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Diffstat (limited to '3rdparty/pybind11/docs/advanced/pycpp/object.rst')
| -rw-r--r-- | 3rdparty/pybind11/docs/advanced/pycpp/object.rst | 95 |
1 files changed, 88 insertions, 7 deletions
diff --git a/3rdparty/pybind11/docs/advanced/pycpp/object.rst b/3rdparty/pybind11/docs/advanced/pycpp/object.rst index 117131ed..6c7525ce 100644 --- a/3rdparty/pybind11/docs/advanced/pycpp/object.rst +++ b/3rdparty/pybind11/docs/advanced/pycpp/object.rst @@ -1,6 +1,8 @@ Python types ############ +.. _wrappers: + Available wrappers ================== @@ -13,6 +15,13 @@ Available types include :class:`handle`, :class:`object`, :class:`bool_`, :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: + Casting back and forth ====================== @@ -47,20 +56,21 @@ 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"); + py::object Decimal = py::module_::import("decimal").attr("Decimal"); .. code-block:: cpp // Try to import scipy - py::object scipy = py::module::import("scipy"); + py::object scipy = py::module_::import("scipy"); return scipy.attr("__version__"); + .. _calling_python_functions: Calling Python functions ======================== -It is also possible to call Python classes, functions and methods +It is also possible to call Python classes, functions and methods via ``operator()``. .. code-block:: cpp @@ -71,11 +81,11 @@ via ``operator()``. .. code-block:: cpp // Use Python to make our directories - py::object os = py::module::import("os"); + 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 +One can convert the result obtained from Python to a pure C++ version if a ``py::class_`` or type conversion is defined. .. code-block:: cpp @@ -99,8 +109,8 @@ Python method. 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`` +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 @@ -168,3 +178,74 @@ Generalized unpacking according to PEP448_ is also supported: 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). |