Manage Package Metadata

Warning: This project is very early alpha release, so all APIs could change with little or no notice.

Python packages have metadata describing information like the package name, version, etc. The details of this metadata are defined in the Packaging User Guide. When stored in a file (either on the filesystem, or in a package distribution) the metadata is saved in a format which is based on email headers.

This library transforms that metadata to and from a JSON-compatible form, as defined in PEP 566. The JSON form is easier to use in a programming context. Two functions are provided:

• msg_to_json(msg) - convert the email header format to JSON. The msg argument is the metadata in email format, as an email.message.Message object. Returns a dictionary following the layout in the "json" form.
• json_to_msg(json) - convert the JSON form back to email headers. The json argument is a dictionary following the "json" form. Returns a (Unicode) string with the message form.

Note the discrepancy between the two msg forms: a Message object and a string. This is something that may change, as it's a bit of an awkward discrepancy, but there are reasons for this approach:

1. When reading metadata, the file is supposed to be in the UTF-8 encoding, but historically this has not always been the case. By using a Message as the input, this can be constructed from either text or bytes (message_from_string or message_from_bytes) which allows the email package to handle encoding issues. If a project uses non-UTF8 metadata, it's likely that this approach will result in mojibake, but at least the data will be usable.
2. When writing metadata, using a Message object results in unwanted header fields, because the object assumes this is a "real" email, and not just data re-using that format. So it is more reliable to simply return the output in string format. It can then be written to a file (in UTF-8) as required.

An example of using the library:

with open(metadata_file, "r", encoding="utf-8") as f:
    msg = email.message_from_file(f)
metadata = pkg_metadata.msg_to_json(msg)
metadata["keywords"] = ["example", "artificial"]
with open(metadata_file, "w", encoding="utf-8") as f:
    f.write(json_to_msg(metadata))

In addition to the metadata file format, project metadata can also be specified in the pyproject.toml file, in TOML format as specified in PEP 621. This library provides a function to read the [project] section of pyproject.toml and convert it into a ("JSON format") metadata dictionary.

• pyproject_to_json(pyproject) - convert pyproject.toml metadata into a metadata dictionary. The pyproject argument is a dictionary representing the data in the [project] section of pyproject.toml.

Example:

with open("pyproject.toml", "rb") as f:
    pyproject_data = tomli.load(f)

metadata = pkg_metadata.pyproject_to_json(pyproject_data["project"])