python 是否有任何内置功能来通知字典更新时哪些字典元素发生了变化？例如，我正在寻找这样的一些功能：
>>> a = {'a':'hamburger', 'b':'fries', 'c':'coke'}
>>> b = {'b':'fries', 'c':'pepsi', 'd':'ice cream'}
>>> a.diff(b)
{'c':'pepsi', 'd':'ice cream'}
>>> a.update(b)
>>> a
{'a':'hamburger', 'b':'fries', 'c':'pepsi', 'd':'ice cream'}
我希望得到一个更改值的字典，如 a.diff(b) 的结果所示
【问题讨论】：
标签： python diff dictionary
不是内置的，但您可以迭代 dict 的键并进行比较。不过可能会很慢。
更好的解决方案可能是构建更复杂的数据结构并使用字典作为底层表示。
【讨论】：
不，它没有。但是写个字典diff函数并不难：
def diff(a, b):
diff = {}
for key in b.keys():
if (not a.has_key(key)) or (a.has_key(key) and a[key] != b[key]):
diff[key] = b[key]
return diff
【讨论】：
不，但您可以将 dict 子类化以提供更改通知。
class ObservableDict( dict ):
def __init__( self, *args, **kw ):
self.observers= []
super( ObservableDict, self ).__init__( *args, **kw )
def observe( self, observer ):
self.observers.append( observer )
def __setitem__( self, key, value ):
for o in self.observers:
o.notify( self, key, self[key], value )
super( ObservableDict, self ).__setitem__( key, value )
def update( self, anotherDict ):
for k in anotherDict:
self[k]= anotherDict[k]
class Watcher( object ):
def notify( self, observable, key, old, new ):
print "Change to ", observable, "at", key
w= Watcher()
a= ObservableDict( {'a':'hamburger', 'b':'fries', 'c':'coke'} )
a.observe( w )
b = {'b':'fries', 'c':'pepsi'}
a.update( b )
请注意，此处定义的超类 Watcher 不会检查是否存在“真正的”变化；它只是指出发生了变化。
【讨论】：
def diff_update(dict_to_update, updater):
changes=dict((k,updater[k]) for k in filter(lambda k:(k not in dict_to_update or updater[k] != dict_to_update[k]), updater.iterkeys()))
dict_to_update.update(updater)
return changes
a = {'a':'hamburger', 'b':'fries', 'c':'coke'}
b = {'b':'fries', 'c':'pepsi'}
>>> print diff_update(a, b)
{'c': 'pepsi'}
>>> print a
{'a': 'hamburger', 'c': 'pepsi', 'b': 'fries'}
【讨论】：
一个简单的差异函数很容易编写。根据您需要它的频率，它可能比 S.Lott 更优雅的 ObservableDict 更快。
def dict_diff(a, b):
"""Return differences from dictionaries a to b.
Return a tuple of three dicts: (removed, added, changed).
'removed' has all keys and values removed from a. 'added' has
all keys and values that were added to b. 'changed' has all
keys and their values in b that are different from the corresponding
key in a.
"""
removed = dict()
added = dict()
changed = dict()
for key, value in a.iteritems():
if key not in b:
removed[key] = value
elif b[key] != value:
changed[key] = b[key]
for key, value in b.iteritems():
if key not in a:
added[key] = value
return removed, added, changed
if __name__ == "__main__":
print dict_diff({'foo': 1, 'bar': 2, 'yo': 4 },
{'foo': 0, 'foobar': 3, 'yo': 4 })
【讨论】：
我喜欢以下解决方案：
>>> def dictdiff(d1, d2):
return dict(set(d2.iteritems()) - set(d1.iteritems()))
...
>>> a = {'a':'hamburger', 'b':'fries', 'c':'coke'}
>>> b = {'b':'fries', 'c':'pepsi', 'd':'ice cream'}
>>> dictdiff(a, b)
{'c': 'pepsi', 'd': 'ice cream'}
【讨论】：