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Add py::object casting example to embedding docs (#2466)

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* Add py::object casting example to embedding docs

* Move implicit cast example to object.rst

* Move to bottom and improve implicit casting text

* Fix xref

* Improve wording as per @bstaletic's suggestion
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Holger Kohr 2020-09-09 16:39:20 +02:00 committed by GitHub
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docs/advanced/pycpp/object.rst
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@ -20,6 +20,8 @@ Available types include :class:`handle`, :class:`object`, :class:`bool_`,
Be sure to review the :ref:`pytypes_gotchas` before using this heavily in
your C++ API.
.. _casting_back_and_forth:
Casting back and forth
======================
@ -62,6 +64,7 @@ This example obtains a reference to the Python ``Decimal`` class.
py::object scipy = py::module::import("scipy");
return scipy.attr("__version__");
.. _calling_python_functions:
Calling Python functions
@ -176,6 +179,47 @@ Generalized unpacking according to PEP448_ is also supported:
.. _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
===================