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# 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 <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();
    });

Warning

The implementation in `pybind11/iostream.h` is NOT thread safe. Multiple threads writing to a redirected ostream concurrently cause data races and potentially buffer overflows. Therefore it is currently a requirement that all (possibly) concurrent redirected ostream writes are protected by a mutex. #HelpAppreciated: Work on iostream.h thread safety. For more background see the discussions under PR #2982 and PR #2995.

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 <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() <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>`.