A private list variable is inadvertently shared between instance objects

Question

I created many instances of a PlotHandler class. An instance must keep it\'s variables private. But the way I managed them led to a hard to detect problem,

Answer 1

Class attributes are shared between instances. If you want to define an instance attribute (so each object have its own reference to the variable) you have to define it in __init__

class PlotHandler(wx.Frame):
    __crop_section = None
    __projection   = None
    __crop_xcord   = None

    def __init__(self, **kwargs):
        self._band_data = [] #THIS IS NOT SHARED

Answer 2

See this question, this one, and tons of other stuff you can find by googling "Python class variables shared", "Python FAQ class variables", etc.

The short answer is: variables defined directly in the class body are class variables, not instance variables, and are thus shared among instances of the class. If you want instance variables you must assign them from within a method, where you have access to self.