Setting Property via a String

Question

I'm trying to set a Python class property outside of the class via the setattr(self, item, value) function.

class MyClass:
    def getMyProperty(self):
        return self.__my_property

    def setMyProperty(self, value):
        if value is None:
            value = ''
        self.__my_property = value

    my_property = property( getMyProperty, setMyProperty )

And in another script, I create an instance and want to specify the property and let the property mutator handle the simple validation.

myClass = MyClass()
new_value = None

# notice the property in quotes
setattr(myClass, 'my_property', new_value)

The problem is that it doesn't appear to be calling the setMyProperty(self, value) mutator. For a quick test to verify that it doesn't get called, I change the mutator to:

    def setMyProperty(self, value):
        raise ValueError('WTF! Why are you not being called?')
        if value is None:
            value = ''
        self.__my_property = value

I'm fairly new to Python, and perhaps there's another way to do what I'm trying to do, but can someone explain why the mutator isn't being called when setattr(self, item, value) is called?

Is there another way to set a property via a string? I need the validation inside the mutator to be executed when setting the property value.

Do you actually define a property using your setter and getter functions? Your code does not show such a definition. How is Python supposed to know what getter and setter to use for my_property? — Sven Marnach
– Sven Marnach, Commented Mar 12, 2011 at 22:54
@Sven Marnach: Oops, forgot to add that in the example. But, Yes, I'm defining that in the actual code. — E-rich
– E-rich, Commented Mar 12, 2011 at 22:56

Claudiu · Accepted Answer · 2011-03-12 22:59:50Z

4

Works for me:

>>> class MyClass(object):
...   def get(self): return 10
...   def setprop(self, val): raise ValueError("hax%s"%str(val))
...   prop = property(get, setprop)
...
>>> i = MyClass()
>>> i.prop =4
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in setprop
ValueError: hax4
>>> i.prop
10
>>> setattr(i, 'prop', 12)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in setprop
ValueError: hax12

The code you pasted seems to do the same as mine, except that my class inherits from object, but that's cause I'm running Python 2.6 and I thought that in 2.7 all classes automatically inherit from object. Try that, though, and see if it helps.

To make it even clearer: try just doing myClass.my_property = 4. Does that raise an exception? If not then it's an issue with inheriting from object - properties only work for new-style classes, i.e. classes that inherit from object.

answered Mar 12, 2011 at 22:59

Claudiu

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2 Comments

Sven Marnach Over a year ago

Actually, the last paragraph is the solution: Python 2.7 still distinguishes between old-style and new-style classes.

E-rich Over a year ago

The issue was not specifying that it inherits from object. Thank you.

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