Just thinking the first case might get quite a few false positives. Maybe that would still be acceptable, I dunno.

> - the str.encode method is called (redirect to codecs.encode to handle arbitrary input types in a forward compatible way)

I guess you are trying to catch cases like this, which I have come across quite a few times:

data.encode("hex")  # data is a byte string

But I think you would also catch cases that depend on Python 2 “str” objects automatically converting to Unicode. Here are some examples taken from real code:

file_name.encode("utf-8")  # File name parameter may be str or unicode

# Code meant to be compatible with both Python 2 and 3:
"""<?xml . . . encoding="iso-8859-1"?>""".encode("iso-8859-1")
("data %s\n" % len(...)).encode("ascii")