ConfigItems does not work as expected and results in a list of items that contains only the same values.

Reproduciton

plugin.py

class TestPlugin(BasePlugin):
    config_scheme = (
        (
            "items", mkdocs.config.config_options.ConfigItems(
                ("value", mkdocs.config.config_options.Type(str)),
            ),
        ),
    )

    def on_config(self, config):
        print(self.config)

mkdocs.yml

site_name: Test Site

plugins:
    - test:
        items:
            - value: a
            - value: b

This results in:

INFO     -  Building documentation...
{'items': [{'value': 'b'}, {'value': 'b'}]}
INFO     -  Cleaning site directory
INFO     -  Documentation built in 0.05 seconds
INFO     -  [12:21:39] Watching paths for changes: 'docs', 'mkdocs.yml'
INFO     -  [12:21:39] Serving on http://127.0.0.1:8000/

Note that both "value" values are b rather than the first one being a.

Cause

ConfigItems creates a single SubConfig instance when it is instantiated:

It then uses this instance to do validation:

The SubConfig instance uses load_dict to populate self.data before validation.

The load_dict function uses update to make changes to self.data on the SubConfig instance:

Since there is only one SubConfig instance, when this dictionary gets updated it gets updated across all references to it.

Fix

A number of popular plugins already create their own ConfigItems to address this by making item_config a property that returns a separate SubConfig instance each time it is used. This seems like a reasonable approach.

https://github.com/pieterdavid/mkdocs-doxygen-plugin/blob/49b841b649acf58b668c0b805ab27f9e454920f4/mkdocsdoxygen/configitems.py#L24-L26