Re: Multiple inheritance and a broken super() chain
On 7/3/23 12:13, Mats Wichmann via Python-list wrote: To natter on a bit, and possibly muddy the waters even further... Now, as I see it, from the super()'s point of view, there are two inheritance chains, one starting at Left and the other at Right. But *Right.__init__()* is called twice. No: each class has just a single inheritance chain, built up when the class object is constructed, to avoid going insane. Yes, the chain for Left and for Right are different, but you're not instantiating *either* of those classes when you make a Bottom, so they don't matter here. You're just filling out a Bottom: it looks for init, finds it, and so would stop hunting - but then the super() call there sends it to the next class object in the chain to look for the method you told it to (also init), where it would stop since it found it, except you sent it on with another super(), and so on. Python is a bit... different :) (compared to other languages with class definitions) -- https://mail.python.org/mailman/listinfo/python-list
Re: Multiple inheritance and a broken super() chain
On Tue, 4 Jul 2023 at 03:39, Peter Slížik via Python-list wrote: > > Hello. > > The legacy code I'm working with uses a classic diamond inheritance. Let me > call the classes *Top*, *Left*, *Right*, and *Bottom*. > This is a trivial textbook example. The classes were written in the > pre-super() era, so all of them initialized their parents and Bottom > initialized both Left and Right in this order. > > The result was expected: *Top* was initialized twice: > > Top.__init__() Left.__init__() Top.__init__() Right.__init__() > Bottom.__init__() What happens when Top is initialized twice? This seems like a problem waiting to happen, and when you moved to using super(), you more than likely simplified things and fixed things. There are not two instances of Top to be initialized, only one. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Multiple inheritance and a broken super() chain
On 7/3/23 12:01, Richard Damon via Python-list wrote:
On 7/3/23 1:38 PM, Peter Slížik via Python-list wrote:
Hello.
The legacy code I'm working with uses a classic diamond inheritance.
Let me
call the classes *Top*, *Left*, *Right*, and *Bottom*.
This is a trivial textbook example. The classes were written in the
pre-super() era, so all of them initialized their parents and Bottom
initialized both Left and Right in this order.
The result was expected: *Top* was initialized twice:
Top.__init__() Left.__init__() Top.__init__() Right.__init__()
Bottom.__init__()
Now I replaced all parent init calls with *super()*. After this, Top was
initialized only once.
Top.__init__() Right.__init__() Left.__init__() Bottom.__init__()
But at this point, I freaked out. The code is complex and I don't have
the
time to examine its inner workings. And before, everything worked
correctly
even though Top was initialized twice. So I decided to break the
superclass
chain and use super() only in classes inheriting from a single parent. My
intent was to keep the original behavior but use super() where
possible to
make the code more readable.
class Top:
def __init__(self):
print("Top.__init__()")
class Left(Top):
def __init__(self):
super().__init__()
print("Left.__init__()")
class Right(Top):
def __init__(self):
super().__init__()
print("Right.__init__()")
class Bottom(Left, Right):
def __init__(self):
Left.__init__(self) # Here I'm calling both parents manually
Right.__init__(self)
print("Bottom.__init__()")
b = Bottom()
The result has surprised me:
Top.__init__() Right.__init__() Left.__init__() Top.__init__()
Right.__init__() Bottom.__init__()
Now, as I see it, from the super()'s point of view, there are two
inheritance chains, one starting at Left and the other at Right. But
*Right.__init__()* is called twice. What's going on here?
Thanks,
Peter
Because the MRO from Bottom is [Bottom, Left, Right, Top] so super() in
Left is Right. It doesn't go to Top as the MRO knows that Right should
go to Top, so Left needs to go to Right to init everything, and then
Bottom messes things up by calling Right again.
And you can see this a little better in your toy example by using begin
*and* end prints in your initializers.
Also, you might find that because of the MRO, super() in your Bottom
class would actually give you what you want.
And if not sure, print out Bottom.__mro__
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Re: Multiple inheritance and a broken super() chain
On 7/3/23 1:38 PM, Peter Slížik via Python-list wrote:
Hello.
The legacy code I'm working with uses a classic diamond inheritance. Let me
call the classes *Top*, *Left*, *Right*, and *Bottom*.
This is a trivial textbook example. The classes were written in the
pre-super() era, so all of them initialized their parents and Bottom
initialized both Left and Right in this order.
The result was expected: *Top* was initialized twice:
Top.__init__() Left.__init__() Top.__init__() Right.__init__()
Bottom.__init__()
Now I replaced all parent init calls with *super()*. After this, Top was
initialized only once.
Top.__init__() Right.__init__() Left.__init__() Bottom.__init__()
But at this point, I freaked out. The code is complex and I don't have the
time to examine its inner workings. And before, everything worked correctly
even though Top was initialized twice. So I decided to break the superclass
chain and use super() only in classes inheriting from a single parent. My
intent was to keep the original behavior but use super() where possible to
make the code more readable.
class Top:
def __init__(self):
print("Top.__init__()")
class Left(Top):
def __init__(self):
super().__init__()
print("Left.__init__()")
class Right(Top):
def __init__(self):
super().__init__()
print("Right.__init__()")
class Bottom(Left, Right):
def __init__(self):
Left.__init__(self) # Here I'm calling both parents manually
Right.__init__(self)
print("Bottom.__init__()")
b = Bottom()
The result has surprised me:
Top.__init__() Right.__init__() Left.__init__() Top.__init__()
Right.__init__() Bottom.__init__()
Now, as I see it, from the super()'s point of view, there are two
inheritance chains, one starting at Left and the other at Right. But
*Right.__init__()* is called twice. What's going on here?
Thanks,
Peter
Because the MRO from Bottom is [Bottom, Left, Right, Top] so super() in
Left is Right. It doesn't go to Top as the MRO knows that Right should
go to Top, so Left needs to go to Right to init everything, and then
Bottom messes things up by calling Right again.
--
Richard Damon
--
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Multiple inheritance and a broken super() chain
Hello.
The legacy code I'm working with uses a classic diamond inheritance. Let me
call the classes *Top*, *Left*, *Right*, and *Bottom*.
This is a trivial textbook example. The classes were written in the
pre-super() era, so all of them initialized their parents and Bottom
initialized both Left and Right in this order.
The result was expected: *Top* was initialized twice:
Top.__init__() Left.__init__() Top.__init__() Right.__init__()
Bottom.__init__()
Now I replaced all parent init calls with *super()*. After this, Top was
initialized only once.
Top.__init__() Right.__init__() Left.__init__() Bottom.__init__()
But at this point, I freaked out. The code is complex and I don't have the
time to examine its inner workings. And before, everything worked correctly
even though Top was initialized twice. So I decided to break the superclass
chain and use super() only in classes inheriting from a single parent. My
intent was to keep the original behavior but use super() where possible to
make the code more readable.
class Top:
def __init__(self):
print("Top.__init__()")
class Left(Top):
def __init__(self):
super().__init__()
print("Left.__init__()")
class Right(Top):
def __init__(self):
super().__init__()
print("Right.__init__()")
class Bottom(Left, Right):
def __init__(self):
Left.__init__(self) # Here I'm calling both parents manually
Right.__init__(self)
print("Bottom.__init__()")
b = Bottom()
The result has surprised me:
Top.__init__() Right.__init__() Left.__init__() Top.__init__()
Right.__init__() Bottom.__init__()
Now, as I see it, from the super()'s point of view, there are two
inheritance chains, one starting at Left and the other at Right. But
*Right.__init__()* is called twice. What's going on here?
Thanks,
Peter
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