Coverage for python/lsst/utils/classes.py: 59%

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# 

12 

13"""Utilities to help with class creation. 

14""" 

15 

16from __future__ import annotations 

17 

18__all__ = ["Singleton", "cached_getter", "immutable"] 

19 

20import functools 

21from typing import Any, Callable, Dict, Type, TypeVar 

22 

23 

24class Singleton(type): 

25 """Metaclass to convert a class to a Singleton. 

26 

27 If this metaclass is used the constructor for the singleton class must 

28 take no arguments. This is because a singleton class will only accept 

29 the arguments the first time an instance is instantiated. 

30 Therefore since you do not know if the constructor has been called yet it 

31 is safer to always call it with no arguments and then call a method to 

32 adjust state of the singleton. 

33 """ 

34 

35 _instances: Dict[Type, Any] = {} 

36 

37 # Signature is intentionally not substitutable for type.__call__ (no *args, 

38 # **kwargs) to require classes that use this metaclass to have no 

39 # constructor arguments. 

40 def __call__(cls) -> Any: # type: ignore 

41 if cls not in cls._instances: 

42 cls._instances[cls] = super(Singleton, cls).__call__() 

43 return cls._instances[cls] 

44 

45 

46_T = TypeVar("_T", bound="Type") 

47 

48 

49def immutable(cls: _T) -> _T: 

50 """Decorate a class to simulate a simple form of immutability. 

51 

52 A class decorated as `immutable` may only set each of its attributes once; 

53 any attempts to set an already-set attribute will raise `AttributeError`. 

54 

55 Notes 

56 ----- 

57 Subclasses of classes marked with ``@immutable`` are also immutable. 

58 

59 Because this behavior interferes with the default implementation for the 

60 ``pickle`` modules, `immutable` provides implementations of 

61 ``__getstate__`` and ``__setstate__`` that override this behavior. 

62 Immutable classes can then implement pickle via ``__reduce__`` or 

63 ``__getnewargs__``. 

64 

65 Following the example of Python's built-in immutable types, such as `str` 

66 and `tuple`, the `immutable` decorator provides a ``__copy__`` 

67 implementation that just returns ``self``, because there is no reason to 

68 actually copy an object if none of its shared owners can modify it. 

69 

70 Similarly, objects that are recursively (i.e. are themselves immutable and 

71 have only recursively immutable attributes) should also reimplement 

72 ``__deepcopy__`` to return ``self``. This is not done by the decorator, as 

73 it has no way of checking for recursive immutability. 

74 """ 

75 

76 def __setattr__(self: _T, name: str, value: Any) -> None: # noqa: N807 

77 if hasattr(self, name): 

78 raise AttributeError(f"{cls.__name__} instances are immutable.") 

79 object.__setattr__(self, name, value) 

80 

81 # mypy says the variable here has signature (str, Any) i.e. no "self"; 

82 # I think it's just confused by descriptor stuff. 

83 cls.__setattr__ = __setattr__ # type: ignore 

84 

85 def __getstate__(self: _T) -> dict: # noqa: N807 

86 # Disable default state-setting when unpickled. 

87 return {} 

88 

89 cls.__getstate__ = __getstate__ 

90 

91 def __setstate__(self: _T, state: Any) -> None: # noqa: N807 

92 # Disable default state-setting when copied. 

93 # Sadly what works for pickle doesn't work for copy. 

94 assert not state 

95 

96 cls.__setstate__ = __setstate__ 

97 

98 def __copy__(self: _T) -> _T: # noqa: N807 

99 return self 

100 

101 cls.__copy__ = __copy__ 

102 return cls 

103 

104 

105_S = TypeVar("_S") 

106_R = TypeVar("_R") 

107 

108 

109def cached_getter(func: Callable[[_S], _R]) -> Callable[[_S], _R]: 

110 """Decorate a method to cache the result. 

111 

112 Only works on methods that take only ``self`` 

113 as an argument, and returns the cached result on subsequent calls. 

114 

115 Notes 

116 ----- 

117 This is intended primarily as a stopgap for Python 3.8's more sophisticated 

118 ``functools.cached_property``, but it is also explicitly compatible with 

119 the `immutable` decorator, which may not be true of ``cached_property``. 

120 

121 `cached_getter` guarantees that the cached value will be stored in 

122 an attribute named ``_cached_{name-of-decorated-function}``. Classes that 

123 use `cached_getter` are responsible for guaranteeing that this name is not 

124 otherwise used, and is included if ``__slots__`` is defined. 

125 """ 

126 attribute = f"_cached_{func.__name__}" 

127 

128 @functools.wraps(func) 

129 def inner(self: _S) -> _R: 

130 if not hasattr(self, attribute): 

131 object.__setattr__(self, attribute, func(self)) 

132 return getattr(self, attribute) 

133 

134 return inner