def category()

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