Convert some std error types to PyErr

[messense]

Address #21

[fafhrd91]

This is highly unsafe, if you call this conversions outside of GIL, python would crash. In general case ToPyObject trait does py memory allocation, which is require Gil

[fafhrd91]

This should be more precise, check utils.rs from async-tokio

[fafhrd91]

Maybe we can change PyResult type and include unconverted errors. Then we can use custom form of From trait in callback handler when we have python object available. Also we can use unstable try_traiy for implementing ?

[fafhrd91]

Another option is to modify PyErr and just store errors unconverted. And convert them inside PyErr::restore()

[messense]

I known it's unsafe, but without it you would more likely to just unwrap() to workaround it which it not good either.

I'd like to look into other options you mentioned to make it safer.

[messense]

I added a ToPyErr trait just like that in async-tokio.

The ideal way would be

fnmy_py_fn(py:Python,path:&str) -> PyResult<String>{let f = File::open(path)?;
...}

But that's a little complicated to implement now. With this PR we can do:

use pyo3::ToPyErr;fnmy_py_fn(py:Python,path:&str) -> PyResult<String>{let f = File::open(path).map_err(|e| e.to_pyerr(py))?;
...}

which should be good enough for most cases.

[fafhrd91]

did you look into this feature? rust-lang/rust#42275

[fafhrd91]

Lgtm. Could you also update async-tokio