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77 lines
2.5 KiB
ReStructuredText
77 lines
2.5 KiB
ReStructuredText
Utilities
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#########
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Using Python's print function in C++
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====================================
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The usual way to write output in C++ is using ``std::cout`` while in Python one
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would use ``print``. Since these methods use different buffers, mixing them can
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lead to output order issues. To resolve this, pybind11 modules can use the
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:func:`py::print` function which writes to Python's ``sys.stdout`` for consistency.
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Python's ``print`` function is replicated in the C++ API including optional
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keyword arguments ``sep``, ``end``, ``file``, ``flush``. Everything works as
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expected in Python:
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.. code-block:: cpp
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py::print(1, 2.0, "three"); // 1 2.0 three
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py::print(1, 2.0, "three", "sep"_a="-"); // 1-2.0-three
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auto args = py::make_tuple("unpacked", true);
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py::print("->", *args, "end"_a="<-"); // -> unpacked True <-
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Evaluating Python expressions from strings and files
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====================================================
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pybind11 provides the `eval`, `exec` and `eval_file` functions to evaluate
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Python expressions and statements. The following example illustrates how they
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can be used.
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.. code-block:: cpp
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// At beginning of file
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#include <pybind11/eval.h>
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...
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// Evaluate in scope of main module
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py::object scope = py::module::import("__main__").attr("__dict__");
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// Evaluate an isolated expression
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int result = py::eval("my_variable + 10", scope).cast<int>();
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// Evaluate a sequence of statements
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py::exec(
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"print('Hello')\n"
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"print('world!');",
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scope);
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// Evaluate the statements in an separate Python file on disk
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py::eval_file("script.py", scope);
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C++11 raw string literals are also supported and quite handy for this purpose.
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The only requirement is that the first statement must be on a new line following
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the raw string delimiter ``R"(``, ensuring all lines have common leading indent:
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.. code-block:: cpp
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py::exec(R"(
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x = get_answer()
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if x == 42:
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print('Hello World!')
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else:
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print('Bye!')
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)", scope
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);
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.. note::
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`eval` and `eval_file` accept a template parameter that describes how the
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string/file should be interpreted. Possible choices include ``eval_expr``
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(isolated expression), ``eval_single_statement`` (a single statement, return
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value is always ``none``), and ``eval_statements`` (sequence of statements,
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return value is always ``none``). `eval` defaults to ``eval_expr``,
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`eval_file` defaults to ``eval_statements`` and `exec` is just a shortcut
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for ``eval<eval_statements>``.
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