我想创建一个Attribute类，所以我可以做类似的事情：
class Creature:
def __init__(self, health=100, armor=0):
self.health = Attribute(health)
self.armor = Attribute(armor)
现在我做的时候
c1 = Creature()
c1.health += 10
它实际上并没有改变实际值，但它将health的 base 值保持为100，同时给它调整+10。
以下是我尝试过的内容，但它使用了healt.get()和health.set()，这是不需要的：
class Attribute:
def __init__(self, base=0, adjustments=None):
self._base = base
self._adjustments = adjustments or []
def set(self, value):
difference = value - self._base
if difference:
self._adjustments.append(difference)
def get(self):
return self._base + sum(self._adjustments)
c1 = Creature(50)
c1.health.set(60) # Adds +10 adjustment
但我希望能够做到这一点：
c1 = Creature(50)
c1.health = 60
# Or just:
c1.health += 10
这可能吗？
答案 0 :(得分：3)
以下是可能满足您需求的四种方法。
Descriptors允许您在隐藏底层实现时提供直接属性访问。
class AttributeDescriptor(object):
def __init__(self):
self.initialized = False
self.base = 0
self.adjustments = []
def compute(self):
return self.base + sum(self.adjustments)
def __set__(self, inst, value):
if not self.initialized:
self.base = value
self.initialized = True
print("Attribute initialized to %s" % value)
else:
# Calculate delta
delta = (value - self.compute())
self.adjustments.append(delta)
print("Adjustment added: %s" % delta)
def __get__(self, inst, owner):
return self.compute()
class Creature(object):
health = AttributeDescriptor()
armor = AttributeDescriptor()
def __init__(self, health=100, armor=0):
self.health = health
self.armor = armor
c1 = Creature(50)
c1.health = 60 # Adds a +10 adjustment
print c1.health # 60
c1.health += 10 # Add a +10 adjustment
print c1.health # 70
#print c1.health.adjustments # This won't work ('int' object has no attribute 'adjustments')
输出：
Attribute initialized to 50 Attribute initialized to 0 Adjustment added: 10 60 Adjustment added: 10 70
这种方法的问题在于您没有简单的方法来访问描述符的内部。因此，在这种情况下，您无法检查adjustments列表。但是，您可以直接将c1.health = X分配给它，就像它是普通属性一样。
注意：正如Veedrac在评论中所指出的，这些属性是在类级定义的，将在Creature类的所有实例之间共享。仅仅因为这个原因，它不是一个解决方案，但无论如何它都不是一个好的解决方案。
您可以使用实现“augmented assignment”魔法__iadd__()和__isub__()
class AttributeObject(object):
def __init__(self, base):
self.base = base
self.adjustments = []
print("Attribute initialized to %s" % base)
def __compute(self):
return self.base + sum(self.adjustments)
def __int__(self):
return self.__compute()
def __iadd__(self, delta):
print("Adjustment added: %s" % delta)
self.adjustments.append(delta)
return self
def __isub__(self, delta):
print("Adjustment added: %s" % -delta)
self.adjustments.append(-delta)
return self
class Creature(object):
def __init__(self, health=100, armor=0):
self.health = AttributeObject(health)
self.armor = AttributeObject(armor)
c1 = Creature(50)
#c1.health = 60 # Can't do this, because it will override the AttributeObject
print int(c1.health) # 60
c1.health += 10 # Add a +10 adjustment
print int(c1.health) # 70
print c1.health.adjustments # [10]
输出：
Attribute initialized to 50 Attribute initialized to 0 50 Adjustment added: 10 60 [10]
此方法的问题在于，如果不覆盖属性，则无法直接分配属性。换句话说，c1.health = X会将health属性的值覆盖为等于X - 您将失去之前的任何内容。
但是通过这种方法，您可以访问adjustments列表：print c1.health.adjustments
请注意，c1.health是AdjustmentTracker的实例，而不是您可能期望的数字类型（尝试print c1.health）。您可以通过多种方式访问/提取数值，在我使用int(c1.health)类型转换的示例中（可能是因为我实现了__int__）。
使用上述两种方法的组合，您可以使用列出的所有语法。
class AttributeDescriptor(object):
def __init__(self, attr):
self.attr = attr
def __set__(self, inst, value):
getattr(inst, self.attr).update(value)
def __get__(self, inst, owner):
return getattr(inst, self.attr).compute()
class AdjustmentTracker(object):
def __init__(self, base):
print("Attribute initialized to %s" % base)
self.base = base
self.adjustments = []
def compute(self):
return self.base + sum(self.adjustments)
def update(self, value):
delta = (value - self.compute())
print("Adjustment added: %s" % delta)
self.adjustments.append(delta)
class Creature(object):
health = AttributeDescriptor('_health')
armor = AttributeDescriptor('_armor')
def __init__(self, health=100, armor=0):
self._health = AdjustmentTracker(health)
self._armor = AdjustmentTracker(armor)
c1 = Creature(50)
c1.health = 60 # Adds a +10 adjustment
print c1.health # 60
c1.health += 10 # Add a +10 adjustment
print c1.health # 70
print c1._health.adjustments # [10, 10]
输出：
Attribute initialized to 50 Attribute initialized to 0 Adjustment added: 10 60 Adjustment added: 10 70 [10, 10]
在这里，描述符不会跟踪基本和调整列表本身，而是将它们用作与AdjustmentTracker obejcts接口的代理。有了这个，您可以直接分配（例如c1.health = 60）和访问基础初始基础/调整（例如c1._health.adjustments）。
与前面的示例一样，我们使用AdjustmentTracker个对象来保存属性的状态。但在此示例中，您可以使用properties来屏蔽属性，而不是使用显式描述符。
class AdjustmentTracker(object):
def __init__(self, base):
print("Attribute initialized to %s" % base)
self.base = base
self.adjustments = []
def compute(self):
return self.base + sum(self.adjustments)
def update(self, value):
delta = (value - self.compute())
print("Adjustment added: %s" % delta)
self.adjustments.append(delta)
class Creature(object):
@property
def health(self): return self._health.compute()
@health.setter
def health(self, value): self._health.update(value)
@property
def armor(self): return self._armor.compute()
@armor.setter
def armor(self, value): self._armor.update(value)
def __init__(self, health=100, armor=0):
self._health = AdjustmentTracker(health)
self._armor = AdjustmentTracker(armor)
c1 = Creature(50)
c1.health = 60 # Adds a +10 adjustment
print c1.health # 60
c1.health += 10 # Add a +10 adjustment
print c1.health # 70
print c1._health.adjustments # [10, 10]
输出：
Attribute initialized to 50 Attribute initialized to 0 Adjustment added: 10 60 Adjustment added: 10 70 [10, 10]
此示例与前一个示例基本相同，因为它使用了属性，功能完全相同，所以只需更少的代码行。
答案 1 :(得分：1)
查看属性 https://docs.python.org/3/library/functions.html#property
在下面的示例中，属性被用作装饰器。
class Creature:
def __init__(self, health):
self._base_health = health
self._modifications = []
@property
def health(self):
return self._base_health + sum(self._modifications)
@health.setter
def health(self, value):
self._modifications.append(value - self._base_health - sum(self._modifications))
每次检索健康属性时，都会调用getter函数（标有property装饰器的函数）。类似地，当设置了health属性时，将调用setter函数（标有health.setter装饰器的函数）。
c1 = Creature(50)
c1.health = 60
c1.health += 10
print(c1.health)
c1.health = 40
print(c1.health)
输出
70
40