我正在尝试使用类似列表的属性创建一个对象。这就是我的意思。
class Unit:
def __init__(self):
self.val = 0
class Bundle:
def __init__(self, N=3):
self.units = [ Unit() for i in range(N) ]
def getvals(self):
return [ unit.val for unit in self.units ]
def setvals(self, vals):
for i, val in enumerate(vals):
self.units[i].val = val
vals = property(getvals, setvals)
现在，此对象的行为与预期不符。
>>> b = Bundle()
>>> b.setvals([1,2,3])
>>> print b.vals, b.getvals()
[1, 2, 3] [1, 2, 3]
>>> b.vals = [4,5,6]
>>> print b.vals, b.getvals()
[4, 5, 6] [1, 2, 3]
所以语句“b.vals = x”和“b.setvals（x）”不等同。你能告诉我为什么，以及如何让它表现得正常吗？
答案 0 :(得分：8)
在Python 2中，property仅适用于新样式对象;您的Bundle类必须继承自object：
class Bundle(object):
...
一旦您进行了更正，该物业将按预期运作：
>>> b.vals = [4,5,6]
>>> b.vals
[4, 5, 6]
>>> b.getvals()
[4, 5, 6]
>>> [unit.val for unit in b.units]
[4, 5, 6]
答案 1 :(得分：0)
只是为了完成Martijn Pieters answer，这可能是你想要的答案，我不禁展示你可以用Python做的一些非常奇特的事情，当我发现它们时让我非常惊讶。我认为你可能会觉得它们很有用，或者给你一些未来的想法。
list是一个对象，因此您可以从list 这将使您的“Bundle”对象继承内置list对象中的所有方法，并允许您向其添加新方法。
#!/usr/bin/env python
# -*- coding: utf-8 -*-
class Unit(object):
def __init__(self):
self.val = 0
def __repr__(self):
return "%s %s val=%s" % (type(self), hex(id(self)), self.val)
class Bundle(list):
def __init__(self, N=3):
super(Bundle, self).__init__()
self.extend([ Unit() for i in range(N) ])
@property
def vals(self):
return [ unit.val for unit in self]
@vals.setter
def vals(self, vals):
vals = vals[0:min(len(self), len(vals))] # if len(vals) > len(self), self[i] would break
for i, val in enumerate(vals):
self[i].val = val
if __name__ == "__main__":
bundle = Bundle()
print "Bundle: %s" % bundle
newUnit = Unit()
bundle.append(newUnit)
print "Bundle: %s" % bundle
bundle.vals = [1, 2, 3, 4, 5, 6]
print "Bundle (reassigned): %s" % bundle
请注意，我更改了属性定义，使它们成为装饰器，但基本思想保持不变。
list的对象表现为list：请注意，此代码仅用于示例目的。它有一个可怕的设计，一个可怕的OOP使用，它的行为将是非常混乱的每个人（甚至对于Guido van Rossum ......好吧，也许不适合他，但我敢肯定，如果他看到它，他会哭真正的程序）这是......糟糕的代码...但我认为它有助于理解你可以做什么来覆盖内置方法。此外，它缺少很多方法可以覆盖Bundle类作为真正的list对象，但我有点累： - ）检查Emulating container types以及Python文档的下一个点Additional methods for emulation of sequence types以获得完整的参考。
#!/usr/bin/env python
# -*- coding: utf-8 -*-
class Unit(object):
def __init__(self):
self.val = 0
def __repr__(self):
return "%s %s val=%s" % (type(self), hex(id(self)), self.val)
class OtherUnit(object):
def __init__(self):
self.whatever = "hello"
def __repr__(self):
return "%s %s whatever=%s" % (type(self), hex(id(self)), self.whatever)
class Bundle(object):
def __init__(self, N=3):
self.units = [ Unit() for i in range(N) ]
self.otherUnits = [ OtherUnit() for i in range(N) ]
def __repr__(self):
return "%s" % (self.units + self.otherUnits)
def __len__(self):
return len(self.units) + len(self.otherUnits)
def __iter__(self):
for item in (self.units + self.otherUnits):
yield item
def __contains__(self, value):
if isinstance(value, Unit):
return value in self.units
elif isinstance(value, OtherUnit):
return value in self.otherUnits
elif isinstance(value, int):
return value in [unit.val for unit in self.units]
elif isinstance(value, str):
return value in [otherUnit.whatever for otherUnit in self.otherUnits]
else:
return False
def __getitem__(self, index):
assert index >= 0, "Can't accept negative indexes (%s)" % indexes
if index < len(self.units):
return self.units[index]
else:
return self.otherUnits[index - len(self.units)] #Will raise index error if too big
def append(self, thing):
if isinstance(thing, Unit):
self.units.append(thing)
elif isinstance(thing, OtherUnit):
self.otherUnits.append(thing)
else:
raise TypeError("Can't accept %s" % type(thing))
@property
def vals(self):
return [ unit.val for unit in self.units] + [ otherUnit.whatever for otherUnit in self.otherUnits ]
@vals.setter
def vals(self, vals):
insertionPointUnits = 0
insertionPointOtherUnits = 0
for i, val in enumerate(vals):
if isinstance(val, int):
self.units[insertionPointUnits].val = val
insertionPointUnits += 1
elif isinstance(val, str):
self.otherUnits[insertionPointOtherUnits].whatever = val
insertionPointOtherUnits += 1
if __name__ == "__main__":
bundle = Bundle()
print "Bundle: %s" % bundle
newUnit = Unit()
bundle.append(newUnit)
print "Bundle: %s" % bundle
bundle.vals = [1, 2, "bye", 3, "how are you", 4, "doing ok"]
print "Bundle (reassigned): %s" % bundle
print "Bundle has %s items" % len(bundle) #Thanks to overwritting __len__
for i, item in enumerate(bundle):
print "bundle[%s]: %s" % (i, item) #Thanks to overwritting __iter__
print "Does 'bundle' contain 'bye'?: %s" % ('bye'in bundle) #Thanks to overwritting __contains__
print "Does 'bundle' contain 5?: %s" % (5 in bundle) #Thanks to overwritting __contains__
print "Item 1 (should be Unit with val '2': %s" % bundle[1] #Thanks to overwritting __getitem__
print "Item 5 (should be OtherUnit with val 'how are you' (4 Units + 1 OtherUnit... then ours!): %s" % bundle[5] #Thanks to overwritting __getitem__
try:
print "Item 9 (should raise IndexError): %s" % bundle[9]
except IndexError, ie:
print "Wooops: %s" % ie
希望这有点帮助。享受Python的乐趣！