Python __getattr\uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu

我正试图使用神奇的方法

\uuuuu getattr\uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu（如果值为int，则应仅允许设置
x
和
y
的值，并在用户尝试设置属性
区域
、
周长
和
距离
到
圆
时，提高
属性错误
），我的
\uuuu getattr\uuuu
抛出最大递归错误。当我注释掉它时，
\uuuu getattr\uuuuu
就可以正常工作了

from math import pi, hypot, sqrt
'''
Circle class using __getattr__, and __setattr__ (rename circle2)
'''


# __getattr__(self, name): Automatically called when the attribute name
#       is accessed and the object has no such attribute.
# __setattr__(self, name, value): Automatically called when an attempt is made to bind the attribute name to value.

class Circle:
    def __init__(self, x, y, r):
        self.x = x
        self.y = y
        self.r = 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)

    def __getattr__(self, name):
        if name in ["x", "y", "r", "area", "circumference", "distance_to_origin"]:
            print('__get if statement') # check getattr working
            return getattr(self, name)
        else:
            print('Not an attribute')
            return None
    '''
    def __setattr__(self, name, value):
        print(name, value)
        if name in ['x', 'y']:
            if isinstance(value, int):
                print('we can set x,y')
                self.__dict__[name] = value
            else:  # value isn't an int
                raise TypeError('Expected an int')
        elif name in ['area', 'circumference', 'distance_to_origin']:
            raise RuntimeError('Cannot set attribute')
    '''

if __name__ == '__main__':

    circle = Circle(x=3, y=4, r=5)
    # print(circle.x)
    print(circle.__getattr__('x'))
    # print(circle.y)
    print(circle.__getattr__('y'))
    # print(circle.r)
    print(circle.__getattr__('r'))
    # print(circle.area)
    print(circle.__getattr__('area'))
    # print(circle.circumference)
    print(circle.__getattr__('circumference'))
    # print(circle.distance_to_origin)
    print(circle.__getattr__('distance_to_origin'))
    # print(circle.test)
    '''
    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])
    '''

注释掉
\uuuu setattr\uuuu
后，测试代码：

if __name__ == '__main__':

    circle = Circle(x=3, y=4, r=5)
    # print(circle.x)
    print(circle.__getattr__('x'))
    # print(circle.y)
    print(circle.__getattr__('y'))
    # print(circle.r)
    print(circle.__getattr__('r'))
    # print(circle.area)
    print(circle.__getattr__('area'))
    # print(circle.circumference)
    print(circle.__getattr__('circumference'))
    # print(circle.distance_to_origin)
    print(circle.__getattr__('distance_to_origin'))

打印出：

__get if statement
3
__get if statement
4
__get if statement
5
__get if statement
78.53981633974483
__get if statement
31.41592653589793
__get if statement
0.0

改进的解决方案
根据这里的讨论，这是一个较短的改进版本。实现了与原始解决方案相同的功能：

from math import pi, hypot, sqrt


class Circle:
    def __init__(self, x, y, r):
        self.x = x
        self.y = y
        super().__setattr__('r', r)
        super().__setattr__('area', pi * self.r * self.r)
        super().__setattr__('circumference', 2 * pi * self.r)
        super().__setattr__('distance_to_origin',
                            abs(sqrt(self.x * self.x + self.y * self.y) - self.r))

    def __setattr__(self, name, value):
        if name in ['x', 'y']:
            if isinstance(value, int):
                print('we can set x,y')
                super().__setattr__(name, value)
            else:  # value isn't an int
                raise TypeError('Expected an int for: {}'.format(name))
        else:
            raise AttributeError('Cannot set attribute: {}'.format(name))

解决方案
避免将
\uuuu getattr\uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu

from math import pi, hypot, sqrt
'''
Circle class using __getattr__, and __setattr__ (rename circle2)
'''


# __getattr__(self, name): Automatically called when the attribute name
#       is accessed and the object has no such attribute.
# __setattr__(self, name, value): Automatically called when an attempt is made to bind the attribute name to value.

class Circle:
    def __init__(self, x, y, r):
        self._intialized = False
        self.x = x
        self.y = y
        self.r = 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)
        self._intialized = True


    def __setattr__(self, name, value):
        if name in ['_intialized']:
            self.__dict__[name] = value
            return
        if name in ['x', 'y']:
            if isinstance(value, int):
                print('we can set x,y')
                self.__dict__[name] = value
            else:  # value isn't an int
                raise TypeError('Expected an int for: {}'.format(name))
        elif not self._intialized:
            self.__dict__[name] = value

        elif name in ['area', 'circumference', 'distance_to_origin']:
            raise AttributeError('Cannot set attribute: {}'.format(name))

if __name__ == '__main__':

    circle = Circle(x=3, y=4, r=5)
    print('x:', circle.x)
    print('y:', circle.y)
    print('r:', circle.r)
    print('area:', circle.area)
    print('circumference:', circle.circumference)
    print('distance_to_origin:', 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])

输出看起来不错：

python get_set_attr.py 
we can set x,y
we can set x,y
x: 3
y: 4
r: 5
area: 78.53981633974483
circumference: 31.41592653589793
distance_to_origin: 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
Printing circle.z fails

变异
这将允许使用任何其他名称设置属性：

circle.xyz = 100

但事实并非如此：

circle.xyz


Traceback (most recent call last):
  File "get_set_attr.py", line 62, in <module>
    circle.xyz
 AttributeError: 'Circle' object has no attribute 'xyz'

何时使用
\uuu getattr\uuuu（）
？
当您访问一个不存在的属性时，Python会引发一个
AttributeError
：

class A:
    pass
a = A()
a.xyz

....
AttributeError: 'A' object has no attribute 'xyz'

只有当属性不存在时，Python才会调用
\uuu getattr\uuu（）
。
一个用例是围绕另一个对象的包装器，而不是使用继承。
例如，我们可以定义一个
ListWrapper
，它使用列表，但只允许白名单属性：

class ListWrapper:
    _allowed_attrs = set(['append', 'extend'])
    def __init__(self, value=None):
        self._wrapped = list(value) if value is not None else []
    def __getattr__(self, name):
        if name in self._allowed_attrs:
            return getattr(self._wrapped, name)
        else:
            raise AttributeError('No attribute {}.'.format(name))
    def __repr__(self):
        return repr(self._wrapped)

我们可以像列表一样使用它：

>>> my_list = ListWrapper('abc')
>>> my_list
['a', 'b', 'c']

附加元素：

>>> my_list.append('x')
>>> my_list
['a', 'b', 'c', 'x']

但是除了在
\u allowed\u attrs
中定义的属性外，我们不能使用任何其他属性：

my_list.index('a')
...

AttributeError: No attribute index.

发言人说：

当属性查找未在常规位置找到属性时调用（即，它不是实例属性，也未在类树中找到）。name是属性名称。此方法应返回（计算的）属性值或引发
AttributeError
异常

注意，如果该属性是通过正常机制找到的，则不会调用
\uuuuGetAttr\uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu（）
（这是
\uuuuuuuuuuuuuuuuuuuuuuuu
将无法访问实例的其他属性。请注意，至少对于实例变量，您可以通过不在实例属性字典中插入任何值（而是将其插入另一个对象）来伪造total control。请参阅
下面的方法，以获得对属性访问的完全控制权

您可能对代码中导致问题的两个问题感兴趣

您不能在
\uuuu init\uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu

self.r = 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)

您没有在
中检查
r
。这导致
r
被默默忽略，然后当访问
r
时，在
中设置
区域
，
初始化
，
getattr（）
调用
getattr（）
调用
getattr（）
等等（因为未设置
r
），这导致了递归

elif name in ['area', 'circumference', 'distance_to_origin']:
    raise RuntimeError('Cannot set attribute')

这是固定代码。更改已在下面的注释中标记为
mod

#!/usr/bin/python3

from math import pi, hypot, sqrt
'''
Circle class using __getattr__, and __setattr__ (rename circle2)
'''


# __getattr__(self, name): Automatically called when the attribute name
#       is accessed and the object has no such attribute.
# __setattr__(self, name, value): Automatically called when an attempt is made to bind the attribute name to value.

class Circle:
    def __init__(self, x, y, r):
        self.x = x
        self.y = y
        # mod : can't set via self.__getattr__
        super().__setattr__("r", r)
        super().__setattr__("area", pi * self.r * self.r)
        super().__setattr__("circumference", 2 * pi * self.r)
        super().__setattr__("distance_to_origin", abs(sqrt((self.x - 0)*(self.x - 0) + (self.y - 0) * (self.y - 0)) - self.r))

    def __getattr__(self, name):
        print("===== get:", name)
        if name in ["x", "y", "r", "area", "circumference", "distance_to_origin"]:
            print('__get if statement') # check getattr working
            return getattr(self, name)
        else:
            print('Not an attribute')
            return None

    def __setattr__(self, name, value):
        print("===== set:", name, value)
        if name in ['x', 'y']:
            if isinstance(value, int):
                print('we can set x,y')
                super().__setattr__(name, value) # mod : better
            else:  # value isn't an int
                raise TypeError('Expected an int')
        elif name in ['r', 'area', 'circumference', 'distance_to_origin']: # mod : add 'r'
            raise RuntimeError('Cannot set attribute')

if __name__ == '__main__':

    circle = Circle(x=3, y=4, r=5)
    # print(circle.x)
    print(circle.__getattr__('x'))
    # print(circle.y)
    print(circle.__getattr__('y'))
    # print(circle.r)
    print(circle.__getattr__('r'))
    # print(circle.area)
    print(circle.__getattr__('area'))
    # print(circle.circumference)
    print(circle.__getattr__('circumference'))
    # print(circle.distance_to_origin)
    print(circle.__getattr__('distance_to_origin'))
    # print(circle.test)
    '''
    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])
    '''

如果name==…
几乎肯定应该是
如果name in…
，
如果value为int:
几乎肯定应该是
如果isinstance（value，int）：
。这不是递归错误的原因，但这是非常糟糕的代码。另外，在
\uuuuu getattr\uuuuuuu>中使用
它本身应该会导致递归错误。@ShadowRanger我一直在和==之间，这个版本碰巧仍然有==，但后来我更改了它。你能重复你的第二条评论吗？我一直在重读它，但我不明白你说的话。
getattr（x，'abc'））
通过与
x.abc
完全相同的代码路径。如果
x.abc
不存在，则调用
\uuuuu getattr\uuuuu
以尝试满足缺少的属性（这样，如果您首先在
\uu getattr\uuuuu
中，您就知道该属性不存在）。如果“找不到属性处理程序”然后转过身，不做任何修改，在自身上请求相同的属性，最终结果是可预测的。你不能在没有设置值的情况下再次请求它；该属性仍然不存在。@ShadowRanger好的，所以只有当我试图访问一个不存在的属性时才应该调用
\u getattr\uuuuu
执行
循环。test
然后运行
\uuu getattr\uuuu
以尝试查找属性
test
？为什么最好避免使用
\uu getattr\uuuuu
？如果要执行
循环，您不需要它吗？test
（循环的一个不存在的属性）若要返回该属性，则不退出？Python会为您执行此操作。请尝试对我的变体执行
circle.test
。它会引发
AttributeError:“circle”对象没有属性“test”
。无需自行实现。那么何时需要同时实现
\uu getattr\uuuuuuuuuuuuuuuuuuuu
和
\uuuuuuu setattr\uuuuuuuuuuuu
？我在想您“我想把
\uuuuu setattr\uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu_
elif name in ['area', 'circumference', 'distance_to_origin']:
    raise RuntimeError('Cannot set attribute')

#!/usr/bin/python3

from math import pi, hypot, sqrt
'''
Circle class using __getattr__, and __setattr__ (rename circle2)
'''


# __getattr__(self, name): Automatically called when the attribute name
#       is accessed and the object has no such attribute.
# __setattr__(self, name, value): Automatically called when an attempt is made to bind the attribute name to value.

class Circle:
    def __init__(self, x, y, r):
        self.x = x
        self.y = y
        # mod : can't set via self.__getattr__
        super().__setattr__("r", r)
        super().__setattr__("area", pi * self.r * self.r)
        super().__setattr__("circumference", 2 * pi * self.r)
        super().__setattr__("distance_to_origin", abs(sqrt((self.x - 0)*(self.x - 0) + (self.y - 0) * (self.y - 0)) - self.r))

    def __getattr__(self, name):
        print("===== get:", name)
        if name in ["x", "y", "r", "area", "circumference", "distance_to_origin"]:
            print('__get if statement') # check getattr working
            return getattr(self, name)
        else:
            print('Not an attribute')
            return None

    def __setattr__(self, name, value):
        print("===== set:", name, value)
        if name in ['x', 'y']:
            if isinstance(value, int):
                print('we can set x,y')
                super().__setattr__(name, value) # mod : better
            else:  # value isn't an int
                raise TypeError('Expected an int')
        elif name in ['r', 'area', 'circumference', 'distance_to_origin']: # mod : add 'r'
            raise RuntimeError('Cannot set attribute')

if __name__ == '__main__':

    circle = Circle(x=3, y=4, r=5)
    # print(circle.x)
    print(circle.__getattr__('x'))
    # print(circle.y)
    print(circle.__getattr__('y'))
    # print(circle.r)
    print(circle.__getattr__('r'))
    # print(circle.area)
    print(circle.__getattr__('area'))
    # print(circle.circumference)
    print(circle.__getattr__('circumference'))
    # print(circle.distance_to_origin)
    print(circle.__getattr__('distance_to_origin'))
    # print(circle.test)
    '''
    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])
    '''