mirror of
https://github.com/pybind/pybind11.git
synced 2024-11-22 05:05:11 +00:00
556 lines
24 KiB
ReStructuredText
556 lines
24 KiB
ReStructuredText
Functions
|
|
#########
|
|
|
|
Before proceeding with this section, make sure that you are already familiar
|
|
with the basics of binding functions and classes, as explained in :doc:`/basics`
|
|
and :doc:`/classes`. The following guide is applicable to both free and member
|
|
functions, i.e. *methods* in Python.
|
|
|
|
.. _return_value_policies:
|
|
|
|
Return value policies
|
|
=====================
|
|
|
|
Python and C++ use fundamentally different ways of managing the memory and
|
|
lifetime of objects managed by them. This can lead to issues when creating
|
|
bindings for functions that return a non-trivial type. Just by looking at the
|
|
type information, it is not clear whether Python should take charge of the
|
|
returned value and eventually free its resources, or if this is handled on the
|
|
C++ side. For this reason, pybind11 provides a several *return value policy*
|
|
annotations that can be passed to the :func:`module_::def` and
|
|
:func:`class_::def` functions. The default policy is
|
|
:enum:`return_value_policy::automatic`.
|
|
|
|
Return value policies are tricky, and it's very important to get them right.
|
|
Just to illustrate what can go wrong, consider the following simple example:
|
|
|
|
.. code-block:: cpp
|
|
|
|
/* Function declaration */
|
|
Data *get_data() { return _data; /* (pointer to a static data structure) */ }
|
|
...
|
|
|
|
/* Binding code */
|
|
m.def("get_data", &get_data); // <-- KABOOM, will cause crash when called from Python
|
|
|
|
What's going on here? When ``get_data()`` is called from Python, the return
|
|
value (a native C++ type) must be wrapped to turn it into a usable Python type.
|
|
In this case, the default return value policy (:enum:`return_value_policy::automatic`)
|
|
causes pybind11 to assume ownership of the static ``_data`` instance.
|
|
|
|
When Python's garbage collector eventually deletes the Python
|
|
wrapper, pybind11 will also attempt to delete the C++ instance (via ``operator
|
|
delete()``) due to the implied ownership. At this point, the entire application
|
|
will come crashing down, though errors could also be more subtle and involve
|
|
silent data corruption.
|
|
|
|
In the above example, the policy :enum:`return_value_policy::reference` should have
|
|
been specified so that the global data instance is only *referenced* without any
|
|
implied transfer of ownership, i.e.:
|
|
|
|
.. code-block:: cpp
|
|
|
|
m.def("get_data", &get_data, return_value_policy::reference);
|
|
|
|
On the other hand, this is not the right policy for many other situations,
|
|
where ignoring ownership could lead to resource leaks.
|
|
As a developer using pybind11, it's important to be familiar with the different
|
|
return value policies, including which situation calls for which one of them.
|
|
The following table provides an overview of available policies:
|
|
|
|
.. tabularcolumns:: |p{0.5\textwidth}|p{0.45\textwidth}|
|
|
|
|
+--------------------------------------------------+----------------------------------------------------------------------------+
|
|
| Return value policy | Description |
|
|
+==================================================+============================================================================+
|
|
| :enum:`return_value_policy::take_ownership` | Reference an existing object (i.e. do not create a new copy) and take |
|
|
| | ownership. Python will call the destructor and delete operator when the |
|
|
| | object's reference count reaches zero. Undefined behavior ensues when the |
|
|
| | C++ side does the same, or when the data was not dynamically allocated. |
|
|
+--------------------------------------------------+----------------------------------------------------------------------------+
|
|
| :enum:`return_value_policy::copy` | Create a new copy of the returned object, which will be owned by Python. |
|
|
| | This policy is comparably safe because the lifetimes of the two instances |
|
|
| | are decoupled. |
|
|
+--------------------------------------------------+----------------------------------------------------------------------------+
|
|
| :enum:`return_value_policy::move` | Use ``std::move`` to move the return value contents into a new instance |
|
|
| | that will be owned by Python. This policy is comparably safe because the |
|
|
| | lifetimes of the two instances (move source and destination) are decoupled.|
|
|
+--------------------------------------------------+----------------------------------------------------------------------------+
|
|
| :enum:`return_value_policy::reference` | Reference an existing object, but do not take ownership. The C++ side is |
|
|
| | responsible for managing the object's lifetime and deallocating it when |
|
|
| | it is no longer used. Warning: undefined behavior will ensue when the C++ |
|
|
| | side deletes an object that is still referenced and used by Python. |
|
|
+--------------------------------------------------+----------------------------------------------------------------------------+
|
|
| :enum:`return_value_policy::reference_internal` | Indicates that the lifetime of the return value is tied to the lifetime |
|
|
| | of a parent object, namely the implicit ``this``, or ``self`` argument of |
|
|
| | the called method or property. Internally, this policy works just like |
|
|
| | :enum:`return_value_policy::reference` but additionally applies a |
|
|
| | ``keep_alive<0, 1>`` *call policy* (described in the next section) that |
|
|
| | prevents the parent object from being garbage collected as long as the |
|
|
| | return value is referenced by Python. This is the default policy for |
|
|
| | property getters created via ``def_property``, ``def_readwrite``, etc. |
|
|
+--------------------------------------------------+----------------------------------------------------------------------------+
|
|
| :enum:`return_value_policy::automatic` | **Default policy.** This policy falls back to the policy |
|
|
| | :enum:`return_value_policy::take_ownership` when the return value is a |
|
|
| | pointer. Otherwise, it uses :enum:`return_value_policy::move` or |
|
|
| | :enum:`return_value_policy::copy` for rvalue and lvalue references, |
|
|
| | respectively. See above for a description of what all of these different |
|
|
| | policies do. |
|
|
+--------------------------------------------------+----------------------------------------------------------------------------+
|
|
| :enum:`return_value_policy::automatic_reference` | As above, but use policy :enum:`return_value_policy::reference` when the |
|
|
| | return value is a pointer. This is the default conversion policy for |
|
|
| | function arguments when calling Python functions manually from C++ code |
|
|
| | (i.e. via handle::operator()). You probably won't need to use this. |
|
|
+--------------------------------------------------+----------------------------------------------------------------------------+
|
|
|
|
Return value policies can also be applied to properties:
|
|
|
|
.. code-block:: cpp
|
|
|
|
class_<MyClass>(m, "MyClass")
|
|
.def_property("data", &MyClass::getData, &MyClass::setData,
|
|
py::return_value_policy::copy);
|
|
|
|
Technically, the code above applies the policy to both the getter and the
|
|
setter function, however, the setter doesn't really care about *return*
|
|
value policies which makes this a convenient terse syntax. Alternatively,
|
|
targeted arguments can be passed through the :class:`cpp_function` constructor:
|
|
|
|
.. code-block:: cpp
|
|
|
|
class_<MyClass>(m, "MyClass")
|
|
.def_property("data"
|
|
py::cpp_function(&MyClass::getData, py::return_value_policy::copy),
|
|
py::cpp_function(&MyClass::setData)
|
|
);
|
|
|
|
.. warning::
|
|
|
|
Code with invalid return value policies might access uninitialized memory or
|
|
free data structures multiple times, which can lead to hard-to-debug
|
|
non-determinism and segmentation faults, hence it is worth spending the
|
|
time to understand all the different options in the table above.
|
|
|
|
.. note::
|
|
|
|
One important aspect of the above policies is that they only apply to
|
|
instances which pybind11 has *not* seen before, in which case the policy
|
|
clarifies essential questions about the return value's lifetime and
|
|
ownership. When pybind11 knows the instance already (as identified by its
|
|
type and address in memory), it will return the existing Python object
|
|
wrapper rather than creating a new copy.
|
|
|
|
.. note::
|
|
|
|
The next section on :ref:`call_policies` discusses *call policies* that can be
|
|
specified *in addition* to a return value policy from the list above. Call
|
|
policies indicate reference relationships that can involve both return values
|
|
and parameters of functions.
|
|
|
|
.. note::
|
|
|
|
As an alternative to elaborate call policies and lifetime management logic,
|
|
consider using smart pointers (see the section on :ref:`smart_pointers` for
|
|
details). Smart pointers can tell whether an object is still referenced from
|
|
C++ or Python, which generally eliminates the kinds of inconsistencies that
|
|
can lead to crashes or undefined behavior. For functions returning smart
|
|
pointers, it is not necessary to specify a return value policy.
|
|
|
|
.. _call_policies:
|
|
|
|
Additional call policies
|
|
========================
|
|
|
|
In addition to the above return value policies, further *call policies* can be
|
|
specified to indicate dependencies between parameters or ensure a certain state
|
|
for the function call.
|
|
|
|
Keep alive
|
|
----------
|
|
|
|
In general, this policy is required when the C++ object is any kind of container
|
|
and another object is being added to the container. ``keep_alive<Nurse, Patient>``
|
|
indicates that the argument with index ``Patient`` should be kept alive at least
|
|
until the argument with index ``Nurse`` is freed by the garbage collector. Argument
|
|
indices start at one, while zero refers to the return value. For methods, index
|
|
``1`` refers to the implicit ``this`` pointer, while regular arguments begin at
|
|
index ``2``. Arbitrarily many call policies can be specified. When a ``Nurse``
|
|
with value ``None`` is detected at runtime, the call policy does nothing.
|
|
|
|
When the nurse is not a pybind11-registered type, the implementation internally
|
|
relies on the ability to create a *weak reference* to the nurse object. When
|
|
the nurse object is not a pybind11-registered type and does not support weak
|
|
references, an exception will be thrown.
|
|
|
|
Consider the following example: here, the binding code for a list append
|
|
operation ties the lifetime of the newly added element to the underlying
|
|
container:
|
|
|
|
.. code-block:: cpp
|
|
|
|
py::class_<List>(m, "List")
|
|
.def("append", &List::append, py::keep_alive<1, 2>());
|
|
|
|
For consistency, the argument indexing is identical for constructors. Index
|
|
``1`` still refers to the implicit ``this`` pointer, i.e. the object which is
|
|
being constructed. Index ``0`` refers to the return type which is presumed to
|
|
be ``void`` when a constructor is viewed like a function. The following example
|
|
ties the lifetime of the constructor element to the constructed object:
|
|
|
|
.. code-block:: cpp
|
|
|
|
py::class_<Nurse>(m, "Nurse")
|
|
.def(py::init<Patient &>(), py::keep_alive<1, 2>());
|
|
|
|
.. note::
|
|
|
|
``keep_alive`` is analogous to the ``with_custodian_and_ward`` (if Nurse,
|
|
Patient != 0) and ``with_custodian_and_ward_postcall`` (if Nurse/Patient ==
|
|
0) policies from Boost.Python.
|
|
|
|
Call guard
|
|
----------
|
|
|
|
The ``call_guard<T>`` policy allows any scope guard type ``T`` to be placed
|
|
around the function call. For example, this definition:
|
|
|
|
.. code-block:: cpp
|
|
|
|
m.def("foo", foo, py::call_guard<T>());
|
|
|
|
is equivalent to the following pseudocode:
|
|
|
|
.. code-block:: cpp
|
|
|
|
m.def("foo", [](args...) {
|
|
T scope_guard;
|
|
return foo(args...); // forwarded arguments
|
|
});
|
|
|
|
The only requirement is that ``T`` is default-constructible, but otherwise any
|
|
scope guard will work. This is very useful in combination with `gil_scoped_release`.
|
|
See :ref:`gil`.
|
|
|
|
Multiple guards can also be specified as ``py::call_guard<T1, T2, T3...>``. The
|
|
constructor order is left to right and destruction happens in reverse.
|
|
|
|
.. seealso::
|
|
|
|
The file :file:`tests/test_call_policies.cpp` contains a complete example
|
|
that demonstrates using `keep_alive` and `call_guard` in more detail.
|
|
|
|
.. _python_objects_as_args:
|
|
|
|
Python objects as arguments
|
|
===========================
|
|
|
|
pybind11 exposes all major Python types using thin C++ wrapper classes. These
|
|
wrapper classes can also be used as parameters of functions in bindings, which
|
|
makes it possible to directly work with native Python types on the C++ side.
|
|
For instance, the following statement iterates over a Python ``dict``:
|
|
|
|
.. code-block:: cpp
|
|
|
|
void print_dict(py::dict dict) {
|
|
/* Easily interact with Python types */
|
|
for (auto item : dict)
|
|
std::cout << "key=" << std::string(py::str(item.first)) << ", "
|
|
<< "value=" << std::string(py::str(item.second)) << std::endl;
|
|
}
|
|
|
|
It can be exported:
|
|
|
|
.. code-block:: cpp
|
|
|
|
m.def("print_dict", &print_dict);
|
|
|
|
And used in Python as usual:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> print_dict({'foo': 123, 'bar': 'hello'})
|
|
key=foo, value=123
|
|
key=bar, value=hello
|
|
|
|
For more information on using Python objects in C++, see :doc:`/advanced/pycpp/index`.
|
|
|
|
Accepting \*args and \*\*kwargs
|
|
===============================
|
|
|
|
Python provides a useful mechanism to define functions that accept arbitrary
|
|
numbers of arguments and keyword arguments:
|
|
|
|
.. code-block:: python
|
|
|
|
def generic(*args, **kwargs):
|
|
... # do something with args and kwargs
|
|
|
|
Such functions can also be created using pybind11:
|
|
|
|
.. code-block:: cpp
|
|
|
|
void generic(py::args args, py::kwargs kwargs) {
|
|
/// .. do something with args
|
|
if (kwargs)
|
|
/// .. do something with kwargs
|
|
}
|
|
|
|
/// Binding code
|
|
m.def("generic", &generic);
|
|
|
|
The class ``py::args`` derives from ``py::tuple`` and ``py::kwargs`` derives
|
|
from ``py::dict``.
|
|
|
|
You may also use just one or the other, and may combine these with other
|
|
arguments as long as the ``py::args`` and ``py::kwargs`` arguments are the last
|
|
arguments accepted by the function.
|
|
|
|
Please refer to the other examples for details on how to iterate over these,
|
|
and on how to cast their entries into C++ objects. A demonstration is also
|
|
available in ``tests/test_kwargs_and_defaults.cpp``.
|
|
|
|
.. note::
|
|
|
|
When combining \*args or \*\*kwargs with :ref:`keyword_args` you should
|
|
*not* include ``py::arg`` tags for the ``py::args`` and ``py::kwargs``
|
|
arguments.
|
|
|
|
Default arguments revisited
|
|
===========================
|
|
|
|
The section on :ref:`default_args` previously discussed basic usage of default
|
|
arguments using pybind11. One noteworthy aspect of their implementation is that
|
|
default arguments are converted to Python objects right at declaration time.
|
|
Consider the following example:
|
|
|
|
.. code-block:: cpp
|
|
|
|
py::class_<MyClass>("MyClass")
|
|
.def("myFunction", py::arg("arg") = SomeType(123));
|
|
|
|
In this case, pybind11 must already be set up to deal with values of the type
|
|
``SomeType`` (via a prior instantiation of ``py::class_<SomeType>``), or an
|
|
exception will be thrown.
|
|
|
|
Another aspect worth highlighting is that the "preview" of the default argument
|
|
in the function signature is generated using the object's ``__repr__`` method.
|
|
If not available, the signature may not be very helpful, e.g.:
|
|
|
|
.. code-block:: pycon
|
|
|
|
FUNCTIONS
|
|
...
|
|
| myFunction(...)
|
|
| Signature : (MyClass, arg : SomeType = <SomeType object at 0x101b7b080>) -> NoneType
|
|
...
|
|
|
|
The first way of addressing this is by defining ``SomeType.__repr__``.
|
|
Alternatively, it is possible to specify the human-readable preview of the
|
|
default argument manually using the ``arg_v`` notation:
|
|
|
|
.. code-block:: cpp
|
|
|
|
py::class_<MyClass>("MyClass")
|
|
.def("myFunction", py::arg_v("arg", SomeType(123), "SomeType(123)"));
|
|
|
|
Sometimes it may be necessary to pass a null pointer value as a default
|
|
argument. In this case, remember to cast it to the underlying type in question,
|
|
like so:
|
|
|
|
.. code-block:: cpp
|
|
|
|
py::class_<MyClass>("MyClass")
|
|
.def("myFunction", py::arg("arg") = static_cast<SomeType *>(nullptr));
|
|
|
|
Keyword-only arguments
|
|
======================
|
|
|
|
Python 3 introduced keyword-only arguments by specifying an unnamed ``*``
|
|
argument in a function definition:
|
|
|
|
.. code-block:: python
|
|
|
|
def f(a, *, b): # a can be positional or via keyword; b must be via keyword
|
|
pass
|
|
|
|
f(a=1, b=2) # good
|
|
f(b=2, a=1) # good
|
|
f(1, b=2) # good
|
|
f(1, 2) # TypeError: f() takes 1 positional argument but 2 were given
|
|
|
|
Pybind11 provides a ``py::kw_only`` object that allows you to implement
|
|
the same behaviour by specifying the object between positional and keyword-only
|
|
argument annotations when registering the function:
|
|
|
|
.. code-block:: cpp
|
|
|
|
m.def("f", [](int a, int b) { /* ... */ },
|
|
py::arg("a"), py::kw_only(), py::arg("b"));
|
|
|
|
Note that you currently cannot combine this with a ``py::args`` argument. This
|
|
feature does *not* require Python 3 to work.
|
|
|
|
.. versionadded:: 2.6
|
|
|
|
Positional-only arguments
|
|
=========================
|
|
|
|
Python 3.8 introduced a new positional-only argument syntax, using ``/`` in the
|
|
function definition (note that this has been a convention for CPython
|
|
positional arguments, such as in ``pow()``, since Python 2). You can
|
|
do the same thing in any version of Python using ``py::pos_only()``:
|
|
|
|
.. code-block:: cpp
|
|
|
|
m.def("f", [](int a, int b) { /* ... */ },
|
|
py::arg("a"), py::pos_only(), py::arg("b"));
|
|
|
|
You now cannot give argument ``a`` by keyword. This can be combined with
|
|
keyword-only arguments, as well.
|
|
|
|
.. versionadded:: 2.6
|
|
|
|
.. _nonconverting_arguments:
|
|
|
|
Non-converting arguments
|
|
========================
|
|
|
|
Certain argument types may support conversion from one type to another. Some
|
|
examples of conversions are:
|
|
|
|
* :ref:`implicit_conversions` declared using ``py::implicitly_convertible<A,B>()``
|
|
* Calling a method accepting a double with an integer argument
|
|
* Calling a ``std::complex<float>`` argument with a non-complex python type
|
|
(for example, with a float). (Requires the optional ``pybind11/complex.h``
|
|
header).
|
|
* Calling a function taking an Eigen matrix reference with a numpy array of the
|
|
wrong type or of an incompatible data layout. (Requires the optional
|
|
``pybind11/eigen.h`` header).
|
|
|
|
This behaviour is sometimes undesirable: the binding code may prefer to raise
|
|
an error rather than convert the argument. This behaviour can be obtained
|
|
through ``py::arg`` by calling the ``.noconvert()`` method of the ``py::arg``
|
|
object, such as:
|
|
|
|
.. code-block:: cpp
|
|
|
|
m.def("floats_only", [](double f) { return 0.5 * f; }, py::arg("f").noconvert());
|
|
m.def("floats_preferred", [](double f) { return 0.5 * f; }, py::arg("f"));
|
|
|
|
Attempting the call the second function (the one without ``.noconvert()``) with
|
|
an integer will succeed, but attempting to call the ``.noconvert()`` version
|
|
will fail with a ``TypeError``:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> floats_preferred(4)
|
|
2.0
|
|
>>> floats_only(4)
|
|
Traceback (most recent call last):
|
|
File "<stdin>", line 1, in <module>
|
|
TypeError: floats_only(): incompatible function arguments. The following argument types are supported:
|
|
1. (f: float) -> float
|
|
|
|
Invoked with: 4
|
|
|
|
You may, of course, combine this with the :var:`_a` shorthand notation (see
|
|
:ref:`keyword_args`) and/or :ref:`default_args`. It is also permitted to omit
|
|
the argument name by using the ``py::arg()`` constructor without an argument
|
|
name, i.e. by specifying ``py::arg().noconvert()``.
|
|
|
|
.. note::
|
|
|
|
When specifying ``py::arg`` options it is necessary to provide the same
|
|
number of options as the bound function has arguments. Thus if you want to
|
|
enable no-convert behaviour for just one of several arguments, you will
|
|
need to specify a ``py::arg()`` annotation for each argument with the
|
|
no-convert argument modified to ``py::arg().noconvert()``.
|
|
|
|
.. _none_arguments:
|
|
|
|
Allow/Prohibiting None arguments
|
|
================================
|
|
|
|
When a C++ type registered with :class:`py::class_` is passed as an argument to
|
|
a function taking the instance as pointer or shared holder (e.g. ``shared_ptr``
|
|
or a custom, copyable holder as described in :ref:`smart_pointers`), pybind
|
|
allows ``None`` to be passed from Python which results in calling the C++
|
|
function with ``nullptr`` (or an empty holder) for the argument.
|
|
|
|
To explicitly enable or disable this behaviour, using the
|
|
``.none`` method of the :class:`py::arg` object:
|
|
|
|
.. code-block:: cpp
|
|
|
|
py::class_<Dog>(m, "Dog").def(py::init<>());
|
|
py::class_<Cat>(m, "Cat").def(py::init<>());
|
|
m.def("bark", [](Dog *dog) -> std::string {
|
|
if (dog) return "woof!"; /* Called with a Dog instance */
|
|
else return "(no dog)"; /* Called with None, dog == nullptr */
|
|
}, py::arg("dog").none(true));
|
|
m.def("meow", [](Cat *cat) -> std::string {
|
|
// Can't be called with None argument
|
|
return "meow";
|
|
}, py::arg("cat").none(false));
|
|
|
|
With the above, the Python call ``bark(None)`` will return the string ``"(no
|
|
dog)"``, while attempting to call ``meow(None)`` will raise a ``TypeError``:
|
|
|
|
.. code-block:: pycon
|
|
|
|
>>> from animals import Dog, Cat, bark, meow
|
|
>>> bark(Dog())
|
|
'woof!'
|
|
>>> meow(Cat())
|
|
'meow'
|
|
>>> bark(None)
|
|
'(no dog)'
|
|
>>> meow(None)
|
|
Traceback (most recent call last):
|
|
File "<stdin>", line 1, in <module>
|
|
TypeError: meow(): incompatible function arguments. The following argument types are supported:
|
|
1. (cat: animals.Cat) -> str
|
|
|
|
Invoked with: None
|
|
|
|
The default behaviour when the tag is unspecified is to allow ``None``.
|
|
|
|
.. note::
|
|
|
|
Even when ``.none(true)`` is specified for an argument, ``None`` will be converted to a
|
|
``nullptr`` *only* for custom and :ref:`opaque <opaque>` types. Pointers to built-in types
|
|
(``double *``, ``int *``, ...) and STL types (``std::vector<T> *``, ...; if ``pybind11/stl.h``
|
|
is included) are copied when converted to C++ (see :doc:`/advanced/cast/overview`) and will
|
|
not allow ``None`` as argument. To pass optional argument of these copied types consider
|
|
using ``std::optional<T>``
|
|
|
|
Overload resolution order
|
|
=========================
|
|
|
|
When a function or method with multiple overloads is called from Python,
|
|
pybind11 determines which overload to call in two passes. The first pass
|
|
attempts to call each overload without allowing argument conversion (as if
|
|
every argument had been specified as ``py::arg().noconvert()`` as described
|
|
above).
|
|
|
|
If no overload succeeds in the no-conversion first pass, a second pass is
|
|
attempted in which argument conversion is allowed (except where prohibited via
|
|
an explicit ``py::arg().noconvert()`` attribute in the function definition).
|
|
|
|
If the second pass also fails a ``TypeError`` is raised.
|
|
|
|
Within each pass, overloads are tried in the order they were registered with
|
|
pybind11.
|
|
|
|
What this means in practice is that pybind11 will prefer any overload that does
|
|
not require conversion of arguments to an overload that does, but otherwise prefers
|
|
earlier-defined overloads to later-defined ones.
|
|
|
|
.. note::
|
|
|
|
pybind11 does *not* further prioritize based on the number/pattern of
|
|
overloaded arguments. That is, pybind11 does not prioritize a function
|
|
requiring one conversion over one requiring three, but only prioritizes
|
|
overloads requiring no conversion at all to overloads that require
|
|
conversion of at least one argument.
|