当用户尝试更改对象的值时，使用描述符 class 引发 RuntimeError
Using descriptor class to raise RuntimeError when user tries to change object's value
我写了一个圆 class 使用描述符允许用户设置圆的 x、y 和 r 的值并检查 x 和 y 的值是否为整数。如果用户输入一个非整数，则会引发 TypeError，现在我想创建另一个描述符 class，允许用户获取圆的面积和周长的值，但无法设置它。我想我的 __get__ 方法工作正常，但 __set__ 方法不正确。
class Integer:
def __init__(self, name):
self.name = name # stores name of the managed object's attribute
def __get__(self, instance, cls):
if instance is None:
return self
else:
return instance.__dict__[self.name]
def __set__(self, instance, value):
if not isinstance(value, int):
raise TypeError('Expected an int')
else:
instance.__dict__[self.name] = value
class Computations(object):
def __init__(self, name):
self.name = name # default value for area, circumference, distance to origin
def __get__(self, instance, cls):
if instance is None:
print('this is the __get__ if statement running')
return self
else:
print('this is the __get__ else statement running')
return instance.__dict__[self.name]
def __set__(self, instance, value):
if isinstance(value, int):
raise RuntimeError('Cant set formulas')
else:
instance.__dict__[self.name] = value
class Circle:
x = Integer('_x') # Use _x and _y as the __dict__ key of a Point
y = Integer('_y') # These will be the storage names for a Point
r = Integer('_r')
area = Computations('_area') # class variable of Computations
circumference = Computations('_circumference')
distance_to_origin = Computations('_distance_to_origin')
def __init__(self, x, y, r):
self.x = x # invokes Integer.x.__set__
self.y = y # invokes Integer.y.__set__
self.r = r # for radius/invokes Integer.r.
self.area = pi * self.r * self.r
self.circumference = 2 * pi * self.r
self.distance_to_origin = abs(sqrt((self.x - 0)*(self.x - 0) + (self.y - 0) * (self.y - 0)) - self.r)
# Testing code
if __name__ == '__main__':
circle = Circle(x=3, y=4, r=5)
print(circle.x)
print(circle.y)
print(circle.r)
print(circle.area)
# circle.area = 12
print(circle.area)
print(circle.circumference)
print(circle.distance_to_origin)
tests = [('circle.x = 12.3', "print('Setting circle.x to non-integer fails')"),
('circle.y = 23.4', "print('Setting circle.y to non-integer fails')"),
('circle.area = 23.4', "print('Setting circle.area fails')"),
('circle.circumference = 23.4', "print('Setting circle.circumference fails')"),
('circle.distance_to_origin = 23.4', "print('Setting circle.distance_to_origin fails')"),
('circle.z = 5.6', "print('Setting circle.z fails')"),
('print(circle.z)', "print('Printing circle.z fails')")]
for test in tests:
try:
exec(test[0])
except:
exec(test[1])
程序运行，但它允许设置面积、周长和distance_to_origin，这不是我想要的。我知道 "if isinstance(value, int): raise RuntimeError('Can't set formulas')" 行意味着如果用户输入的值不是整数，则会引发错误。我只是不确定用什么来代替它，这样它就会抛出错误并且不允许用户更改值。
测试代码正确输出如下：
78.53981633974483
31.41592653589793
0.0
Setting circle.x to non-integer fails
Setting circle.y to non-integer fails
Setting circle.area fails
Setting circle.circumference fails
Setting circle.distance_to_origin fails
5.6
改进的描述符计算
这将只允许 Computations 描述符的初始设置：
class Computations(object):
def __init__(self, name):
self.name = name # default value for area, circumference, distance to origin
def __get__(self, instance, cls):
if instance is None:
print('this is the __get__ if statement running')
return self
else:
print('this is the __get__ else statement running')
return instance.__dict__[self.name]
def __set__(self, instance, value):
if hasattr(instance, self.name + '_is_set'):
raise ValueError('Cannot set {} again.'.format(self.name[1:]))
if isinstance(value, int):
raise RuntimeError('Cant set formulas')
else:
instance.__dict__[self.name] = value
setattr(instance, self.name + '_is_set', True)
诀窍是在实例上生成一个新属性：
setattr(instance, self.name + '_is_set', True)
对于实例 circle 和属性 circumference 这意味着：
circle._circumference_is_set = True
检查此属性是否存在：
if hasattr(instance, self.name + '_is_set')
对于我们的案例，这意味着：
if hasattr(circle, '_circumference_is_set')
第一次为 circumference 调用 __set__ 是在 class Circle:
self.circumference = 2 * pi * self.r
现在 _circumference_is_set 存在，下次尝试设置它会导致异常。
综合起来
你的代码和我的新描述符 Computation:
from math import pi, sqrt
class Integer(object):
def __init__(self, name):
self.name = name # stores name of the managed object's attribute
def __get__(self, instance, cls):
if instance is None:
return self
else:
return instance.__dict__[self.name]
def __set__(self, instance, value):
if not isinstance(value, int):
raise TypeError('Expected an int')
else:
instance.__dict__[self.name] = value
class Computations(object):
def __init__(self, name):
self.name = name # default value for area, circumference, distance to origin
def __get__(self, instance, cls):
if instance is None:
print('this is the __get__ if statement running')
return self
else:
print('this is the __get__ else statement running')
return instance.__dict__[self.name]
def __set__(self, instance, value):
if hasattr(instance, self.name + 'is_set'):
raise ValueError('Cannot set {} again.'.format(self.name[1:]))
if isinstance(value, int):
raise RuntimeError('Cant set formulas')
else:
instance.__dict__[self.name] = value
setattr(instance, self.name + 'is_set', True)
class Circle(object):
x = Integer('_x') # Use _x and _y as the __dict__ key of a Point
y = Integer('_y') # These will be the storage names for a Point
r = Integer('_r')
area = Computations('_area') # class variable of Computations
circumference = Computations('_circumference')
distance_to_origin = Computations('_distance_to_origin')
def __init__(self, x, y, r):
self.x = x # invokes Integer.x.__set__
self.y = y # invokes Integer.y.__set__
self.r = r # for radius/invokes Integer.r.
self.area = pi * self.r * self.r
self.circumference = 2 * pi * self.r
self.distance_to_origin = abs(sqrt(self.x * self.x + self.y * self.y) - self.r)
测试
现在尝试设置 circle.circumference 引发异常：
# Testing code
if __name__ == '__main__':
circle = Circle(x=3, y=4, r=5)
print('circumference', circle.circumference)
print('try setting circumference')
circle.circumference = 12.5
输出：
this is the __get__ else statement running
circumference 31.41592653589793
try setting circumference
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-227-316199bab738> in <module>()
64
65 print('try setting circumference')
---> 66 circle.circumference = 12.5
<ipython-input-227-316199bab738> in __set__(self, instance, value)
31 def __set__(self, instance, value):
32 if hasattr(instance, self.name + 'is_set'):
---> 33 raise ValueError('Cannot set {} again.'.format(self.name[1:]))
34 if isinstance(value, int):
35 raise RuntimeError('Cant set formulas')
ValueError: Cannot set circumference again.
您的测试：
if __name__ == '__main__':
circle = Circle(x=3, y=4, r=5)
print(circle.x)
print(circle.y)
print(circle.r)
print(circle.area)
# circle.area = 12
print(circle.area)
print(circle.circumference)
print(circle.distance_to_origin)
tests = [('circle.x = 12.3', "print('Setting circle.x to non-integer fails')"),
('circle.y = 23.4', "print('Setting circle.y to non-integer fails')"),
('circle.area = 23.4', "print('Setting circle.area fails')"),
('circle.circumference = 23.4', "print('Setting circle.circumference fails')"),
('circle.distance_to_origin = 23.4', "print('Setting circle.distance_to_origin fails')"),
('circle.z = 5.6', "print('Setting circle.z fails')"),
('print(circle.z)', "print('Printing circle.z fails')")]
for test in tests:
try:
exec(test[0])
except:
exec(test[1])
生成此输出：
3
4
5
this is the __get__ else statement running
78.53981633974483
this is the __get__ else statement running
78.53981633974483
this is the __get__ else statement running
31.41592653589793
this is the __get__ else statement running
0.0
Setting circle.x to non-integer fails
Setting circle.y to non-integer fails
Setting circle.area fails
Setting circle.circumference fails
Setting circle.distance_to_origin fails
5.6
不能正确使用浮点数
您的 __set__ 函数目前允许设置一个值，只要它不是整数：
def __set__(self, instance, value):
if isinstance(value, int):
raise RuntimeError('Cant set formulas')
else:
instance.__dict__[self.name] = value
然后，您在 class 本身和测试中都将 circumference 设置为浮点值，因此它通过了：
self.circumference = 2 * pi * self.r
和：
circle.circumference = 23.4
但是，设置整数值会失败：
circle.circumference = 10
--> RuntimeError: Cant set formulas
修复 __set__
要解决该问题，您可以使 __set__ 始终失败：
def __set__(self, instance, value):
raise RuntimeError('Cant set formulas')
您仍然希望周长可以从 class 中写入，但是为此，您可以使用 private_circumference:
self._area = pi * self.r * self.r
self._circumference = 2 * pi * self.r
self._distance_to_origin = abs(sqrt((self.x - 0)*(self.x - 0) + (self.y - 0) * (self.y - 0)) - self.r)
更简单的解决方案
以上修复只是理论上的，以表明它是可以修复的。使用 property 提供自定义 getters 和 setters 通常要简单得多：
def __init__(self, ...):
...
self._circumference = 2 * pi * self.r
@property
def circumference(self):
return self._circumference
# if you wanted to allow setting (but you don't), you would add this:
#@circumference.setter
#def circumference(self, value):
# self._circumference = value
Computations class 根本不需要。
更好的设计
如果您打算拥有自己的 getter 和 setter 作为计算值，那么按需计算它是有意义的，而无需记住该值：
@property
def circumference(self):
return 2 * pi * self.r
# there is no self._circumference!
这样做的好处是，当r发生变化时，周长仍然是正确的（使用原始代码，circumference始终具有__init__的原始值）。
注：这是Python3!
之前没说清楚，这里说一下。此代码适用于 Python 3.
如果 运行 和 Python 2，由于旧式 class 定义，描述符将被忽略。要使代码与 Python 2 一起工作，只需确保所有 classes 都继承自 object，例如
class Circle(object):
...
注2：类型检查
在 Python 中实施类型检查之前请三思。 Not 检查类型通常会使代码更好。在您的情况下，您将值限制为整数，这意味着：float 不起作用，Decimal 不起作用，long 不起作用（在 Python 2 中），MySpecialNumericType 不起作用。尽管只要代码没有明确禁止它就可以工作。
我写了一个圆 class 使用描述符允许用户设置圆的 x、y 和 r 的值并检查 x 和 y 的值是否为整数。如果用户输入一个非整数，则会引发 TypeError，现在我想创建另一个描述符 class，允许用户获取圆的面积和周长的值，但无法设置它。我想我的 __get__ 方法工作正常，但 __set__ 方法不正确。
class Integer:
def __init__(self, name):
self.name = name # stores name of the managed object's attribute
def __get__(self, instance, cls):
if instance is None:
return self
else:
return instance.__dict__[self.name]
def __set__(self, instance, value):
if not isinstance(value, int):
raise TypeError('Expected an int')
else:
instance.__dict__[self.name] = value
class Computations(object):
def __init__(self, name):
self.name = name # default value for area, circumference, distance to origin
def __get__(self, instance, cls):
if instance is None:
print('this is the __get__ if statement running')
return self
else:
print('this is the __get__ else statement running')
return instance.__dict__[self.name]
def __set__(self, instance, value):
if isinstance(value, int):
raise RuntimeError('Cant set formulas')
else:
instance.__dict__[self.name] = value
class Circle:
x = Integer('_x') # Use _x and _y as the __dict__ key of a Point
y = Integer('_y') # These will be the storage names for a Point
r = Integer('_r')
area = Computations('_area') # class variable of Computations
circumference = Computations('_circumference')
distance_to_origin = Computations('_distance_to_origin')
def __init__(self, x, y, r):
self.x = x # invokes Integer.x.__set__
self.y = y # invokes Integer.y.__set__
self.r = r # for radius/invokes Integer.r.
self.area = pi * self.r * self.r
self.circumference = 2 * pi * self.r
self.distance_to_origin = abs(sqrt((self.x - 0)*(self.x - 0) + (self.y - 0) * (self.y - 0)) - self.r)
# Testing code
if __name__ == '__main__':
circle = Circle(x=3, y=4, r=5)
print(circle.x)
print(circle.y)
print(circle.r)
print(circle.area)
# circle.area = 12
print(circle.area)
print(circle.circumference)
print(circle.distance_to_origin)
tests = [('circle.x = 12.3', "print('Setting circle.x to non-integer fails')"),
('circle.y = 23.4', "print('Setting circle.y to non-integer fails')"),
('circle.area = 23.4', "print('Setting circle.area fails')"),
('circle.circumference = 23.4', "print('Setting circle.circumference fails')"),
('circle.distance_to_origin = 23.4', "print('Setting circle.distance_to_origin fails')"),
('circle.z = 5.6', "print('Setting circle.z fails')"),
('print(circle.z)', "print('Printing circle.z fails')")]
for test in tests:
try:
exec(test[0])
except:
exec(test[1])
程序运行，但它允许设置面积、周长和distance_to_origin，这不是我想要的。我知道 "if isinstance(value, int): raise RuntimeError('Can't set formulas')" 行意味着如果用户输入的值不是整数，则会引发错误。我只是不确定用什么来代替它，这样它就会抛出错误并且不允许用户更改值。
测试代码正确输出如下：
78.53981633974483
31.41592653589793
0.0
Setting circle.x to non-integer fails
Setting circle.y to non-integer fails
Setting circle.area fails
Setting circle.circumference fails
Setting circle.distance_to_origin fails
5.6
改进的描述符计算
这将只允许 Computations 描述符的初始设置：
class Computations(object):
def __init__(self, name):
self.name = name # default value for area, circumference, distance to origin
def __get__(self, instance, cls):
if instance is None:
print('this is the __get__ if statement running')
return self
else:
print('this is the __get__ else statement running')
return instance.__dict__[self.name]
def __set__(self, instance, value):
if hasattr(instance, self.name + '_is_set'):
raise ValueError('Cannot set {} again.'.format(self.name[1:]))
if isinstance(value, int):
raise RuntimeError('Cant set formulas')
else:
instance.__dict__[self.name] = value
setattr(instance, self.name + '_is_set', True)
诀窍是在实例上生成一个新属性：
setattr(instance, self.name + '_is_set', True)
对于实例 circle 和属性 circumference 这意味着：
circle._circumference_is_set = True
检查此属性是否存在：
if hasattr(instance, self.name + '_is_set')
对于我们的案例，这意味着：
if hasattr(circle, '_circumference_is_set')
第一次为 circumference 调用 __set__ 是在 class Circle:
self.circumference = 2 * pi * self.r
现在 _circumference_is_set 存在，下次尝试设置它会导致异常。
综合起来
你的代码和我的新描述符 Computation:
from math import pi, sqrt
class Integer(object):
def __init__(self, name):
self.name = name # stores name of the managed object's attribute
def __get__(self, instance, cls):
if instance is None:
return self
else:
return instance.__dict__[self.name]
def __set__(self, instance, value):
if not isinstance(value, int):
raise TypeError('Expected an int')
else:
instance.__dict__[self.name] = value
class Computations(object):
def __init__(self, name):
self.name = name # default value for area, circumference, distance to origin
def __get__(self, instance, cls):
if instance is None:
print('this is the __get__ if statement running')
return self
else:
print('this is the __get__ else statement running')
return instance.__dict__[self.name]
def __set__(self, instance, value):
if hasattr(instance, self.name + 'is_set'):
raise ValueError('Cannot set {} again.'.format(self.name[1:]))
if isinstance(value, int):
raise RuntimeError('Cant set formulas')
else:
instance.__dict__[self.name] = value
setattr(instance, self.name + 'is_set', True)
class Circle(object):
x = Integer('_x') # Use _x and _y as the __dict__ key of a Point
y = Integer('_y') # These will be the storage names for a Point
r = Integer('_r')
area = Computations('_area') # class variable of Computations
circumference = Computations('_circumference')
distance_to_origin = Computations('_distance_to_origin')
def __init__(self, x, y, r):
self.x = x # invokes Integer.x.__set__
self.y = y # invokes Integer.y.__set__
self.r = r # for radius/invokes Integer.r.
self.area = pi * self.r * self.r
self.circumference = 2 * pi * self.r
self.distance_to_origin = abs(sqrt(self.x * self.x + self.y * self.y) - self.r)
测试
现在尝试设置 circle.circumference 引发异常：
# Testing code
if __name__ == '__main__':
circle = Circle(x=3, y=4, r=5)
print('circumference', circle.circumference)
print('try setting circumference')
circle.circumference = 12.5
输出：
this is the __get__ else statement running
circumference 31.41592653589793
try setting circumference
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-227-316199bab738> in <module>()
64
65 print('try setting circumference')
---> 66 circle.circumference = 12.5
<ipython-input-227-316199bab738> in __set__(self, instance, value)
31 def __set__(self, instance, value):
32 if hasattr(instance, self.name + 'is_set'):
---> 33 raise ValueError('Cannot set {} again.'.format(self.name[1:]))
34 if isinstance(value, int):
35 raise RuntimeError('Cant set formulas')
ValueError: Cannot set circumference again.
您的测试：
if __name__ == '__main__':
circle = Circle(x=3, y=4, r=5)
print(circle.x)
print(circle.y)
print(circle.r)
print(circle.area)
# circle.area = 12
print(circle.area)
print(circle.circumference)
print(circle.distance_to_origin)
tests = [('circle.x = 12.3', "print('Setting circle.x to non-integer fails')"),
('circle.y = 23.4', "print('Setting circle.y to non-integer fails')"),
('circle.area = 23.4', "print('Setting circle.area fails')"),
('circle.circumference = 23.4', "print('Setting circle.circumference fails')"),
('circle.distance_to_origin = 23.4', "print('Setting circle.distance_to_origin fails')"),
('circle.z = 5.6', "print('Setting circle.z fails')"),
('print(circle.z)', "print('Printing circle.z fails')")]
for test in tests:
try:
exec(test[0])
except:
exec(test[1])
生成此输出：
3
4
5
this is the __get__ else statement running
78.53981633974483
this is the __get__ else statement running
78.53981633974483
this is the __get__ else statement running
31.41592653589793
this is the __get__ else statement running
0.0
Setting circle.x to non-integer fails
Setting circle.y to non-integer fails
Setting circle.area fails
Setting circle.circumference fails
Setting circle.distance_to_origin fails
5.6
不能正确使用浮点数
您的 __set__ 函数目前允许设置一个值，只要它不是整数：
def __set__(self, instance, value):
if isinstance(value, int):
raise RuntimeError('Cant set formulas')
else:
instance.__dict__[self.name] = value
然后，您在 class 本身和测试中都将 circumference 设置为浮点值，因此它通过了：
self.circumference = 2 * pi * self.r
和：
circle.circumference = 23.4
但是，设置整数值会失败：
circle.circumference = 10
--> RuntimeError: Cant set formulas
修复 __set__
要解决该问题，您可以使 __set__ 始终失败：
def __set__(self, instance, value):
raise RuntimeError('Cant set formulas')
您仍然希望周长可以从 class 中写入，但是为此，您可以使用 private_circumference:
self._area = pi * self.r * self.r
self._circumference = 2 * pi * self.r
self._distance_to_origin = abs(sqrt((self.x - 0)*(self.x - 0) + (self.y - 0) * (self.y - 0)) - self.r)
更简单的解决方案
以上修复只是理论上的，以表明它是可以修复的。使用 property 提供自定义 getters 和 setters 通常要简单得多：
def __init__(self, ...):
...
self._circumference = 2 * pi * self.r
@property
def circumference(self):
return self._circumference
# if you wanted to allow setting (but you don't), you would add this:
#@circumference.setter
#def circumference(self, value):
# self._circumference = value
Computations class 根本不需要。
更好的设计
如果您打算拥有自己的 getter 和 setter 作为计算值，那么按需计算它是有意义的，而无需记住该值：
@property
def circumference(self):
return 2 * pi * self.r
# there is no self._circumference!
这样做的好处是，当r发生变化时，周长仍然是正确的（使用原始代码，circumference始终具有__init__的原始值）。
注：这是Python3!
之前没说清楚，这里说一下。此代码适用于 Python 3.
如果 运行 和 Python 2，由于旧式 class 定义，描述符将被忽略。要使代码与 Python 2 一起工作，只需确保所有 classes 都继承自 object，例如
class Circle(object):
...
注2：类型检查
在 Python 中实施类型检查之前请三思。 Not 检查类型通常会使代码更好。在您的情况下，您将值限制为整数，这意味着：float 不起作用，Decimal 不起作用，long 不起作用（在 Python 2 中），MySpecialNumericType 不起作用。尽管只要代码没有明确禁止它就可以工作。