将none传递给属性会导致崩溃

Question

我正在使用属性以便在我的类中获取/设置变量，但是当变量设置为None时，下一次设置变量时程序崩溃-就像下面的代码一样:

class Abc(object):
    def __init__(self, a=None):
        self.a = a
    def set_a(self, value):
        self._a = value*5
    def get_a(self):
        return self._a
    a = property(get_a, set_a)
A = Abc()
A.a = 4
print A.a

当我运行这个时，我得到:

Traceback (most recent call last):
  File "<string>", line 13, in <module>
  File "<string>", line 3, in __init__
  File "<string>", line 6, in set_a
TypeError: unsupported operand type(s) for *: 'NoneType' and 'int'

如何写代码来阻止这个错误的发生?

Answer 1

Set self._a, not self.a;后者使用属性设置器:

class Abc(object):
    def __init__(self, a=None):
        self._a = a

或使用数字默认值:

class Abc(object):
    def __init__(self, a=0):
        self.a = a