Python's method-resolution algorithm: An implementation question

[Ian Kelly]

On Tue, Aug 15, 2017 at 9:56 AM, Evan Aad <oddeveneven at gmail.com> wrote:
> According to the description of Python's method resolution order (mro)
> (https://www.python.org/download/releases/2.3/mro/), a.k.a. C3
> linearization (see Wikipedia), the algorithm can be described as
> follows:
>
> "the linearization of C is the sum of C plus the merge of the
> linearizations of the parents and the list of the parents."
>
> Symbolically, the algorithm is defined recursively thus:
>
> L(O) = <O>
> L(C) = <C> + merge(L(B1),..., L(Bn), <B1,...,Bn>)
>
> where
>
> * O is the class from which every class inherits.
> * C is a class that inherits directly from B1, ..., Bn, in this order.
> * < and > are list delimiters.
> * + is the list-concatenation operator.
> * 'merge' merges its list arguments into a single list with no
> repetitions in the manner described in the next paragraph.
>
> Consider the head of the first list, i.e L(B1)[0]: if it is a good
> head, i.e. if it is not in the proper tail of any of the other lists,
> add it to the linearization of C, and remove it from all the lists in
> the merge. Otherwise, consider the head of the next list, etc. Repeat
> until no more classes, or no good heads. In the latter case, it is
> impossible to construct the merge.
>
>
>
> Why is the list <B1,...,Bn> of parents given as an argument to
> 'merge'? Will the algorithm produce different results if this argument
> is omitted?

It ensures that the parents are linearized in that order. Without that
list, it would be possible for B2 to appear before B1 in the MRO of C.