mirror of
https://github.com/pybind/pybind11.git
synced 2024-11-25 14:45:12 +00:00
79 lines
2.5 KiB
ReStructuredText
79 lines
2.5 KiB
ReStructuredText
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 <-
|
|
|
|
.. _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 <pybind11/eval.h>
|
|
|
|
...
|
|
|
|
// 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<int>();
|
|
|
|
// 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<eval_statements>``.
|