why is my text not aligning properly in wxPython?

Edit: Although everything commented below works on windows, the first option would not work on, for example, Ubuntu due to maybe a bug. A previous post given in the comments indicate that the same problem is found in OSX.
In any case, the second option using vertical sizers works both in Ubuntu and windows so you could try it on OSX.

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Your text has the instruction to align in the way you want with wx.ALIGN... and, in fact, it is aligned. However the size of the StaticText is not that of the panel but just the size of the text. Having restricted its position to its own size, you can not see the diference between the alignment modes.

You have two options to solve the problem:

Option 1. Expand the StaticText widget size and position your text on it

You could expand the size of your StaticText widget using its size parameter. This is a bad solution except for fixed size parents or frames you are not going to change size or reuse in other applications. If the size of the widget containing the text changes then the relative position of the text will be also modified because its size stays fixed. So it is always better to organize your widgets by means of sizers.

The proportion of the available space the widget occupies in the sizer slot is given by the second parameter in sizer.Add() (0 is minimal size, 1 is full occupation):

sizer_2.Add(self.label_1, 0, 0, 0)

To see the text aligned in the panel as you want you have to tell the StaticText to expand to all the space available:

sizer_2.Add(self.label_1, 1, 0, 0)

Here you have the relevant code:

class MyFrame(wx.Frame):
    def __init__(self, *args, **kwds):
        wx.Frame.__init__(self, *args, **kwds)
        self.panel_1 = wx.Panel(self, -1)
        self.label_1 = wx.StaticText(self.panel_1, -1, "label_1", style=wx.ALIGN_RIGHT)
        self.panel_2 = wx.Panel(self, -1)
        self.label_2 = wx.StaticText(self.panel_2, -1, "label_2", style=wx.ALIGN_CENTRE)
        self.panel_3 = wx.Panel(self, -1)
        self.label_3 = wx.StaticText(self.panel_3, -1, "label_3")

        self.panel_1.SetBackgroundColour(wx.Colour(0, 255, 0))
        self.panel_2.SetBackgroundColour(wx.Colour(0, 255, 255))
        self.panel_3.SetBackgroundColour(wx.Colour(219, 112, 147))

        sizer_1 = wx.BoxSizer(wx.HORIZONTAL)
        sizer_2 = wx.BoxSizer(wx.HORIZONTAL)
        sizer_3 = wx.BoxSizer(wx.HORIZONTAL)
        sizer_4 = wx.BoxSizer(wx.HORIZONTAL)

        sizer_2.Add(self.label_1, 1, 0, 0)
        sizer_3.Add(self.label_2, 1, 0, 0)
        sizer_4.Add(self.label_3, 1, 0, 0)

        self.panel_1.SetSizer(sizer_2)
        self.panel_2.SetSizer(sizer_3)
        self.panel_3.SetSizer(sizer_4)

        sizer_1.Add(self.panel_1, 1, wx.EXPAND, 0)
        sizer_1.Add(self.panel_2, 1, wx.EXPAND, 0)
        sizer_1.Add(self.panel_3, 1, wx.EXPAND, 0)

        self.SetSizer(sizer_1)
        sizer_1.Fit(self)
        self.Layout()

Note the code is longer than needed in order to mimick your example with three panels. You obtain the same frame view using one only panel. In fact it could be simplified further not using panels and setting the StaticText directly on the sizer:

class MyFrame2(wx.Frame):
    def __init__(self, *args, **kwds):
        wx.Frame.__init__(self, *args, **kwds)
        self.label_1 = wx.StaticText(self, -1, "label_1", style=wx.ALIGN_RIGHT)
        self.label_2 = wx.StaticText(self, -1, "label_2", style=wx.ALIGN_CENTRE)
        self.label_3 = wx.StaticText(self, -1, "label_3")

        self.label_1.SetBackgroundColour(wx.Colour(127, 255, 0))
        self.label_2.SetBackgroundColour(wx.Colour(0, 255, 255))
        self.label_3.SetBackgroundColour(wx.Colour(219, 112, 147))

        sizer = wx.BoxSizer(wx.HORIZONTAL)
        sizer.Add(self.label_1, 1, wx.EXPAND, 0)
        sizer.Add(self.label_2, 1, wx.EXPAND, 0)
        sizer.Add(self.label_3, 1, wx.EXPAND, 0)
        self.SetSizer(sizer)
        sizer.Fit(self)
        self.Layout()

Option 2. Locate the widget itself in the desired position at the available space of the sizer. You could use the position parameter of StaticText for that purpose. But this would have the same problems indicated above for the use of size. So again you want to control the geometry of your views with sizers. You position the widget in the sizer using one of:

sizer_6.Add(self.label_5, 0, wx.ALIGN_RIGHT, 0)

or

sizer_7.Add(self.label_6, 0, wx.ALIGN_CENTER_HORIZONTAL, 0)

For some reason, for this to work you need a vertical BoxSizer (and in the same way, if you would like to use wx.ALIGN_CENTER_VERTICAL you will need an horizontal BoxSizer:

class MyFrame(wx.Frame):
    def __init__(self, *args, **kwds):
        wx.Frame.__init__(self, *args, **kwds)
        self.panel_4 = wx.Panel(self, -1)
        self.label_5 = wx.StaticText(self.panel_4, -1, "label_5")
        self.panel_5 = wx.Panel(self, -1)
        self.label_6 = wx.StaticText(self.panel_5, -1, "label_6")
        self.panel_6 = wx.Panel(self, -1)
        self.label_7 = wx.StaticText(self.panel_6, -1, "label_7")

        self.panel_4.SetBackgroundColour(wx.Colour(0, 255, 255))
        self.panel_5.SetBackgroundColour(wx.Colour(127, 255, 0))
        self.panel_6.SetBackgroundColour(wx.Colour(219, 112, 219))

        sizer_1 = wx.BoxSizer(wx.HORIZONTAL)
        sizer_8 = wx.BoxSizer(wx.VERTICAL)
        sizer_7 = wx.BoxSizer(wx.VERTICAL)
        sizer_6 = wx.BoxSizer(wx.VERTICAL)

        sizer_6.Add(self.label_5, 0, wx.ALIGN_RIGHT, 0)
        sizer_7.Add(self.label_6, 0, wx.ALIGN_CENTER_HORIZONTAL, 0)
        sizer_8.Add(self.label_7, 0, 0, 0)

        self.panel_4.SetSizer(sizer_6)
        self.panel_5.SetSizer(sizer_7)
        self.panel_6.SetSizer(sizer_8)

        sizer_1.Add(self.panel_4, 1, wx.EXPAND, 0)
        sizer_1.Add(self.panel_5, 1, wx.EXPAND, 0)
        sizer_1.Add(self.panel_6, 1, wx.EXPAND, 0)

        self.SetSizer(sizer_1)
        sizer_1.Fit(self)
        self.Layout()

This option implies a combination of panels and sizers that produces a code that is more difficult to simplify than that shown for the first option.