子类没有在Python 2.x中初始化父类

Question

I'm having some problems initialising the parent class inside one subclass in Python 2. What I'm trying to do is override parent class attributes with properties in the child class.

Somehow, when I don't use the method _update_rect(self) in the child class setters (eg _set_grab(self, ps_value) and _set_grab(self, ps_value) ), everything works as expected. But as soon as I use it the initialization of the parent class fails (the print '+++ END GfxObject initialisation' is not reached) and I get AttributeError: 'GfxRect' object has no attribute '_s_grab' .

As I said at the beginning, the code looks correct to me and the problematic method _update_rect only uses real attributes of itself so I have no idea where the error comes. I could avoid using inheritance in the child class as a workaround but I really want to understand what the problem is.

Regards and thanks in advance.

# Extra code to simplify test code
#=================================
class pygame:
    class Rect:
        def __init__(self, x, y, w, h):
            self.x = x
            self.y = y
            self.w = w
            self.h = h


# Classes
#========
class GfxObject(object):
    """
    Generic Graphical Object which is the parent class of all the sub-classes below.
    """
    def __init__(self):
        self.i_x = 0
        self.i_y = 0
        self.s_grab = 'nw'


class GfxRect(GfxObject):
    """
    Class to draw a rectangle.
    """
    def __init__(self):
        print '--- START GfxObject initialisation'
        super(GfxRect, self).__init__()
        print '+++ END GfxObject initialisation'

        self._i_x = 0
        self._s_grab = 'nw'
        self._o_rect = None

        print self

        self._update_rect()

    def __str__(self):
        return unicode(self).encode('utf8')

    def __unicode__(self):
        u_out = u'<GfxRect>\n'
        u_out += u'  .i_x:     %s\n' % self.i_x
        u_out += u'  ._i_x:    %s\n' % self._i_x
        u_out += u'  .s_grab:  %s\n' % self.s_grab
        u_out += u'  ._s_grab: %s\n' % self._s_grab
        return u_out

    def _get_grab(self):
        return self._s_grab

    def _get_x(self):
        return self._i_x

    def _set_grab(self, ps_value):
        self._s_grab = ps_value
        #self._update_rect()
        self._b_redraw = True

    def _set_x(self, i_value):
        self._i_x = i_value
        self._update_rect()
        self._b_redraw = True

    def _update_rect(self):
        """
        Method to update the pygame rectangle object.
        :return:
        """

        # [1/?] Calculating the deltas for (x,y) based on the grab position
        #------------------------------------------------------------------
        if self._s_grab == 'nw':
            i_dx = 0
        elif self._s_grab == 'n':
            i_dx = -800 / 2
        else:
            raise ValueError('Invalid grab value "%s"' % self._s_grab)

        # [2/?] Applying the deltas
        #--------------------------
        i_x = self._i_x + i_dx

        self._o_rect = pygame.Rect(i_x, 0, 800, 600)

    i_x = property(fget=_get_x, fset=_set_x)
    s_grab = property(fget=_get_grab, fset=_set_grab)


# Main code
#==========
if __name__ == '__main__':
    o_progbar = GfxRect()

UPDATE: Moving the initialisation of the parent class in the child class after the internal properties seems to fix the problem which it's even more weird for me.

Before (doesn't work)

def __init__(self):
    print '--- START GfxObject initialisation'
    super(GfxRect, self).__init__()
    print '+++ END GfxObject initialisation'

    self._i_x = 0
    self._s_grab = 'nw'
    self._o_rect = None

    self._update_rect()

After (works)

def __init__(self):
    self._i_x = 0
    self._s_grab = 'nw'
    self._o_rect = None

    print '--- START GfxObject initialisation'
    super(GfxRect, self).__init__()
    print '+++ END GfxObject initialisation'

    self._update_rect()

...but something wrong seems to be happening under the hood. If I add print 'child class "_update_rect" called' to the _update_rect method, I get this output when running the script:

--- START GfxObject initialisation
child class "_update_rect" called  <-- ERROR!?
child class "_update_rect" called  <-- ERROR!?
+++ END GfxObject initialisation
child class "_update_rect" called  <-- this is correct
...

Which means the parent class is calling the child methods when being initialised!?

UPDATE 2: It seems this is the workflow when initialising the child class.

[1] Child.__init__()
[2] Parent.__init__()
[3] self.i_x = 0
[4] Child._set_x(0)
[5] Child._update_rect()
[6] Child._s_grab = 'foo'

The problem appears in step [6] because ._s_grab attribute hasn't been created yet since the initialisation of the Child class is still initialising the Parent class. To me, it's counter-intuitive (and I would say that it's even a bug) the step [3]-[4] when setting the attribute .i_x of the Parent class triggers the property of the Child class.

By moving the initialisation of the parent class at the end of the child class or adding the missing attribute as a Parent class global (not to the instances), the problem disappear.

Answer 1

Your issue is that you're trying to initialise non-existing class properties/variables (you're calling self.i_x , but you've initialised self._i_x , for example) with a method call.

Changing GfxObject to:

class GfxObject(object):
    """
    Generic Graphical Object which is the parent class of all the sub-classes below.
    """
    def __init__(self):
        self._i_x = 0
        self._i_y = 0
        self._s_grab = 'nw'

Executed script on my machine. Result output:

 $ python2 issue.py --- START GfxObject initialisation +++ END GfxObject initialisation <GfxRect> .i_x: 0 ._i_x: 0 .s_grab: nw ._s_grab: nw 

EDIT

Funny thing, once I moved i_x and s_grab from GfxObject() 's init, your code worked as a charm. Basically I only changed that class to this:

class GfxObject(object):
    """
    Generic Graphical Object which is the parent class of all the sub-classes below.
    """
    i_x = None
    s_grab = None
    def __init__(self):
    #    self.i_x = None
    #    self.s_grab = None
        #self.s_grab = 'nw'
        pass

So, it seems that you're expperiencing same issue I had with Python3 and @property decorator - if I didn't set an object to None or some other value before defining it as a property, it threw no attribute/not defined errors first time I'd try to use that getter/setter.