The above config in mypy linter config causes python_executable to be hard coded to whatever the first executable in path. From mypy's website

When options conflict, the precedence order for configuration is:

1. Inline configuration in the source file
2. Sections with concrete module names (foo.bar)
3. Sections with “unstructured” wildcard patterns (foo.*.baz), with sections later in the configuration file overriding sections earlier.
4. Sections with “well-structured” wildcard patterns (foo.bar.*), with more specific overriding more general.
5. Command line options.
6. Top-level configuration file options.

While other hardcoded config are required for it to play nicely with nvim's diagnostics, python_executable is a user preferred setting. It doesn't have to be in the path and can be anywhere in the user system. It should be left upto the user to configure it using config files.