Module import fails when creating and importing from Rust

[flickpp]

This is probably just a documentation issue, I can't find the answer on pyo3.rs.

This code:

use pyo3::prelude::*;

fn main() {
    Python::with_gil(|py| -> PyResult<()> {
        let mod1 = PyModule::new(py, "my_module")?;
        let mod2 = PyModule::import(py, "my_module")?;

        Ok(())
    })
    .expect("didn't import module");
}

produces

thread 'main' panicked at 'didn't import module: PyErr { type: <class 'ModuleNotFoundError'>, value: ModuleNotFoundError("No module named 'my_module'"), traceback: None }', src/main.rs:10:6

what I expected to happen
mod2 should be a reference to the python module.

what I'm trying to do
I'm trying to call Python code from inside Rust, which itself calls back into Rust.
Roughly I'm trying to do this.

use pyo3::prelude::*;

fn main() {
    Python::with_gil(|py| -> PyResult<()> {
        let mod1 = PyModule::new(py, "my_module")?;
        py.run("import my_module", None, None)?;

        Ok(())
    })
    .expect("didn't import module");
}

Suggestion
On the "calling Python from rust" section of pyo3.rs, add either an example or some docs on how/why this won't work.

[birkenfeld]

Creating a new module does not make it known to the import machinery. You'll also have to add it to the module registry (sys.modules) since import will not find it in the usual way (looking for files on sys.path).

[flickpp]

Thanks! Works as expected.

I get this is obvious to people who know Python internals, but is it worth adding something to the docs?

[mejrs]

I agree.

It's in the maturin starter example (https://github.com/PyO3/pyo3/blob/main/examples/maturin-starter/src/lib.rs#L30), but I wouldn't expect someone to sift through the examples. It'd be nice to mention it somewhere on the pymodule page

[flickpp]

If this done come to pass - I'd like to volunteer myself.

[mejrs]

If you're willing to write it up that would be appreciated :)

[birkenfeld]

Looks like there is a valid issue here, sorry for converting it prematurely.