Python setter does not change variable

Question

May be I do not completely understand the concept of properties in python, but I am confused by the behaviour of my Python program.

I have a class like this:

class MyClass():
  def __init__(self, value):
    self._value = value

  @property
  def value(self):
    return self._value

  @value.setter
  def value(self, value):
    self._value = value

What I would expect is, that calling MyClass.value = ... changes the content of _value. But what actually happened is this:

my_class = MyClass(1)
assert my_class.value == 1 # true
assert my_class._value == 1 # true

my_class.value = 2

assert my_class.value == 2 # true
assert my_class._value == 2 # false! _value is still 1

Did I make a mistake while writing the properties or is this really the correct behaviour? I know that I should not call my_class._value for reading the value, but nevertheless I would expect that it should work, anyway. I am using Python 2.7.

falsetru · Accepted Answer · 2014-07-02 15:18:41Z

2

The class should inherit object class (in other word, the class should be new-style class) to use value.setter. Otherwise setter method is not called.

class MyClass(object):
              ^^^^^^

answered Jul 2, 2014 at 15:18

falsetru

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