C3 Superclass Linearization (Optional)

In Python 3.8.0, it is not even allowed to create the Y class in this example:

>>> class X(object):
...     def __init__(self):
...             print("X")
...
>>> class Y(object, X):
...     def __init__(self):
...             print("Y")
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Cannot create a consistent method resolution
order (MRO) for bases object, X

Thanks for this class, Austin. It is indeed very interesting and really cool to know about this!

last example form cyclical inheritance how?

because when you work out the linearization algorithm, you get:

L[y] = [Y] + merge(L(object), L(X), [object, x]) = [y] + merge([object], [X, object], [object, X])

Since X and object are both the last element of one of the lists, none of the elements can get merged.

Great video,

Could you please elaborate more how the merge function works or point me to some article about algorithm if you have some recommendations?

Didn’t quite understand this part:

04:42 It’s going to search the first item in the first list, called the head.

04:48 As long as that first item isn’t not the first item in the remaining lists, it will pull that item out of all the lists and merge it with the list on the very left

Does this mean that it will always merge the first item, as long as the first item is not in the first list and list count > 2?

Thank you

Can I get some help with getting the MRO of TemporarySecretary with a different inheritance order? Pleaase see below where I get stuck. I can output what employees.TemporarySecretary’s MRO is*(<class 'employees.TemporarySecretary'>, <class 'employees.Secretary'>, <class 'employees.SalaryEmployee'>, <class 'employees.HourlyEmployee'>, <class 'employees.Employee'>, <class 'object'>)* but how do I get that result? What’s the rulewhen we do not have a good candidate like in the previous examples?

# MRO practice
TemporarySecretary(Secretary, HourlyEmployee)
L(TS)     = [TS] + merge(L(S), L(HE))
            [TS ] + merge([S, SE, E], [HE, E])
            [ TS, 

Please ignore my comment above

Here is how I finally managed to get it right

TemporarySecretary(HourlyEmployee, Secretary)
L(TS)     = [TS] + merge(L(HE), L(S), [HE, S])
          = [TS ] + merge([HE, E], [S, SE, E], [HE, S])
          = [ TS, HE ] + merge([E], [S, SE, E], [S])
          = [ TS, HE, S ] + merge([E], [SE, E])
          = [ TS, HE, S, SE ] + merge([E])
          = [ TS, HE, S, SE, E ]

The double negative comment on the merge for secretary is a bit confusing :)

It’s not clear for me what is the cyclic problem of C3 algorithm in example:

class X(object):
class Y(X, object)

Why not to resolve it as following?

L(Y) = [Y] + merge(L(object), L(X)) = [Y] + merge([object], [X,object]) #object is not the first in 2'nd = [Y,X] + merge([object]) = [Y,X,object]