    Annotation for arguments

    Subclassed by arg_v

    Public Functions

    inline explicit constexpr arg(const char *name = nullptr)

      Constructs an argument with the name of the argument; if null or omitted, this is a positional argument.

    template<typename T>
    arg_v operator=(T &&value) const

      Assign a value to this argument.

    inline arg &noconvert(bool flag = true)

      Indicate that the type should not be converted in the type caster.

    inline arg &none(bool flag = true)

      Indicates that the argument should/shouldn’t allow None (e.g. for nullable pointer args)

    Public Members

    const char *name

      If non-null, this is a named kwargs argument.

    bool flag_noconvert

      If set, do not allow conversion (requires a supporting type caster!)

    bool flag_none

      If set (the default), allow None to be passed to this argument.

  struct arg_v : public arg
    #include <cast.h>

    Annotation for arguments with values

    Public Functions

    template<typename T>
    inline arg_v(const char *name, T &&x, const char *descr = nullptr)

      Direct construction with name, default, and description.

    template<typename T>
    inline arg_v(const arg &base, T &&x, const char *descr = nullptr)

      Called internally when invoking `py::arg("a") = value`

    inline arg_v &noconvert(bool flag = true)

      Same as `arg::noconvert()`, but returns *this as arg_v&, not arg&.

    inline arg_v &none(bool flag = true)

      Same as `arg::nonone()`, but returns *this as arg_v&, not arg&.

    Public Members

    object value

      The default value.

    const char *descr

      The (optional) description of the default value.

    std::string type

      The C++ type name of the default value (only available when compiled in debug mode)

  struct kw_only
    #include <cast.h>

    Annotation indicating that all following arguments are keyword-only; the is the equivalent of an unnamed ‘*’ argument (in Python 3)

  struct pos_only
    #include <cast.h>

    Annotation indicating that all previous arguments are positional-only; the is the equivalent of an unnamed ‘/’ argument (in Python 3.8)

## Embedding the interpreter

PYBIND11_EMBEDDED_MODULE(name, variable)

  Add a new module to the table of builtins for the interpreter. Must be defined in global scope. The first macro parameter is the name of the module (without quotes). The second parameter is the variable which will be used as the interface to add functions and classes to the module.

      PYBIND11_EMBEDDED_MODULE(example, m) {
          // ... initialize functions and classes here
          m.def("foo", []() {
              return "Hello, World!";
          });
      }

inline void initialize_interpreter(bool init_signal_handlers = true, int argc = 0, const char *const *argv = nullptr, bool add_program_dir_to_path = true)

  Initialize the Python interpreter. No other pybind11 or CPython API functions can be called before this is done; with the exception of `PYBIND11_EMBEDDED_MODULE`. The optional `init_signal_handlers` parameter can be used to skip the registration of signal handlers (see the Python documentation for details). Calling this function again after the interpreter has already been initialized is a fatal error.

  If initializing the Python interpreter fails, then the program is terminated. (This is controlled by the CPython runtime and is an exception to pybind11’s normal behavior of throwing exceptions on errors.)

  The remaining optional parameters, `argc`, `argv`, and `add_program_dir_to_path` are used to populate `sys.argv` and `sys.path`. See the `PySys_SetArgvEx` documentation for details.

inline void finalize_interpreter()

  Shut down the Python interpreter. No pybind11 or CPython API functions can be called after this. In addition, pybind11 objects must not outlive the interpreter:

      { // BAD
          py::initialize_interpreter();
          auto hello = py::str("Hello, World!");
          py::finalize_interpreter();
      } // <-- BOOM, hello's destructor is called after interpreter shutdown

      { // GOOD
          py::initialize_interpreter();
          { // scoped
              auto hello = py::str("Hello, World!");
          } // <-- OK, hello is cleaned up properly
          py::finalize_interpreter();
      }

      { // BETTER
          py::scoped_interpreter guard{};
          auto hello = py::str("Hello, World!");
      }

  Warning

  The interpreter can be restarted by calling `initialize_interpreter()` again. Modules created using pybind11 can be safely re-initialized. However, Python itself cannot completely unload binary extension modules and there are several caveats with regard to interpreter restarting. All the details can be found in the CPython documentation. In short, not all interpreter memory may be freed, either due to reference cycles or user-created global data.

class scoped_interpreter

  Scope guard version of `initialize_interpreter()` and `finalize_interpreter()`. This a move-only guard and only a single instance can exist.

  See `initialize_interpreter()` for a discussion of its constructor arguments.

      #include <pybind11/embed.h>

      int main() {
          py::scoped_interpreter guard{};
          py::print(Hello, World!);
      } // <-- interpreter shutdown

## Redirecting C++ streams

class scoped_ostream_redirect

  This a move-only guard that redirects output.

      #include <pybind11/iostream.h>

      ...

      {
          py::scoped_ostream_redirect output;
          std::cout << "Hello, World!"; // Python stdout
      } // <-- return std::cout to normal

  You can explicitly pass the c++ stream and the python object, for example to guard stderr instead.

      {
          py::scoped_ostream_redirect output{std::cerr, py::module::import("sys").attr("stderr")};
          std::cout << "Hello, World!";
      }

  Subclassed by scoped_estream_redirect

class scoped_estream_redirect : public scoped_ostream_redirect

  Like `scoped_ostream_redirect`, but redirects cerr by default. This class is provided primary to make `py::call_guard` easier to make.

      m.def("noisy_func", &noisy_func,
            py::call_guard<scoped_ostream_redirect,
                           scoped_estream_redirect>());

inline class_<detail::OstreamRedirect> add_ostream_redirect(module_ m, const std::string &name = "ostream_redirect")

  This is a helper function to add a C++ redirect context manager to Python instead of using a C++ guard. To use it, add the following to your binding code:

      #include <pybind11/iostream.h>

      ...

      py::add_ostream_redirect(m, "ostream_redirect");

  You now have a Python context manager that redirects your output:

      with m.ostream_redirect():
          m.print_to_cout_function()

  This manager can optionally be told which streams to operate on:

      with m.ostream_redirect(stdout=true, stderr=true):
          m.noisy_function_with_error_printing()

## Python built-in functions

group python_builtins

  Unless stated otherwise, the following C++ functions behave the same as their Python counterparts.

  Functions

  inline dict globals()

    Return a dictionary representing the global variables in the current execution frame, or `__main__.__dict__` if there is no frame (usually when the interpreter is embedded).

  template<typename T, detail::enable_if_t<std::is_base_of<object, T>::value, int> = 0>
  bool isinstance(handle obj)

    Return true if `obj` is an instance of `T`. Type `T` must be a subclass of `object` or a class which was exposed to Python as `py::class_<T>`.

  inline bool isinstance(handle obj, handle type)

    Return true if `obj` is an instance of the `type`.

  inline bool hasattr(handle obj, handle name)
  inline bool hasattr(handle obj, const char *name)
  inline void delattr(handle obj, handle name)
  inline void delattr(handle obj, const char *name)
  inline object getattr(handle obj, handle name)
  inline object getattr(handle obj, const char *name)
  inline object getattr(handle obj, handle name, handle default_)
  inline object getattr(handle obj, const char *name, handle default_)
  inline void setattr(handle obj, handle name, handle value)
  inline void setattr(handle obj, const char *name, handle value)
  inline ssize_t hash(handle obj)
  inline size_t len(handle h)

    Get the length of a Python object.

  inline size_t len_hint(handle h)

    Get the length hint of a Python object. Returns 0 when this cannot be determined.

  inline str repr(handle h)
  inline iterator iter(handle obj)

## Inheritance

See Object-oriented code and Classes for more detail.

PYBIND11_OVERRIDE(ret_type, cname, fn, ...)

  Macro to populate the virtual method in the trampoline class. This macro tries to look up the method from the Python side, deals with the Global Interpreter Lock (GIL) and necessary argument conversions to call this method and return the appropriate type. This macro should be used if the method name in C and in Python are identical. See Overriding virtual functions in Python for more information.

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

          // Trampoline (need one for each virtual function)
          std::string go(int n_times) override {
              PYBIND11_OVERRIDE_PURE(
                  std::string, // Return type (ret_type)
                  Animal,      // Parent class (cname)
                  go,          // Name of function in C++ (must match Python name) (fn)
                  n_times      // Argument(s) (...)
              );
          }
      };

PYBIND11_OVERRIDE_PURE(ret_type, cname, fn, ...)

  Macro for pure virtual functions, this function is identical to `PYBIND11_OVERRIDE`, except that it throws if no override can be found.

PYBIND11_OVERRIDE_NAME(ret_type, cname, name, fn, ...)

  Macro to populate the virtual method in the trampoline class. This macro tries to look up a method named ‘fn’ from the Python side, deals with the Global Interpreter Lock (GIL) and necessary argument conversions to call this method and return the appropriate type. See Overriding virtual functions in Python for more information. This macro should be used when the method name in C is not the same as the method name in Python. For example with `__str__`.

      std::string toString() override {
        PYBIND11_OVERRIDE_NAME(
            std::string, // Return type (ret_type)
            Animal,      // Parent class (cname)
            "__str__",   // Name of method in Python (name)
            toString,    // Name of function in C++ (fn)
        );
      }

PYBIND11_OVERRIDE_PURE_NAME(ret_type, cname, name, fn, ...)

  Macro for pure virtual functions, this function is identical to `PYBIND11_OVERRIDE_NAME`, except that it throws if no override can be found.

template<class T>
function get_override(const T *this_ptr, const char *name)

  Try to retrieve a python method by the provided name from the instance pointed to by the this_ptr.

  This_ptr

    The pointer to the object the overridden method should be retrieved for. This should be the first non-trampoline class encountered in the inheritance chain.

  Name

    The name of the overridden Python method to retrieve.

  Returns

    The Python method by this name from the object or an empty function wrapper.

## Exceptions

class error_already_set : public std::runtime_error

  Fetch and hold an error which was already set in Python. An instance of this is typically thrown to propagate python-side errors back through C++ which can either be caught manually or else falls back to the function dispatcher (which then raises the captured error back to python).

  Public Functions

  inline error_already_set()

    Constructs a new exception from the current Python error indicator, if any. The current Python error indicator will be cleared.

  inline void restore()

    Give the currently-held error back to Python, if any. If there is currently a Python error already set it is cleared first. After this call, the current object no longer stores the error variables (but the `.what()` string is still available).

  inline void discard_as_unraisable(object err_context)

    If it is impossible to raise the currently-held error, such as in a destructor, we can write it out using Python’s unraisable hook (`sys.unraisablehook`). The error context should be some object whose `repr()` helps identify the location of the error. Python already knows the type and value of the error, so there is no need to repeat that. After this call, the current object no longer stores the error variables, and neither does Python.

  inline void discard_as_unraisable(const char *err_context)

    An alternate version of `discard_as_unraisable()`, where a string provides information on the location of the error. For example, `__func__` could be helpful.

  inline bool matches(handle exc) const

    Check if the currently trapped error type matches the given Python exception class (or a subclass thereof). May also be passed a tuple to search for any exception class matches in the given tuple.

class builtin_exception : public std::runtime_error

  C++ bindings of builtin Python exceptions.

  Public Functions

  virtual void set_error() const = 0

    Set the error using the Python C API.

## Literals

namespace literals