1 # Copyright (c) 2013 The Chromium OS Authors. All rights reserved. 2 # Use of this source code is governed by a BSD-style license that can be 3 # found in the LICENSE file. 4 5 import functools 6 import logging 7 8 9 def in_context(context_name): 10 """ 11 Call a method in the context of member variable 'context_name.' 12 13 You can use this like: 14 class Foo(object): 15 def __init__(self): 16 self._mutex = threading.RLock() 17 18 @in_context('_mutex') 19 def bar(self): 20 # Two threads can call Foo.bar safely without 21 # any other synchronization. 22 print 'Locked up tight.' 23 24 def contextless_bar(self): 25 with self._mutex: 26 print 'Locked up tight.' 27 28 With the in_context decorator, self.bar is equivalent to 29 self.contextless_bar. You can use this this to declare synchronized 30 methods in the style of Java. Similar to other locking methods, this 31 can land you in deadlock in a hurry if you're not aware of what you're 32 doing. 33 34 @param context_name string name of the context manager to look up in self. 35 36 """ 37 def wrap(func): 38 """ 39 This function will get called with the instance method pulled off 40 of self. It does not get the self object though, so we wrap yet 41 another nested function. 42 43 @param func Function object that we'll eventually call. 44 45 """ 46 @functools.wraps(func) 47 def wrapped_manager(self, *args, **kwargs): 48 """ Do the actual work of acquiring the context. 49 50 We need this layer of indirection so that we can get at self. 51 We use functools.wraps does some magic so that the function 52 names and docs are set correctly on the wrapped function. 53 54 """ 55 context = getattr(self, context_name) 56 with context: 57 return func(self, *args, **kwargs) 58 return wrapped_manager 59 return wrap 60 61 62 class _Property(object): 63 def __init__(self, func): 64 self._func = func 65 66 def __get__(self, obj, type=None): 67 if not hasattr(obj, '_property_cache'): 68 obj._property_cache = {} 69 if self._func not in obj._property_cache: 70 obj._property_cache[self._func] = self._func(obj) 71 return obj._property_cache[self._func] 72 73 74 def cached_property(func): 75 """ 76 A read-only property that is only run the first time the attribute is 77 accessed, and then the result is saved and returned on each future 78 reference. 79 80 @param func: The function to calculate the property value. 81 @returns: An object that abides by the descriptor protocol. 82 """ 83 return _Property(func) 84 85 86 def test_module_available(module, raise_error=False): 87 """A decorator to test if the given module is available first before 88 calling a function. 89 90 @param module: Module object. The value should be None if the module is 91 failed to be imported. 92 @param raise_error: If true an import error will be raised on call if module 93 is not imported. 94 """ 95 96 def decorator(f): 97 """The actual decorator. 98 99 @param f: The function to call. 100 101 @return: The function to call based on the value of `module` 102 """ 103 104 def dummy_func(*args, **kargs): 105 """A dummy function silently pass.""" 106 logging.debug('Module %s is not found. Call %s is skipped.', module, 107 f) 108 if raise_error: 109 raise ImportError('Module %s is not found.' % module) 110 111 return f if module else dummy_func 112 113 return decorator 114
