So I was following Python's Super Considered Harmful, and went to test out his examples.
However, Example 1-3, which is supposed to show the correct way of calling super when handling __init__ methods that expect different arguments, flat-out doesn't work.
This is what I get:
~ $ python example1-3.py
MRO: ['E', 'C', 'A', 'D', 'B', 'object']
E arg= 10
C arg= 10
A
D arg= 10
B
Traceback (most recent call last):
File "Download/example1-3.py", line 27, in <module>
E(arg=10)
File "Download/example1-3.py", line 24, in __init__
super(E, self).__init__(arg, *args, **kwargs)
File "Download/example1-3.py", line 14, in __init__
super(C, self).__init__(arg, *args, **kwargs)
File "Download/example1-3.py", line 4, in __init__
super(A, self).__init__(*args, **kwargs)
File "Download/example1-3.py", line 19, in __init__
super(D, self).__init__(arg, *args, **kwargs)
File "Download/example1-3.py", line 9, in __init__
super(B, self).__init__(*args, **kwargs)
TypeError: object.__init__() takes no parameters
It seems that object itself violates one of the best practices mentioned in the document, which is that methods which use super must accept *args and **kwargs.
Now, obviously Mr. Knight expected his examples to work, so is this something that was changed in recent versions of Python? I checked 2.6 and 2.7, and it fails on both.
So what is the correct way to deal with this problem?
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Sometimes two classes may have some parameter names in common. In that case, you can't pop the key-value pairs off of **kwargs or remove them from *args. Instead, you can define a Base class which unlike object, absorbs/ignores arguments:
class Base(object):
def __init__(self, *args, **kwargs): pass
class A(Base):
def __init__(self, *args, **kwargs):
print "A"
super(A, self).__init__(*args, **kwargs)
class B(Base):
def __init__(self, *args, **kwargs):
print "B"
super(B, self).__init__(*args, **kwargs)
class C(A):
def __init__(self, arg, *args, **kwargs):
print "C","arg=",arg
super(C, self).__init__(arg, *args, **kwargs)
class D(B):
def __init__(self, arg, *args, **kwargs):
print "D", "arg=",arg
super(D, self).__init__(arg, *args, **kwargs)
class E(C,D):
def __init__(self, arg, *args, **kwargs):
print "E", "arg=",arg
super(E, self).__init__(arg, *args, **kwargs)
print "MRO:", [x.__name__ for x in E.__mro__]
E(10)
yields
MRO: ['E', 'C', 'A', 'D', 'B', 'Base', 'object']
E arg= 10
C arg= 10
A
D arg= 10
B
Note that for this to work, Base must be the penultimate class in the MRO.
If you're going to have a lot of inheritence (that's the case here) I suggest you to pass all parameters using **kwargs, and then pop them right after you use them (unless you need them in upper classes).
class First(object):
def __init__(self, *args, **kwargs):
self.first_arg = kwargs.pop('first_arg')
super(First, self).__init__(*args, **kwargs)
class Second(First):
def __init__(self, *args, **kwargs):
self.second_arg = kwargs.pop('second_arg')
super(Second, self).__init__(*args, **kwargs)
class Third(Second):
def __init__(self, *args, **kwargs):
self.third_arg = kwargs.pop('third_arg')
super(Third, self).__init__(*args, **kwargs)
This is the simplest way to solve those kind of problems.
third = Third(first_arg=1, second_arg=2, third_arg=3)
As explained in Python's super() considered super, one way is to have class eat the arguments it requires, and pass the rest on. Thus, when the call-chain reaches object, all arguments have been eaten, and object.__init__ will be called without arguments (as it expects). So your code should look like this:
class A(object):
def __init__(self, *args, **kwargs):
print "A"
super(A, self).__init__(*args, **kwargs)
class B(object):
def __init__(self, *args, **kwargs):
print "B"
super(B, self).__init__(*args, **kwargs)
class C(A):
def __init__(self, arg, *args, **kwargs):
print "C","arg=",arg
super(C, self).__init__(*args, **kwargs)
class D(B):
def __init__(self, arg, *args, **kwargs):
print "D", "arg=",arg
super(D, self).__init__(*args, **kwargs)
class E(C,D):
def __init__(self, arg, *args, **kwargs):
print "E", "arg=",arg
super(E, self).__init__(*args, **kwargs)
print "MRO:", [x.__name__ for x in E.__mro__]
E(10, 20, 30)
Source: Stackoverflow.com