builtins not available by default in py::eval

[anntzer]

Issue description

builtins are not available by default when using py::eval, i.e. when using an empty dict as scope. Instead, it is necessary to pass e.g. a module's __dict__, which is actually semantically weird because a module's __dict__ does NOT, in fact, contain the builtins (as can be checked manually).

Instead, py::eval should just use the same semantics as the builtin eval, i.e. builtins.__dict__ should always be available in the execution scope, regardless of what gets passed in as globals and locals.

The same remark applies to py::exec, of course.

Reproducible example code

Starting from the python_example

#include <pybind11/pybind11.h>
#include <pybind11/eval.h>

namespace py = pybind11;

PYBIND11_MODULE(python_example, m) {
    // auto scope = py::module::import("__main__").attr("__dict__");  // this works
    auto scope = py::dict{};  // this crashes Py3.6 at import time with a SystemError
    py::eval("print(123)", scope);
}

build, and try to import python_example.

---

#887 appears to be a similar, but different issue.

[bstaletic]

This is essentially calling this thing. As the C API docs show, the py::dict must be a valid dict. A default constructed py::dict holds a NULL pointer.

[bstaletic]

That was a hasty comment. While the above works as expected on python 3.8, python 3.6 is still affected. py::dict{} also is a valid and empty python dictionary, again as expected.

[YannickJadoul]

Change introduced here: python/cpython#13362 (to CPython). I'm not sure pybind11 is to blame for this :-/

I guess we could add __builtins__ if it's not set, but ... pfff, if Python doesn't do it, should we?

[anntzer]

thanks :)

[YannickJadoul]

Thanks for reporting, @anntzer! :-)

[YannickJadoul]

(Oops, 3 years ago, but still)