Passing none to a property causes crashes

Question

I'm using properties in order to get/set variables in my class, but when the variable is set to None, the program crashes the next time the variable is set - like in the following code:

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

When I run this I get:

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'

What's the correct way of writing the code to stop this error occurring?

Answer 1

Set self._a, not self.a; the latter uses the property setter:

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

or use a numeric default instead:

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