Re: Python's method-resolution algorithm: An implementation question
Thanks! On Tue, Aug 15, 2017 at 10:41 PM, Ian Kelly wrote: > > On Tue, Aug 15, 2017 at 12:57 PM, Evan Aad wrote: > > I don't see how, since the L(B*)'s are listed in order in the argument > > list: L(B1), L(B2), ..., and each L(B*) starts with B*: L(B1) = > ...>, L(B2) = , ... > > > > Could you please give a counter-example? > > Sure. > > merge(, ) -> > > vs: > > merge(, , ) -> TypeError > > Or in Python: > > >>> class A1: pass > >>> class A2: pass > >>> class B1(A1): pass > >>> class B2(A2, B1): pass > >>> class C(B1, B2): pass > Traceback (most recent call last): > File "", line 1, in > TypeError: Cannot create a consistent method resolution order (MRO) > for bases B1, B2 > -- > https://mail.python.org/mailman/listinfo/python-list -- https://mail.python.org/mailman/listinfo/python-list
Re: Python's method-resolution algorithm: An implementation question
On Tue, Aug 15, 2017 at 12:57 PM, Evan Aad wrote: > I don't see how, since the L(B*)'s are listed in order in the argument > list: L(B1), L(B2), ..., and each L(B*) starts with B*: L(B1) = ...>, L(B2) = , ... > > Could you please give a counter-example? Sure. merge(, ) -> vs: merge(, , ) -> TypeError Or in Python: >>> class A1: pass >>> class A2: pass >>> class B1(A1): pass >>> class B2(A2, B1): pass >>> class C(B1, B2): pass Traceback (most recent call last): File "", line 1, in TypeError: Cannot create a consistent method resolution order (MRO) for bases B1, B2 -- https://mail.python.org/mailman/listinfo/python-list
Re: Python's method-resolution algorithm: An implementation question
I don't see how, since the L(B*)'s are listed in order in the argument list: L(B1), L(B2), ..., and each L(B*) starts with B*: L(B1) = , L(B2) = , ... Could you please give a counter-example? On Tue, Aug 15, 2017 at 9:44 PM, Ian Kelly wrote: > > On Tue, Aug 15, 2017 at 9:56 AM, Evan Aad 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) = > > L(C) = + merge(L(B1),..., L(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 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. > -- > https://mail.python.org/mailman/listinfo/python-list -- https://mail.python.org/mailman/listinfo/python-list
Re: Python's method-resolution algorithm: An implementation question
On Tue, Aug 15, 2017 at 9:56 AM, Evan Aad 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) = > L(C) = + merge(L(B1),..., L(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 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. -- https://mail.python.org/mailman/listinfo/python-list
Python's method-resolution algorithm: An implementation question
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) = L(C) = + merge(L(B1),..., L(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 of parents given as an argument to 'merge'? Will the algorithm produce different results if this argument is omitted? -- https://mail.python.org/mailman/listinfo/python-list
