Instance variables vs. class variables in Python

python class variables static member
deamon picture deamon · Apr 26, 2010 · Viewed 97.4k times · Source

I have Python classes, of which I need only one instance at runtime, so it would be sufficient to have the attributes only once per class and not per instance. If there would be more than one instance (which won't happen), all instance should have the same configuration. I wonder which of the following options would be better or more "idiomatic" Python.

Class variables:

class MyController(Controller):

  path = "something/"
  children = [AController, BController]

  def action(self, request):
    pass

Instance variables:

class MyController(Controller):

  def __init__(self):
    self.path = "something/"
    self.children = [AController, BController]

  def action(self, request):
    pass

Answer

Alex Martelli picture Alex Martelli · Apr 26, 2010

If you have only one instance anyway, it's best to make all variables per-instance, simply because they will be accessed (a little bit) faster (one less level of "lookup" due to the "inheritance" from class to instance), and there are no downsides to weigh against this small advantage.

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