基本介紹
- 中文名：C3線性化
- 又稱：方法解析順序
- 領域：計算機
定義,描述,例子,例子的Python實現,
定義
名字C3指最終線性化的三種重要的屬性：
- 一致擴展前趨圖，
- 本地前趨序的保持，
- 適於單調標準
1996年的OOPSLA會議上，論文"A Monotonic Superclass Linearization forDylan"首次提出了C3超類線性化。被Python2.3選作方法解析的默認算法。Perl 6Parrot,,Solidity,PGF/TikZ等面向對象語言。也作為可選的、默認不是MRO的語言如Perl 5從版本5.10.0.早期版本Perl 5的一個擴展實現，稱為Class::C3發布於CPAN。
描述
一個類的C3超類線性化是這個類，再加上它的各個父類的線性化與各個父類形成列表的唯一合併的結果。父類列表作為合併過程的最後實參，保持了直接父類的本地前趨序。
各個父類的線性化與各個父類形成列表的合併算法，首先選擇不出現在各個列表的尾部（指除了第一個元素外的其他元素）的第一個元素，該元素可能同時出現在多個列表的頭部。被選中元素從各個列表中刪除並追加到輸出列表中。這個選擇再刪除、追加元素的過程疊代下去直到各個列表被耗盡。如果在某個時候無法選出元素，說明這個類繼承體系的依賴序是不一致的，因而無法線性化。
例子
給定
class Oclass A extends Oclass B extends Oclass C extends Oclass D extends Oclass E extends Oclass K1 extends A, B, Cclass K2 extends D, B, Eclass K3 extends D, Aclass Z extends K1, K2, K3
Z的線性化計算：
L(O) := [O] // the linearization of O is trivially the singleton list [O], because O has no parentsL(A) := [A] + merge(L(O), [O]) // the linearization of A is A plus the merge of its parents' linearizations with the list of parents... = [A] + merge([O], [O]) = [A, O] // ...which simply prepends A to its single parent's linearizationL(B) := [B, O] // linearizations of B, C, D and E are computed similar to that of AL(C) := [C, O]L(D) := [D, O]L(E) := [E, O]L(K1) := [K1] + merge(L(A), L(B), L(C), [A, B, C]) // first, find the linearizations of K1's parents, L(A), L(B), and L(C), and merge them with the parent list [A, B, C] = [K1] + merge([A, O], [B, O], [C, O], [A, B, C]) // class A is a good candidate for the first merge step, because it only appears as the head of the first and last lists = [K1, A] + merge([O], [B, O], [C, O], [B, C]) // class O is not a good candidate for the next merge step, because it also appears in the tails of list 2 and 3; but class B is a good candidate = [K1, A, B] + merge([O], [O], [C, O], [C]) // class C is a good candidate; class O still appears in the tail of list 3 = [K1, A, B, C] + merge([O], [O], [O]) // finally, class O is a valid candidate, which also exhausts all remaining lists = [K1, A, B, C, O]L(K2) := [K2] + merge(L(D), L(B), L(E), [D, B, E]) = [K2] + merge([D, O], [B, O], [E, O], [D, B, E]) // select D = [K2, D] + merge([O], [B, O], [E, O], [B, E]) // fail O, select B = [K2, D, B] + merge([O], [O], [E, O], [E]) // fail O, select E = [K2, D, B, E] + merge([O], [O], [O]) // select O = [K2, D, B, E, O]L(K3) := [K3] + merge(L(D), L(A), [D, A]) = [K3] + merge([D, O], [A, O], [D, A]) // select D = [K3, D] + merge([O], [A, O], [A]) // fail O, select A = [K3, D, A] + merge([O], [O]) // select O = [K3, D, A, O]L(Z) := [Z] + merge(L(K1), L(K2), L(K3), [K1, K2, K3]) = [Z] + merge([K1, A, B, C, O], [K2, D, B, E, O], [K3, D, A, O], [K1, K2, K3]) // select K1 = [Z, K1] + merge([A, B, C, O], [K2, D, B, E, O], [K3, D, A, O], [K2, K3]) // fail A, select K2 = [Z, K1, K2] + merge([A, B, C, O], [D, B, E, O], [K3, D, A, O], [K3]) // fail A, fail D, select K3 = [Z, K1, K2, K3] + merge([A, B, C, O], [D, B, E, O], [D, A, O]) // fail A, select D = [Z, K1, K2, K3, D] + merge([A, B, C, O], [B, E, O], [A, O]) // select A = [Z, K1, K2, K3, D, A] + merge([B, C, O], [B, E, O], [O]) // select B = [Z, K1, K2, K3, D, A, B] + merge([C, O], [E, O], [O]) // select C = [Z, K1, K2, K3, D, A, B, C] + merge([O], [E, O], [O]) // fail O, select E = [Z, K1, K2, K3, D, A, B, C, E] + merge([O], [O], [O]) // select O = [Z, K1, K2, K3, D, A, B, C, E, O] // done
例子的Python實現
首先，metaclass允許對象通過名字簡明表示而不用<class'__main__.A'>：
class Type(type): def __repr__(cls): return cls.__name__A = Type('A', (object,), {})類A表示自身通過名字：
>>> AA
由於Python 3的metaclass語法改變了，為使例子兼任版本2與3，用metaclass創建對象，如同用type。
A = Type('A', (object,), {})B = Type('B', (object,), {})C = Type('C', (object,), {})D = Type('D', (object,), {})E = Type('E', (object,), {})K1 = Type('K1', (A, B, C), {})K2 = Type('K2', (D, B, E), {})K3 = Type('K3', (D, A), {})Z = Type('Z', (K1, K2, K3), {})等價的Python 2 的類定義是：
class Z(K1, K2, K3): __metaclass__ = Type
對於Python 3是
class Z(K1, K2, K3, metaclass=Type): pass
最後有：
>>> Z.mro()[Z, K1, K2, K3, D, A, B, C, E, <type 'object'>]
