human
I wrote the below code

    class ManyRelatedManager(superclass, ):
        def __init__(self, model=None, query_field_name=None, instance=None, symmetrical=None, source_field_name=None, target_field_name=None, reverse=False, through=None, prefetch_cache_name=None):
            super(ManyRelatedManager, self).__init__()
            self.model = model
            self.query_field_name = query_field_name
            source_field = through._meta.get_field(source_field_name)
            source_related_fields = source_field.related_fields
            self.core_filters = {}
            for (lh_field, rh_field) in source_related_fields:
                self.core_filters[('%s__%s' % (query_field_name, rh_field.name))] = getattr(instance, rh_field.attname)
            self.instance = instance
            self.symmetrical = symmetrical
            self.source_field = source_field
            self.source_field_name = source_field_name
            self.target_field_name = target_field_name
            self.reverse = reverse
            self.through = through
            self.prefetch_cache_name = prefetch_cache_name
            self.related_val = source_field.get_foreign_related_value(instance)
            self._fk_val = self.related_val[0]
            if (None in self.related_val):
                raise ValueError(('"%r"    needs    to    have    a    value    for    field    "%s"    before    this    many-to-many    relationship    can    be    used.' % (instance, source_field_name)))
            if (instance.pk is None):
                raise ValueError(('%r    instance    needs    to    have    a    primary    key    value    before    a    many-to-many    relationship    can    be    used.' % instance.__class__.__name__))
        def _get_fk_val(self, obj, field_name):
            "\n                                                Returns    the    correct    value    for    this    relationship's    foreign    key.    This\n                                                might    be    something    else    than    pk    value    when   ...

What's a good function header?
 ... Here's a good function header?
assistant
#Here's a good function header.

def create_many_related_manager(superclass, rel):