From 444e535f000fd7b53dadf6726d5cd29ac34cc75f Mon Sep 17 00:00:00 2001 From: Miodrag Milanovic Date: Thu, 23 Jul 2020 08:58:19 +0200 Subject: Add pybind11 2.5 source --- .../pybind11/docs/advanced/pycpp/utilities.rst | 144 +++++++++++++++++++++ 1 file changed, 144 insertions(+) create mode 100644 3rdparty/pybind11/docs/advanced/pycpp/utilities.rst (limited to '3rdparty/pybind11/docs/advanced/pycpp/utilities.rst') diff --git a/3rdparty/pybind11/docs/advanced/pycpp/utilities.rst b/3rdparty/pybind11/docs/advanced/pycpp/utilities.rst new file mode 100644 index 00000000..369e7c94 --- /dev/null +++ b/3rdparty/pybind11/docs/advanced/pycpp/utilities.rst @@ -0,0 +1,144 @@ +Utilities +######### + +Using Python's print function in C++ +==================================== + +The usual way to write output in C++ is using ``std::cout`` while in Python one +would use ``print``. Since these methods use different buffers, mixing them can +lead to output order issues. To resolve this, pybind11 modules can use the +:func:`py::print` function which writes to Python's ``sys.stdout`` for consistency. + +Python's ``print`` function is replicated in the C++ API including optional +keyword arguments ``sep``, ``end``, ``file``, ``flush``. Everything works as +expected in Python: + +.. code-block:: cpp + + py::print(1, 2.0, "three"); // 1 2.0 three + py::print(1, 2.0, "three", "sep"_a="-"); // 1-2.0-three + + auto args = py::make_tuple("unpacked", true); + py::print("->", *args, "end"_a="<-"); // -> unpacked True <- + +.. _ostream_redirect: + +Capturing standard output from ostream +====================================== + +Often, a library will use the streams ``std::cout`` and ``std::cerr`` to print, +but this does not play well with Python's standard ``sys.stdout`` and ``sys.stderr`` +redirection. Replacing a library's printing with `py::print ` may not +be feasible. This can be fixed using a guard around the library function that +redirects output to the corresponding Python streams: + +.. code-block:: cpp + + #include + + ... + + // Add a scoped redirect for your noisy code + m.def("noisy_func", []() { + py::scoped_ostream_redirect stream( + std::cout, // std::ostream& + py::module::import("sys").attr("stdout") // Python output + ); + call_noisy_func(); + }); + +This method respects flushes on the output streams and will flush if needed +when the scoped guard is destroyed. This allows the output to be redirected in +real time, such as to a Jupyter notebook. The two arguments, the C++ stream and +the Python output, are optional, and default to standard output if not given. An +extra type, `py::scoped_estream_redirect `, is identical +except for defaulting to ``std::cerr`` and ``sys.stderr``; this can be useful with +`py::call_guard`, which allows multiple items, but uses the default constructor: + +.. code-block:: py + + // Alternative: Call single function using call guard + m.def("noisy_func", &call_noisy_function, + py::call_guard()); + +The redirection can also be done in Python with the addition of a context +manager, using the `py::add_ostream_redirect() ` function: + +.. code-block:: cpp + + py::add_ostream_redirect(m, "ostream_redirect"); + +The name in Python defaults to ``ostream_redirect`` if no name is passed. This +creates the following context manager in Python: + +.. code-block:: python + + with ostream_redirect(stdout=True, stderr=True): + noisy_function() + +It defaults to redirecting both streams, though you can use the keyword +arguments to disable one of the streams if needed. + +.. note:: + + The above methods will not redirect C-level output to file descriptors, such + as ``fprintf``. For those cases, you'll need to redirect the file + descriptors either directly in C or with Python's ``os.dup2`` function + in an operating-system dependent way. + +.. _eval: + +Evaluating Python expressions from strings and files +==================================================== + +pybind11 provides the `eval`, `exec` and `eval_file` functions to evaluate +Python expressions and statements. The following example illustrates how they +can be used. + +.. code-block:: cpp + + // At beginning of file + #include + + ... + + // Evaluate in scope of main module + py::object scope = py::module::import("__main__").attr("__dict__"); + + // Evaluate an isolated expression + int result = py::eval("my_variable + 10", scope).cast(); + + // Evaluate a sequence of statements + py::exec( + "print('Hello')\n" + "print('world!');", + scope); + + // Evaluate the statements in an separate Python file on disk + py::eval_file("script.py", scope); + +C++11 raw string literals are also supported and quite handy for this purpose. +The only requirement is that the first statement must be on a new line following +the raw string delimiter ``R"(``, ensuring all lines have common leading indent: + +.. code-block:: cpp + + py::exec(R"( + x = get_answer() + if x == 42: + print('Hello World!') + else: + print('Bye!') + )", scope + ); + +.. note:: + + `eval` and `eval_file` accept a template parameter that describes how the + string/file should be interpreted. Possible choices include ``eval_expr`` + (isolated expression), ``eval_single_statement`` (a single statement, return + value is always ``none``), and ``eval_statements`` (sequence of statements, + return value is always ``none``). `eval` defaults to ``eval_expr``, + `eval_file` defaults to ``eval_statements`` and `exec` is just a shortcut + for ``eval``. -- cgit v1.2.3