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?
This question is related to
python
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)
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)
Source: Stackoverflow.com