| # Licensed to the Apache Software Foundation (ASF) under one | |
| # or more contributor license agreements. See the NOTICE file | |
| # distributed with this work for additional information | |
| # regarding copyright ownership. The ASF licenses this file | |
| # to you under the Apache License, Version 2.0 (the | |
| # "License"); you may not use this file except in compliance | |
| # with the License. You may obtain a copy of the License at | |
| # | |
| # http://www.apache.org/licenses/LICENSE-2.0 | |
| # | |
| # Unless required by applicable law or agreed to in writing, | |
| # software distributed under the License is distributed on an | |
| # "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY | |
| # KIND, either express or implied. See the License for the | |
| # specific language governing permissions and limitations | |
| # under the License. | |
| from typing import ( | |
| Dict, | |
| Iterator, | |
| Mapping, | |
| Optional, | |
| Sequence, | |
| TypeVar, | |
| Union, | |
| cast, | |
| ) | |
| K = TypeVar("K") | |
| V = TypeVar("V") | |
| class LazyDict(Mapping[K, V]): | |
| """Lazily build a dictionary from an array of items.""" | |
| __slots__ = ("_contents", "_dict") | |
| # Since Python's type system is not powerful enough to express the type of the | |
| # contents of the dictionary, we use specify the type as a sequence of either K or V | |
| # values. | |
| # | |
| # Rather than spending the runtime cost of checking the type of each item, we presume | |
| # that the developer has correctly used the class and that the contents are valid. | |
| def __init__(self, contents: Sequence[Sequence[Union[K, V]]]): | |
| self._contents = contents | |
| self._dict: Optional[Dict[K, V]] = None | |
| def _build_dict(self) -> Dict[K, V]: | |
| self._dict = {} | |
| for item in self._contents: | |
| self._dict.update(dict(zip(cast(Sequence[K], item[::2]), cast(Sequence[V], item[1::2])))) | |
| return self._dict | |
| def __getitem__(self, key: K, /) -> V: | |
| """Return the value for the given key.""" | |
| source = self._dict or self._build_dict() | |
| return source[key] | |
| def __iter__(self) -> Iterator[K]: | |
| """Return an iterator over the keys of the dictionary.""" | |
| source = self._dict or self._build_dict() | |
| return iter(source) | |
| def __len__(self) -> int: | |
| """Return the number of items in the dictionary.""" | |
| source = self._dict or self._build_dict() | |
| return len(source) | |
| def __dict__(self) -> Dict[K, V]: # type: ignore | |
| """Convert the lazy dict in a dict.""" | |
| return self._dict or self._build_dict() | |