__only_fields is always none, even if input "values["__only_fields"]" has a value when initializing BaseDocument

I realized something was up when I assigned a value to values["__only_fields"], and nothing was happening. It was only realized when I stepped through the debugger that I saw what was happening

I am talking about this part:

https://github.com/MongoEngine/mongoengine/blob/master/mongoengine/base/document.py#L83

When I assign something to values["__only_fields"] I am still getting none for __only_fields. This is what I see in the console debugger:

code I modified to show what I am talking about:

__auto_convert = values.pop("__auto_convert", True)

g = values.pop("__only_fields", values)

__only_fields = set(g)

when I explore things in the console:

What's happening? When I do name mangling in the python interpreter, the output of set on the list is not None. I think that if you were intending this to be None, then just assign it none. Otherwise it blows my mind that the output of a set is None.

Inspect mongoengine/base/document.py around line 83 and reproduce initialization with values["__only_fields"] set. Compare the debugger result with the expected handling of __only_fields, then define the intended behavior and verify that the issue is resolved with a focused regression test.