Notice

They are initialized by common_types and types_map.

The same is applicable for MimeTypes.types_map_inv. Try

import mimetypes

mimetypes.init()
mimetypes.types_map[".ae1"] = "application/pers.added"
db = mimetypes.MimeTypes()
print(db.types_map[True].get(".ae1"))

it reports

None

Accordingly to the docs I would expect application/pers.added. It can be achieved by commenting out the mimetypes.init() call.

I think, behavior changed when #49213 was fixed, but docs have not been updated.

I see some reason in the documented behavior. Perhaps mimetypes.init() should be more flexible and support both ways to set types_map: from the global dictionary and from the internal data, but likely it should be discussed on the pyhton-ideas mailing list first.

Linked PRs

• gh-115075: Clarify docs for types_map and types_map_inv on mimetypes.MimeTypes #136761