pybind11/docs/advanced/pycpp/object.rst

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Python types
############
.. _wrappers:
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Available wrappers
==================
All major Python types are available as thin C++ wrapper classes. These
can also be used as function parameters -- see :ref:`python_objects_as_args`.
Available types include :class:`handle`, :class:`object`, :class:`bool_`,
:class:`int_`, :class:`float_`, :class:`str`, :class:`bytes`, :class:`tuple`,
:class:`list`, :class:`dict`, :class:`slice`, :class:`none`, :class:`capsule`,
:class:`iterable`, :class:`iterator`, :class:`function`, :class:`buffer`,
:class:`array`, and :class:`array_t`.
.. warning::
Be sure to review the :ref:`pytypes_gotchas` before using this heavily in
your C++ API.
.. _casting_back_and_forth:
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Casting back and forth
======================
In this kind of mixed code, it is often necessary to convert arbitrary C++
types to Python, which can be done using :func:`py::cast`:
.. code-block:: cpp
MyClass *cls = ..;
py::object obj = py::cast(cls);
The reverse direction uses the following syntax:
.. code-block:: cpp
py::object obj = ...;
MyClass *cls = obj.cast<MyClass *>();
When conversion fails, both directions throw the exception :class:`cast_error`.
.. _python_libs:
Accessing Python libraries from C++
===================================
It is also possible to import objects defined in the Python standard
library or available in the current Python environment (``sys.path``) and work
with these in C++.
This example obtains a reference to the Python ``Decimal`` class.
.. code-block:: cpp
// Equivalent to "from decimal import Decimal"
py::object Decimal = py::module_::import("decimal").attr("Decimal");
.. code-block:: cpp
// Try to import scipy
py::object scipy = py::module_::import("scipy");
return scipy.attr("__version__");
.. _calling_python_functions:
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Calling Python functions
========================
It is also possible to call Python classes, functions and methods
via ``operator()``.
.. code-block:: cpp
// Construct a Python object of class Decimal
py::object pi = Decimal("3.14159");
.. code-block:: cpp
// Use Python to make our directories
py::object os = py::module_::import("os");
py::object makedirs = os.attr("makedirs");
makedirs("/tmp/path/to/somewhere");
One can convert the result obtained from Python to a pure C++ version
if a ``py::class_`` or type conversion is defined.
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.. code-block:: cpp
py::function f = <...>;
py::object result_py = f(1234, "hello", some_instance);
MyClass &result = result_py.cast<MyClass>();
.. _calling_python_methods:
Calling Python methods
========================
To call an object's method, one can again use ``.attr`` to obtain access to the
Python method.
.. code-block:: cpp
// Calculate e^π in decimal
py::object exp_pi = pi.attr("exp")();
py::print(py::str(exp_pi));
In the example above ``pi.attr("exp")`` is a *bound method*: it will always call
the method for that same instance of the class. Alternately one can create an
*unbound method* via the Python class (instead of instance) and pass the ``self``
object explicitly, followed by other arguments.
.. code-block:: cpp
py::object decimal_exp = Decimal.attr("exp");
// Compute the e^n for n=0..4
for (int n = 0; n < 5; n++) {
py::print(decimal_exp(Decimal(n));
}
Keyword arguments
=================
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Keyword arguments are also supported. In Python, there is the usual call syntax:
.. code-block:: python
def f(number, say, to):
... # function code
f(1234, say="hello", to=some_instance) # keyword call in Python
In C++, the same call can be made using:
.. code-block:: cpp
using namespace pybind11::literals; // to bring in the `_a` literal
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f(1234, "say"_a="hello", "to"_a=some_instance); // keyword call in C++
Unpacking arguments
===================
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Unpacking of ``*args`` and ``**kwargs`` is also possible and can be mixed with
other arguments:
.. code-block:: cpp
// * unpacking
py::tuple args = py::make_tuple(1234, "hello", some_instance);
f(*args);
// ** unpacking
py::dict kwargs = py::dict("number"_a=1234, "say"_a="hello", "to"_a=some_instance);
f(**kwargs);
// mixed keywords, * and ** unpacking
py::tuple args = py::make_tuple(1234);
py::dict kwargs = py::dict("to"_a=some_instance);
f(*args, "say"_a="hello", **kwargs);
Generalized unpacking according to PEP448_ is also supported:
.. code-block:: cpp
py::dict kwargs1 = py::dict("number"_a=1234);
py::dict kwargs2 = py::dict("to"_a=some_instance);
f(**kwargs1, "say"_a="hello", **kwargs2);
.. seealso::
The file :file:`tests/test_pytypes.cpp` contains a complete
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example that demonstrates passing native Python types in more detail. The
file :file:`tests/test_callbacks.cpp` presents a few examples of calling
Python functions from C++, including keywords arguments and unpacking.
.. _PEP448: https://www.python.org/dev/peps/pep-0448/
.. _implicit_casting:
Implicit casting
================
When using the C++ interface for Python types, or calling Python functions,
objects of type :class:`object` are returned. It is possible to invoke implicit
conversions to subclasses like :class:`dict`. The same holds for the proxy objects
returned by ``operator[]`` or ``obj.attr()``.
Casting to subtypes improves code readability and allows values to be passed to
C++ functions that require a specific subtype rather than a generic :class:`object`.
.. code-block:: cpp
#include <pybind11/numpy.h>
using namespace pybind11::literals;
py::module_ os = py::module_::import("os");
py::module_ path = py::module_::import("os.path"); // like 'import os.path as path'
py::module_ np = py::module_::import("numpy"); // like 'import numpy as np'
py::str curdir_abs = path.attr("abspath")(path.attr("curdir"));
py::print(py::str("Current directory: ") + curdir_abs);
py::dict environ = os.attr("environ");
py::print(environ["HOME"]);
py::array_t<float> arr = np.attr("ones")(3, "dtype"_a="float32");
py::print(py::repr(arr + py::int_(1)));
These implicit conversions are available for subclasses of :class:`object`; there
is no need to call ``obj.cast()`` explicitly as for custom classes, see
:ref:`casting_back_and_forth`.
.. note::
If a trivial conversion via move constructor is not possible, both implicit and
explicit casting (calling ``obj.cast()``) will attempt a "rich" conversion.
For instance, ``py::list env = os.attr("environ");`` will succeed and is
equivalent to the Python code ``env = list(os.environ)`` that produces a
list of the dict keys.
.. TODO: Adapt text once PR #2349 has landed
Handling exceptions
===================
Python exceptions from wrapper classes will be thrown as a ``py::error_already_set``.
See :ref:`Handling exceptions from Python in C++
<handling_python_exceptions_cpp>` for more information on handling exceptions
raised when calling C++ wrapper classes.
.. _pytypes_gotchas:
Gotchas
=======
Default-Constructed Wrappers
----------------------------
When a wrapper type is default-constructed, it is **not** a valid Python object (i.e. it is not ``py::none()``). It is simply the same as
``PyObject*`` null pointer. To check for this, use
``static_cast<bool>(my_wrapper)``.
Assigning py::none() to wrappers
--------------------------------
You may be tempted to use types like ``py::str`` and ``py::dict`` in C++
signatures (either pure C++, or in bound signatures), and assign them default
values of ``py::none()``. However, in a best case scenario, it will fail fast
because ``None`` is not convertible to that type (e.g. ``py::dict``), or in a
worse case scenario, it will silently work but corrupt the types you want to
work with (e.g. ``py::str(py::none())`` will yield ``"None"`` in Python).