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| """Utilities for union (sum type) disambiguation.""" | |
| from collections import OrderedDict | |
| from functools import reduce | |
| from operator import or_ | |
| from typing import ( # noqa: F401, imported for Mypy. | |
| Callable, | |
| Dict, | |
| Mapping, | |
| Optional, | |
| Type, | |
| ) | |
| from attr import fields, NOTHING | |
| from cattr._compat import get_origin | |
| def create_uniq_field_dis_func(*classes: Type) -> Callable: | |
| """Given attr classes, generate a disambiguation function. | |
| The function is based on unique fields.""" | |
| if len(classes) < 2: | |
| raise ValueError("At least two classes required.") | |
| cls_and_attrs = [ | |
| (cl, set(at.name for at in fields(get_origin(cl) or cl))) | |
| for cl in classes | |
| ] | |
| if len([attrs for _, attrs in cls_and_attrs if len(attrs) == 0]) > 1: | |
| raise ValueError("At least two classes have no attributes.") | |
| # TODO: Deal with a single class having no required attrs. | |
| # For each class, attempt to generate a single unique required field. | |
| uniq_attrs_dict = OrderedDict() # type: Dict[str, Type] | |
| cls_and_attrs.sort(key=lambda c_a: -len(c_a[1])) | |
| fallback = None # If none match, try this. | |
| for i, (cl, cl_reqs) in enumerate(cls_and_attrs): | |
| other_classes = cls_and_attrs[i + 1 :] | |
| if other_classes: | |
| other_reqs = reduce(or_, (c_a[1] for c_a in other_classes)) | |
| uniq = cl_reqs - other_reqs | |
| if not uniq: | |
| m = "{} has no usable unique attributes.".format(cl) | |
| raise ValueError(m) | |
| # We need a unique attribute with no default. | |
| cl_fields = fields(get_origin(cl) or cl) | |
| for attr_name in uniq: | |
| if getattr(cl_fields, attr_name).default is NOTHING: | |
| break | |
| else: | |
| raise ValueError(f"{cl} has no usable non-default attributes.") | |
| uniq_attrs_dict[attr_name] = cl | |
| else: | |
| fallback = cl | |
| def dis_func(data): | |
| # type: (Mapping) -> Optional[Type] | |
| if not isinstance(data, Mapping): | |
| raise ValueError("Only input mappings are supported.") | |
| for k, v in uniq_attrs_dict.items(): | |
| if k in data: | |
| return v | |
| return fallback | |
| return dis_func | |