为什么不能在 __init__ 中调用对象属性？

Question

I'm trying to understand why you can call an object's attribute within a class' method, but not within it's constructor. For example:

class Panel1(wx.Panel):

    #include Frame in the constructor
    def __init__(self, parent, frame):
        wx.Panel.__init__(self, parent)
        #set up so objects assigned in Frame (and it's children) can be called
        self.frame = frame
        button = wx.Button(self, label='Test')
        #button.Bind(wx.EVT_BUTTON, lambda event: self.onButton(event))
        button.Bind(wx.EVT_BUTTON, self.onButton)

        pprint(vars(self.frame))


    def onButton(self, event):
        print("Button pressed.")
        pprint(vars(self.frame))
        #the following fails in the constructor as panel2 for some reason is not callable
        a = self.frame.panel2.a
        print("123: " + str(a))

Could someone point me to a link that explains why this isn't possible in the constructor?

Thanks!

*****EDIT****

Below is functional code to help explain my question better. If you try to call Panel2 attributes via the parent within Panel1's constructor, it fails. But it works fine when done inside Panel1's onButton method. I'd like to understand why.

import wx
from pprint import pprint

class Panel1(wx.Panel):

    #include Frame in the constructor
    def __init__(self, parent, frame):
        wx.Panel.__init__(self, parent)

        self.frame = frame
        self.strPanel1 = "Panel1 string"
        self.intPanel1 = 0

        button = wx.Button(self, label='Test')
        button.Bind(wx.EVT_BUTTON, self.onButton)

        self.textbox = wx.TextCtrl(self)

        sizer = wx.BoxSizer(wx.VERTICAL)
        self.SetSizer(sizer)
        sizer.Add(self.textbox, 0, wx.ALL, 5)
        sizer.Add(button, 0, wx.ALL, 5)
        #pprint self.frame attributes fail here

    def onButton(self, event):
        #but not here!?!
        print("Panel1 button pressed.")
        pprint(vars(self.frame))
        Panel2str = self.frame.panel2.strPanel2
        print(Panel2str)
        Panel2int = self.frame.panel2.intPanel2
        print(str(Panel2int))
        #Panel2 button press counter
        self.frame.panel2.intPanel2 += 1
        self.frame.panel2.trigger()

    def trigger(self):
        print("Panel1 has been triggered")

class Panel2(wx.Panel):

    #include Frame in the constructor
    def __init__(self, parent, frame):
        wx.Panel.__init__(self, parent)

        self.frame = frame
        self.strPanel2 = "Panel2 string"
        self.intPanel2 = 0

        button = wx.Button(self, label='Test')
        button.Bind(wx.EVT_BUTTON, self.onButton)

        self.textbox = wx.TextCtrl(self)

        sizer = wx.BoxSizer(wx.VERTICAL)
        self.SetSizer(sizer)
        sizer.Add(self.textbox, 0, wx.ALL, 5)
        sizer.Add(button, 0, wx.ALL, 5)
        #pprint self.frame attributes fail here

    def onButton(self, event):
        #but not here!?!
        print("Panel2 button pressed.")
        pprint(vars(self.frame))
        Panel1str = self.frame.panel1.strPanel1
        print(Panel1str)
        Panel1int = self.frame.panel1.intPanel1
        print(str(Panel1int))
        #Panel1 button press counter
        self.frame.panel1.intPanel1 += 1
        self.frame.panel1.trigger()

    def trigger(self):
        print("Panel2 has been triggered")

class Frame(wx.Frame):

    def __init__(self):
        wx.Frame.__init__(self, None, title="My Frame")

        #Spliiting the frame
        splitter = wx.SplitterWindow(self)

        #Send frame to children
        self.panel1 = Panel1(splitter, self)
        self.panel2 = Panel2(splitter, self) 

        splitter.SplitVertically(self.panel1, self.panel2, 0)
        splitter.SetMinimumPaneSize(200)

        sizer = wx.BoxSizer(wx.VERTICAL)
        sizer.Add(splitter, 1, wx.EXPAND)

        self.SetSizer(sizer)


if __name__ == '__main__':
    app = wx.App()
    frame = Frame()
    frame.Show()
    frame.Centre()
    app.MainLoop()

Answer 1

Below is functional code to help explain my question better. If you try to call Panel2 attributes via the parent within Panel1's constructor, it fails.

It fails, because at the time your Panel1.__init__ is called, frame.panel2 is not set yet. Look at your Frame class:

class Frame(wx.Frame):

    def __init__(self):
        wx.Frame.__init__(self, None, title="My Frame")

        #Spliiting the frame
        splitter = wx.SplitterWindow(self)

        #Send frame to children
        self.panel1 = Panel1(splitter, self)
        self.panel2 = Panel2(splitter, self)

        ...

When your Panel1 is initialized, panel2 is not set yet. It will only be set once Panel1 is done initializing and has been assigned to frame.panel1 . If you absolutely need to access panel2 into Panel1.__init__ , then you could make it so that your Panel1 take care of it. For example:

class Panel1:
    def __init__(self, frame):
        # Initialize panel2 and assign it to our frame
        frame.panel2 = Panel2(frame)

        # Now we can do whatever with panel2
        frame.panel2

class Panel2:
    def __init__(self, frame):
        pass

class Frame:
    def __init__(self):
        # This initialize panel1, and internally initialize panel2
        self.panel1 = Panel1(self)


frame = Frame()