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 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:

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 <-

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:

#include <pybind11/iostream.h>

...

// 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:

// Alternative: Call single function using call guard
m.def("noisy_func", &call_noisy_function,
      py::call_guard<py::scoped_ostream_redirect,
                     py::scoped_estream_redirect>());

The redirection can also be done in Python with the addition of a context manager, using the py::add_ostream_redirect() function:

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:

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.

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.

// 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:

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>.