K
Kent Johnson
Are there any best practice guidelines for when to use
super(Class, self).__init__()
vs
Base.__init__(self)
to call a base class __init__()?
The super() method only works correctly in multiple inheritance when the
base classes are written to expect it, so "Always use super()" seems
like bad advice. OTOH sometimes you need super() to get correct
behaviour. ISTM "Only use super() when you know you need it" might be
the best advice. Is there any conventional wisdom on this?
The question arises from a naive use of super() in a post on the tutor
list. This code gives an AttributeError because Base.__init__() is never
called:
import threading
class Base(object):
def __init__(self):
self.x = 1
class Derived(threading.Thread, Base):
def __init__(self):
super(Derived, self).__init__()
d=Derived()
d.x
If the order of base classes is reversed, the reference to d.x works but
of course threading.Thread.__init__() is never called.
1. One way to fix the code is to call Base.__init__() and
threading.Thread.__init__() explicitly in Derived.__init__().
2. Another fix is for Base.__init__() to call super(Base,
self).__init__() and to list Base first in the list of base classes.
This is fragile - it depends on the order of base classes and adding
another base class would break it.
3. A third fix might be to change both Base and threading.Thread() to
call super(...).__init__(). This might break existing code that is
written in the style of fix 1 (calling both base class __init__()
methods explicitly).
I prefer the first fix, it is explicit and fairly robust - it works if
the order of bases is changed, and it's pretty clear from the body of
Derived.__init__() that if you add another base class, you should change
__init__().
Any other opinions? Any consensus about the "best" way to do this?
BTW I understand what super() does, I know why the original code is
broken, I'm not asking for help with that. I'm wondering what others
think best practices are.
Thanks,
Kent
super(Class, self).__init__()
vs
Base.__init__(self)
to call a base class __init__()?
The super() method only works correctly in multiple inheritance when the
base classes are written to expect it, so "Always use super()" seems
like bad advice. OTOH sometimes you need super() to get correct
behaviour. ISTM "Only use super() when you know you need it" might be
the best advice. Is there any conventional wisdom on this?
The question arises from a naive use of super() in a post on the tutor
list. This code gives an AttributeError because Base.__init__() is never
called:
import threading
class Base(object):
def __init__(self):
self.x = 1
class Derived(threading.Thread, Base):
def __init__(self):
super(Derived, self).__init__()
d=Derived()
d.x
If the order of base classes is reversed, the reference to d.x works but
of course threading.Thread.__init__() is never called.
1. One way to fix the code is to call Base.__init__() and
threading.Thread.__init__() explicitly in Derived.__init__().
2. Another fix is for Base.__init__() to call super(Base,
self).__init__() and to list Base first in the list of base classes.
This is fragile - it depends on the order of base classes and adding
another base class would break it.
3. A third fix might be to change both Base and threading.Thread() to
call super(...).__init__(). This might break existing code that is
written in the style of fix 1 (calling both base class __init__()
methods explicitly).
I prefer the first fix, it is explicit and fairly robust - it works if
the order of bases is changed, and it's pretty clear from the body of
Derived.__init__() that if you add another base class, you should change
__init__().
Any other opinions? Any consensus about the "best" way to do this?
BTW I understand what super() does, I know why the original code is
broken, I'm not asking for help with that. I'm wondering what others
think best practices are.
Thanks,
Kent
The threading.Thread class does not properly call