Detecting class attribute value change and then changing another class attribute

Question

Let's say I have a class called Number

class Number():
  def __init__(self,n):
    self.n=n
    self.changed=None
a = Number(8)
print(a.n) #prints 8
a.n=9
print(a.n) #prints 9

When the n class attribute changes, I want the changed class attribute to be changed to True.

changed to what? changed attribute should be changed to what? — Shivam Jha
– Shivam Jha, Commented Jul 2, 2020 at 21:10
Does this answer your question? Call Python Method on Class Attribute Change — user11837869
– user11837869, Commented Jul 2, 2020 at 21:12
This is an instance attribute, a class attribute is something entirely different. The answer is to make n a @property, so that code runs when it is assigned. — jasonharper
– jasonharper, Commented Jul 2, 2020 at 21:13
Does this answer your question? How do you change the value of one attribute by changing the value of another? (dependent attributes) — mkrieger1
– mkrieger1, Commented Jul 2, 2020 at 21:27

rdas · Accepted Answer · 2020-07-02 21:12:45Z

2

Possible with proper use of @propety

class Number():
  def __init__(self,n):
    self._n=n
    self._changed=None

  @property
  def n(self):
      return self._n

  @property
  def changed(self):
      return self._changed

  @n.setter
  def n(self, val):
      # while setting the value of n, change the 'changed' value as well
      self._n = val
      self._changed = True

a = Number(8)
print(a.n) #prints 8
a.n=9
print(a.n) #prints 9
print(a.changed)

Returns:

8
9
True

answered Jul 2, 2020 at 21:12

rdas

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2 Comments

Hans Musgrave Over a year ago

You wouldn't need a property for changed.

rdas Over a year ago

Added that to keep it consistent with what OP is expecting

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