Observer Pattern (Python recipe) by Jørgen Cederberg

ActiveState Code (http://code.activestate.com/recipes/131499/)

Tightening up... I would humbly suggest removing the Observer class, as it adds no value to the program. Also modifying the attach command to help avoid double additions would be a good thing:

class Subject:
    def attach(self, observer):
        if not observer in self._observers:
            self._observers.append(observer)

Removed observer class. Based on the comment, I removed the Observer Class. Additionally I changed the try-except clause, to except ValueError instead of all errors.

I think it was wrong to remove the Observer class. Observer was a kind of interface that documented the behaviour of an observer. Of course, in practice it isn't needed _because this is python and it is a dynamic language_, but it makes it clearer to the reader/maintainer.

subclassing necessary? I disagree, forcing observers to be subclassed from an IObserver class is unecessary. As long as your Subject class is well documented, there shouldn't be any confusion. If you really can't trust the observers, why not add extra checks to the attach method to enforce the contract.

class Subject:
    def attach(self, observer):
        # must have update method
        assert hasattr(observer,'update')
        # update must accept at least one argument
        assert observer.update.func_code.co_argcount > 0
        if not observer in self._observers:
            self._observers.append(observer)

Those checks don't cover every bad observer, but i'm sure it's possible with some extra work.

Here's an update, 9 years later, to take advantage of some newer language features added to python since you wrote your post.

class Subject(object):
  def __init__(self):
    self._observers = []

  def attach(self, observer):
    if not observer in self._observers:
      self._observers.append(observer)

  def detach(self, observer):
    try:
      self._observers.remove(observer)
    except ValueError:
      pass

  def notify(self, modifier=None):
    for observer in self._observers:
      if modifier != observer:
        observer.update(self)


# Example usage
class Data(Subject):
  def __init__(self, name=''):
    super(Data, self).__init__()
    self.name = name
    self._data = 0

  @property
  def data(self):
    return self._data

  @data.setter
  def data(self, value):
    self._data = value
    self.notify()



class HexViewer(object):
  def update(self, subject):
    print 'HexViewer: Subject %s has data 0x%x' % (subject.name, subject.data)


class DecimalViewer(object):
  def update(self, subject):
    print 'DecimalViewer: Subject %s has data %d' % (subject.name, subject.data)


# Example usage...
def main():
  data1 = Data('Data 1')
  data2 = Data('Data 2')
  view1 = DecimalViewer()
  view2 = HexViewer()
  data1.attach(view1)
  data1.attach(view2)
  data2.attach(view2)
  data2.attach(view1)

  print "Setting Data 1 = 10"
  data1.data = 10
  print "Setting Data 2 = 15"
  data2.data = 15
  print "Setting Data 1 = 3"
  data1.data = 3
  print "Setting Data 2 = 5"
  data2.data = 5
  print "Detach HexViewer from data1 and data2."
  data1.detach(view2)
  data2.detach(view2)
  print "Setting Data 1 = 10"
  data1.data = 10
  print "Setting Data 2 = 15"
  data2.data = 15

if __name__ == '__main__':
  main()