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1# ------------------------------------------------------------------------------ 

2# Copyright (C) 2019 Maximilian Stahlberg 

3# 

4# This file is part of PICOS. 

5# 

6# PICOS is free software: you can redistribute it and/or modify it under the 

7# terms of the GNU General Public License as published by the Free Software 

8# Foundation, either version 3 of the License, or (at your option) any later 

9# version. 

10# 

11# PICOS is distributed in the hope that it will be useful, but WITHOUT ANY 

12# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR 

13# A PARTICULAR PURPOSE. See the GNU General Public License for more details. 

14# 

15# You should have received a copy of the GNU General Public License along with 

16# this program. If not, see <http://www.gnu.org/licenses/>. 

17# ------------------------------------------------------------------------------ 

18 

19"""Caching helpers.""" 

20 

21import functools 

22from contextlib import contextmanager 

23 

24from .apidoc import api_end, api_start 

25 

26_API_START = api_start(globals()) 

27# ------------------------------- 

28 

29 

30#: The prefix used for storing cached values. 

31CACHED_PREFIX = "_cached_" 

32 

33#: An attribute name whose presence unlocks the setter of cached properties. 

34CACHED_PROP_UNLOCKED_TOKEN = "_CACHED_PROPERTIES_ARE_UNLOCKED" 

35 

36 

37@contextmanager 

38def unlocked_cached_properties(obj): 

39 """Unlock the setters of cached instance attributes. 

40 

41 Normally, cached attributes are read-only properties. When the user first 

42 reads them, the cache is populated with the value returned to the user, and 

43 successive reads will return the cached value. 

44 

45 The user is allowed to empty the cache by using ``del`` on the variable, but 

46 they may not assign a value to it. This context allows the programmer to 

47 manually populate the cache by assigning a value to the property. 

48 

49 :Example: 

50 

51 >>> from picos.caching import cached_property, unlocked_cached_properties 

52 >>> class A: 

53 ... @cached_property 

54 ... def p(self): 

55 ... return 1 

56 ... 

57 >>> a = A() 

58 >>> try: 

59 ... a.p = 2 

60 ... except AttributeError: 

61 ... print("Not possible.") 

62 ... 

63 Not possible. 

64 >>> with unlocked_cached_properties(a): 

65 ... a.p = 2 # Populate the cache of a.p. 

66 ... 

67 >>> a.p 

68 2 

69 """ 

70 assert not hasattr(obj, CACHED_PROP_UNLOCKED_TOKEN) 

71 setattr(obj, CACHED_PROP_UNLOCKED_TOKEN, None) 

72 

73 try: 

74 yield 

75 finally: 

76 delattr(obj, CACHED_PROP_UNLOCKED_TOKEN) 

77 

78 

79class cached_property(property): 

80 """A read-only property whose result is cached.""" 

81 

82 def __init__(self, fget=None, fset=None, fdel=None, doc=None): # noqa 

83 if fget is None: 

84 raise NotImplementedError("Unlike normal properties, cached " 

85 "properties must be initialized with a getter.") 

86 elif fget.__name__ == (lambda: None).__name__: 

87 raise NotImplementedError("Cached properties cannot be used with a " 

88 "lambda getter as lambdas do not have a unique name to identify" 

89 " the cached value.") 

90 

91 if fset is not None: 

92 raise AttributeError("Cannot have a custom setter on a cached " 

93 "property as __set__ is reserved for cache population.") 

94 

95 if fdel is not None: 

96 raise AttributeError("Cannot have a custom deleter on a cached " 

97 "property as __delete__ is reserved for cache clearing.") 

98 

99 self._cache_name = CACHED_PREFIX + fget.__name__ 

100 

101 property.__init__(self, fget, fset, fdel, doc) 

102 

103 self.__module__ = fget.__module__ 

104 

105 def __get__(self, obj, objtype=None): 

106 if obj is None: 

107 return self 

108 

109 if self.fget is None: 

110 raise AttributeError("unreadable attribute") # Mimic Python. 

111 

112 if not hasattr(obj, self._cache_name): 

113 # Compute and cache the value. 

114 setattr(obj, self._cache_name, self.fget(obj)) 

115 

116 return getattr(obj, self._cache_name) 

117 

118 def __set__(self, obj, value): 

119 if not hasattr(obj, CACHED_PROP_UNLOCKED_TOKEN): 

120 raise AttributeError("can't set attribute") # Mimic Python. 

121 

122 # Populate the cache. 

123 setattr(obj, self._cache_name, value) 

124 

125 def __delete__(self, obj): 

126 # Empty the cache. 

127 if hasattr(obj, self._cache_name): 

128 delattr(obj, self._cache_name) 

129 

130 def getter(self, fget): # noqa 

131 raise NotImplementedError( 

132 "Cannot change the getter on a cached property.") 

133 

134 def setter(self, fset): # noqa 

135 raise AttributeError("Cannot add a setter to a cached property.") 

136 

137 def deleter(self, fdel): # noqa 

138 return AttributeError("Cannot add a deleter to a cached property.") 

139 

140 

141class cached_selfinverse_property(cached_property): 

142 """A read-only, self-inverse property whose result is cached.""" 

143 

144 def __get__(self, obj, objtype=None): 

145 if obj is None: 

146 return self 

147 

148 if self.fget is None: 

149 raise AttributeError("unreadable attribute") # Mimic Python. 

150 

151 if not hasattr(obj, self._cache_name): 

152 # Compute and cache the value, populate the value's cache. 

153 value = self.fget(obj) 

154 setattr(obj, self._cache_name, value) 

155 setattr(value, self._cache_name, obj) 

156 

157 return getattr(obj, self._cache_name) 

158 

159 

160def cached_unary_operator(operator): 

161 """Make a unary operator method cache its result. 

162 

163 This is supposed to be used for property-like special methods such as 

164 ``__neg__`` where :func:`cached_property` can't be used. 

165 """ 

166 cacheName = CACHED_PREFIX + operator.__name__ 

167 

168 @functools.wraps(operator) 

169 def wrapper(self): 

170 if not hasattr(self, cacheName): 

171 setattr(self, cacheName, operator(self)) 

172 return getattr(self, cacheName) 

173 return wrapper 

174 

175 

176def cached_selfinverse_unary_operator(operator): 

177 """Make a self-inverse unary operator method cache its result. 

178 

179 This is supposed to be used for property-like special methods such as 

180 ``__neg__`` where :func:`cached_property` can't be used. 

181 

182 .. warning:: 

183 The result returned by the wrapped operator must be a fresh object as 

184 it will be modified. 

185 """ 

186 cacheName = CACHED_PREFIX + operator.__name__ 

187 

188 @functools.wraps(operator) 

189 def wrapper(self): 

190 if not hasattr(self, cacheName): 

191 value = operator(self) 

192 setattr(self, cacheName, value) 

193 setattr(value, cacheName, self) 

194 return getattr(self, cacheName) 

195 return wrapper 

196 

197 

198def empty_cache(obj): 

199 """Clear all cached values of an object.""" 

200 for name in dir(obj): 

201 if name.startswith(CACHED_PREFIX): 

202 delattr(obj, name) 

203 

204 

205def borrow_cache(target, source, names): 

206 """Copy cached values from one object to another. 

207 

208 :param target: The object to populate the cache of. 

209 :param source: The object to take cached values from. 

210 :param names: Names of cached properties or functions to borrow. 

211 """ 

212 for name in names: 

213 cacheName = CACHED_PREFIX + name 

214 if hasattr(source, cacheName): 

215 setattr(target, cacheName, getattr(source, cacheName)) 

216 

217 

218# -------------------------------------- 

219__all__ = api_end(_API_START, globals())