Add file

[FlaveFlav20]

I created a file-like type for PyO3. Since file is a fundamental type in Python, I believe it should be supported by this library.

Example:

use pyo3::types::pyo3file::Pyo3File;

#[pyfunction]
fn my_custom_file(mut file: Pyo3File) -> PyResult<String> {
    let mut contents = String::new();

    file.getfile().read_to_string(&mut contents)?;

    Ok(contents)
}

I created a pyo3 type because the Python file uses three parameters that are not directly available in Rust::

• name
• opening mode
• encoding (we can't know the file encoding with rust file type)

I opened this pull request to check if this approach is acceptable.

[davidhewitt]

Thanks for the PR, and sorry for the slow review.

While I'm aware that IO type hierarchy is not mapped out by PyO3 at all, I have a number of concerns about the way this PR is designed which leave me unconvinced it's the way to move forward.

Without this PR, users can already do things like PyModule::import("io") to interact with the IO types at runtime, and anything we'd do in PyO3 would just be a convenience layer which centralises maintenance burden. I'm not sure we have a solid enough proposal to consider taking that on yet.

If there is a particular use case you were trying to solve, perhaps start from there, and we can help you that way?

[davidhewitt]

This typedef doesn't exist in https://github.com/python/cpython/blob/main/Include/cpython/fileobject.h, we should not invent this.

[davidhewitt]

This would allow any object to be cast to PyFile, which is definitely wrong. I don't think there's a good way to typecheck file objects.

[davidhewitt]

There is no such thing as a file object in Python 3:

>>> f = open('foo.txt', 'w')
>>> f
<_io.TextIOWrapper name='foo.txt' mode='w' encoding='UTF-8'>

... we can't expose this as a native type like this. There might be something we can do with the io module, however that module doesn't have any C API to interact with it and we are typically very careful about adding support for anything which doesn't have a C API.

[davidhewitt]

According to the docs for https://docs.python.org/3/c-api/file.html#c.PyFile_FromFd, name is ignored on all our supported Python versions (3.2+).

[davidhewitt]

Needs updating.