in usort/config.py [0:0]
def category(self, dotted_import: str) -> Category:
"""
Given a piece of an import string, return its category for this config.
You can pass in ".foo" or "pkg.foo.bar" or just "os" and it should
categorize.
"""
first_part = dotted_import.split(".")[0]
if first_part == "":
# relative import
return CAT_FIRST_PARTY
elif first_part in self.known:
return self.known[first_part]
else:
return self.default_category