python 字典更新差异

Question

Python 有没有什么内置的功能，可以在字典更新时通知我哪些元素发生了变化？举个例子，我想要一些类似这样的功能：

>>> 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'}

我想得到一个字典，里面包含了变化的值，就像 a.diff(b) 的结果那样。

Answer 1

写一个简单的比较函数其实很简单。如果你经常需要用到这个函数，它的运行速度可能会比S.Lott的那种更优雅的ObservableDict要快。

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 })

Answer 2

一年后

我喜欢下面这个解决方案：

>>> 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'}

Answer 3

不可以，但你可以通过创建一个字典的子类来实现变更通知。

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 )

需要注意的是，这里定义的父类Watcher并不会检查是否真的发生了变化，它只是简单地记录下发生了变化。