Coverage for python/lsst/utils/classes.py: 60%
39 statements
« prev ^ index » next coverage.py v7.2.7, created at 2023-07-08 09:53 +0000
« prev ^ index » next coverage.py v7.2.7, created at 2023-07-08 09:53 +0000
1# This file is part of utils.
2#
3# Developed for the LSST Data Management System.
4# This product includes software developed by the LSST Project
5# (https://www.lsst.org).
6# See the COPYRIGHT file at the top-level directory of this distribution
7# for details of code ownership.
8#
9# Use of this source code is governed by a 3-clause BSD-style
10# license that can be found in the LICENSE file.
11#
13"""Utilities to help with class creation.
14"""
16from __future__ import annotations
18__all__ = ["Singleton", "cached_getter", "immutable"]
20import functools
21from collections.abc import Callable
22from typing import Any, Type, TypeVar
25class Singleton(type):
26 """Metaclass to convert a class to a Singleton.
28 If this metaclass is used the constructor for the singleton class must
29 take no arguments. This is because a singleton class will only accept
30 the arguments the first time an instance is instantiated.
31 Therefore since you do not know if the constructor has been called yet it
32 is safer to always call it with no arguments and then call a method to
33 adjust state of the singleton.
34 """
36 _instances: dict[type, Any] = {}
38 # Signature is intentionally not substitutable for type.__call__ (no *args,
39 # **kwargs) to require classes that use this metaclass to have no
40 # constructor arguments.
41 def __call__(cls) -> Any:
42 if cls not in cls._instances:
43 cls._instances[cls] = super().__call__()
44 return cls._instances[cls]
47_T = TypeVar("_T", bound="Type")
50def immutable(cls: _T) -> _T:
51 """Decorate a class to simulate a simple form of immutability.
53 A class decorated as `immutable` may only set each of its attributes once;
54 any attempts to set an already-set attribute will raise `AttributeError`.
56 Notes
57 -----
58 Subclasses of classes marked with ``@immutable`` are also immutable.
60 Because this behavior interferes with the default implementation for the
61 ``pickle`` modules, `immutable` provides implementations of
62 ``__getstate__`` and ``__setstate__`` that override this behavior.
63 Immutable classes can then implement pickle via ``__reduce__`` or
64 ``__getnewargs__``.
66 Following the example of Python's built-in immutable types, such as `str`
67 and `tuple`, the `immutable` decorator provides a ``__copy__``
68 implementation that just returns ``self``, because there is no reason to
69 actually copy an object if none of its shared owners can modify it.
71 Similarly, objects that are recursively (i.e. are themselves immutable and
72 have only recursively immutable attributes) should also reimplement
73 ``__deepcopy__`` to return ``self``. This is not done by the decorator, as
74 it has no way of checking for recursive immutability.
75 """
77 def __setattr__(self: _T, name: str, value: Any) -> None: # noqa: N807
78 if hasattr(self, name):
79 raise AttributeError(f"{cls.__name__} instances are immutable.")
80 object.__setattr__(self, name, value)
82 # mypy says the variable here has signature (str, Any) i.e. no "self";
83 # I think it's just confused by descriptor stuff.
84 cls.__setattr__ = __setattr__ # type: ignore
86 def __getstate__(self: _T) -> dict: # noqa: N807
87 # Disable default state-setting when unpickled.
88 return {}
90 cls.__getstate__ = __getstate__
92 def __setstate__(self: _T, state: Any) -> None: # noqa: N807
93 # Disable default state-setting when copied.
94 # Sadly what works for pickle doesn't work for copy.
95 assert not state
97 cls.__setstate__ = __setstate__
99 def __copy__(self: _T) -> _T: # noqa: N807
100 return self
102 cls.__copy__ = __copy__
103 return cls
106_S = TypeVar("_S")
107_R = TypeVar("_R")
110def cached_getter(func: Callable[[_S], _R]) -> Callable[[_S], _R]:
111 """Decorate a method to cache the result.
113 Only works on methods that take only ``self``
114 as an argument, and returns the cached result on subsequent calls.
116 Notes
117 -----
118 This is intended primarily as a stopgap for Python 3.8's more sophisticated
119 ``functools.cached_property``, but it is also explicitly compatible with
120 the `immutable` decorator, which may not be true of ``cached_property``.
122 `cached_getter` guarantees that the cached value will be stored in
123 an attribute named ``_cached_{name-of-decorated-function}``. Classes that
124 use `cached_getter` are responsible for guaranteeing that this name is not
125 otherwise used, and is included if ``__slots__`` is defined.
126 """
127 attribute = f"_cached_{func.__name__}"
129 @functools.wraps(func)
130 def inner(self: _S) -> _R:
131 if not hasattr(self, attribute):
132 object.__setattr__(self, attribute, func(self))
133 return getattr(self, attribute)
135 return inner