python - When is "self" required?

Question

I have been using classes for only a short while and when I write a method, I make all variables reference self, e.g. self.foo.

However, I'm looking through the wxPython in Action book and notice that "self" isn't used all the time. For example:

 import wx
 class TextFrame(wx.Frame):
    def __init__(self):
        wx.Frame.__init__(self, None, -1, 'Text Entry Example',
            size=(300, 100))
        panel = wx.Panel(self, -1)
        basicLabel = wx.StaticText(panel, -1, "Basic Control:")
        basicText = wx.TextCtrl(panel, -1, "I've entered some text!",
            size=(175, -1))
        basicText.SetInsertionPoint(0)
        pwdLabel = wx.StaticText(panel, -1, "Password:")
        pwdText = wx.TextCtrl(panel, -1, "password", size=(175, -1),
            style=wx.TE_PASSWORD)
        sizer = wx.FlexGridSizer(cols=2, hgap=6, vgap=6)
        sizer.AddMany([basicLabel, basicText, pwdLabel, pwdText])
        panel.SetSizer(sizer)

The one below does use "self".

import wx
class ButtonFrame(wx.Frame):
    def __init__(self):
        wx.Frame.__init__(self, None, -1, 'Button Example',
            size=(300, 100))
        panel = wx.Panel(self, -1)
        self.button = wx.Button(panel, -1, "Hello", pos=(50, 20))
        self.Bind(wx.EVT_BUTTON, self.OnClick, self.button)
        self.button.SetDefault()
    def OnClick(self, event):
        self.button.SetLabel("Clicked")

If I remember correctly, "self" is reference to a particular instance of the class, so when is it not necessary? Is there a general rule of thumb?

Answer 1

You use self.attribute to reference an attribute of your current instance.

You use wx.Frame.__init__() to reference a method of the parent class.

You don't use self if you only reference a local name (variable) of the method (function) you are in.

These are not "rules of thumb," because there are no exceptions.

---

What is probably confusing you in this particular example is that panel seems to be only a local name in the constructor, so it looks like the panel would disappear, once your constructor returns.

If you look at the documentation to wx.Panel, though, you will see that its constructor attaches the panel to the parent window, so it will continue to exist, even after the constructor returns.

Magic :)