Subclassing Built-In Types Is Tricky



In [5]:

    
class DoppelDict(dict):
    def __setitem__(self, key, value):
        super().__setitem__(key, [value] * 2)



In [6]:

    
dd = DoppelDict(one=1)



In [7]:

    
dd









    Out[7]:





{'one': 1}



In [8]:

    
dd['two'] = 2



In [9]:

    
dd









    Out[9]:





{'one': 1, 'two': [2, 2]}



In [10]:

    
dd.update(three=3)



In [11]:

    
dd









    Out[11]:





{'one': 1, 'three': 3, 'two': [2, 2]}



In [12]:

    
class AnswerDict(dict):
    def __getitem__(self, key):
        return 42



In [13]:

    
ad = AnswerDict(a='foo')
ad['a']









    Out[13]:





42



In [14]:

    
d = {}
d.update(ad)
d['a']









    Out[14]:





'foo'



In [15]:

    
d









    Out[15]:





{'a': 'foo'}



In [16]:

    
import collections



In [17]:

    
class DoppelDict2(collections.UserDict):
    def __setitem__(self, key, value):
        super().__setitem__(key, [value] * 2)



In [18]:

    
dd = DoppelDict2(one=1)
dd









    Out[18]:





{'one': [1, 1]}



In [19]:

    
dd['two'] = 2
dd









    Out[19]:





{'two': [2, 2], 'one': [1, 1]}



In [20]:

    
dd.update(three=3)
dd









    Out[20]:





{'three': [3, 3], 'two': [2, 2], 'one': [1, 1]}



In [21]:

    
class AnswerDict2(collections.UserDict):
    def __getitem__(self, key):
        return 42



In [22]:

    
ad = AnswerDict2(a='foo')
ad['a']









    Out[22]:





42



In [23]:

    
d = {}
d.update(ad)
d['a']









    Out[23]:





42



In [24]:

    
d









    Out[24]:





{'a': 42}

Multiple Inheritance and Method Resolution Order



In [25]:

    
class A:
    def ping(self):
        print('ping', self)



In [26]:

    
class B(A):
    def pong(self):
        print('pong:', self)



In [27]:

    
class C(A):
    def pong(self):
        print('PONG:', self)



In [39]:

    
class D(B, C):
    
    def ping(self):
        super().ping()
        print('post-ping:', self)
        
    def pingpong(self):
        self.ping()
        super().ping()
        self.pong()
        super().pong()
        C.pong(self)



In [40]:

    
d = D()
d.pong()









    



pong: <__main__.D object at 0x000000AD5FC89B00>



In [41]:

    
C.pong(d)









    



PONG: <__main__.D object at 0x000000AD5FC89B00>



In [42]:

    
D.__mro__









    Out[42]:





(__main__.D, __main__.B, __main__.C, __main__.A, object)



In [43]:

    
d.ping()









    



ping <__main__.D object at 0x000000AD5FC89B00>
post-ping: <__main__.D object at 0x000000AD5FC89B00>



In [44]:

    
d.pingpong()









    



ping <__main__.D object at 0x000000AD5FC89B00>
post-ping: <__main__.D object at 0x000000AD5FC89B00>
ping <__main__.D object at 0x000000AD5FC89B00>
pong: <__main__.D object at 0x000000AD5FC89B00>
pong: <__main__.D object at 0x000000AD5FC89B00>
PONG: <__main__.D object at 0x000000AD5FC89B00>



In [45]:

    
bool.__mro__









    Out[45]:





(bool, int, object)



In [46]:

    
def print_mro(cls):
    print(', '.join(c.__name__ for c in cls.__mro__))



In [47]:

    
print_mro(bool)









    



bool, int, object



In [48]:

    
import numbers
print_mro(numbers.Integral)









    



Integral, Rational, Real, Complex, Number, object



In [49]:

    
import io
print_mro(io.BytesIO)









    



BytesIO, _BufferedIOBase, _IOBase, object



In [50]:

    
print_mro(io.TextIOWrapper)









    



TextIOWrapper, _TextIOBase, _IOBase, object



In [51]:

    
import tkinter
print_mro(tkinter.Text)









    



Text, Widget, BaseWidget, Misc, Pack, Place, Grid, XView, YView, object

Multiple Inheritance in the Real World



In [52]:

    
import tkinter



In [53]:

    
print_mro(tkinter.Toplevel)









    



Toplevel, BaseWidget, Misc, Wm, object



In [54]:

    
print_mro(tkinter.Widget)









    



Widget, BaseWidget, Misc, Pack, Place, Grid, object



In [55]:

    
print_mro(tkinter.Button)









    



Button, Widget, BaseWidget, Misc, Pack, Place, Grid, object



In [56]:

    
print_mro(tkinter.Entry)









    



Entry, Widget, BaseWidget, Misc, Pack, Place, Grid, XView, object



In [57]:

    
print_mro(tkinter.Text)









    



Text, Widget, BaseWidget, Misc, Pack, Place, Grid, XView, YView, object



In [ ]: