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# 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.
"""
This is a singleton metaclass that can be used to cache and re-use existing objects.
In the Iceberg codebase we have a lot of objects that are stateless (for example Types such as StringType,
BooleanType etc). FixedTypes have arguments (eg. Fixed[22]) that we also make part of the key when caching
the newly created object.
The Singleton uses a metaclass which essentially defines a new type. When the Type gets created, it will first
evaluate the `__call__` method with all the arguments. If we already initialized a class earlier, we'll just
return it.
More information on metaclasses: https://docs.python.org/3/reference/datamodel.html#metaclasses
"""
from typing import Any, ClassVar, Dict
def _convert_to_hashable_type(element: Any) -> Any:
if isinstance(element, dict):
return tuple((_convert_to_hashable_type(k), _convert_to_hashable_type(v)) for k, v in element.items())
elif isinstance(element, list):
return tuple(map(_convert_to_hashable_type, element))
return element
class Singleton:
_instances: ClassVar[Dict] = {} # type: ignore
def __new__(cls, *args, **kwargs): # type: ignore
key = (cls, tuple(args), _convert_to_hashable_type(kwargs))
if key not in cls._instances:
cls._instances[key] = super().__new__(cls)
return cls._instances[key]
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