Coverage for python/lsst/daf/butler/_utilities/thread_safe_cache.py: 56%

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1# This file is part of daf_butler. 

2# 

3# Developed for the LSST Data Management System. 

4# This product includes software developed by the LSST Project 

5# (http://www.lsst.org). 

6# See the COPYRIGHT file at the top-level directory of this distribution 

7# for details of code ownership. 

8# 

9# This software is dual licensed under the GNU General Public License and also 

10# under a 3-clause BSD license. Recipients may choose which of these licenses 

11# to use; please see the files gpl-3.0.txt and/or bsd_license.txt, 

12# respectively. If you choose the GPL option then the following text applies 

13# (but note that there is still no warranty even if you opt for BSD instead): 

14# 

15# This program is free software: you can redistribute it and/or modify 

16# it under the terms of the GNU General Public License as published by 

17# the Free Software Foundation, either version 3 of the License, or 

18# (at your option) any later version. 

19# 

20# This program is distributed in the hope that it will be useful, 

21# but WITHOUT ANY WARRANTY; without even the implied warranty of 

22# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 

23# GNU General Public License for more details. 

24# 

25# You should have received a copy of the GNU General Public License 

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

27 

28import threading 

29from typing import Generic, TypeVar 

30 

31TKey = TypeVar("TKey") 

32TValue = TypeVar("TValue") 

33 

34 

35class ThreadSafeCache(Generic[TKey, TValue]): 

36 """A simple thread-safe cache. Ensures that once a value is stored for 

37 a key, it does not change. 

38 """ 

39 

40 def __init__(self) -> None: 

41 # This mutex may not actually be necessary for the current version of 

42 # CPython, because both the 'get' and 'set' operation use underlying C 

43 # functions that don't drop the GIL (at first glance anyway.) This is 

44 # not documented anywhere that I can find, however, so better safe than 

45 # sorry. 

46 self._mutex = threading.Lock() 

47 self._cache: dict[TKey, TValue] = dict() 

48 

49 def get(self, key: TKey) -> TValue | None: 

50 """Return the value associated with the given key, or ``None`` if no 

51 value has been assigned to that key. 

52 

53 Parameters 

54 ---------- 

55 key : ``TKey`` 

56 Key used to look up the value. 

57 """ 

58 with self._mutex: 

59 return self._cache.get(key) 

60 

61 def set_or_get(self, key: TKey, value: TValue) -> TValue: 

62 """Set a value for a key if the key does not already have a value. 

63 

64 Parameters 

65 ---------- 

66 key : ``TKey`` 

67 Key used to look up the value. 

68 value : ``TValue`` 

69 Value to store in the cache. 

70 

71 Returns 

72 ------- 

73 value : ``TValue`` 

74 The existing value stored for the key if it was present, or 

75 ``value`` if this was a new key. 

76 """ 

77 with self._mutex: 

78 return self._cache.setdefault(key, value)