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| -rw-r--r-- | Doc/tut/tut.tex | 31 |
1 files changed, 20 insertions, 11 deletions
diff --git a/Doc/tut/tut.tex b/Doc/tut/tut.tex index 342fbca..2981a78 100644 --- a/Doc/tut/tut.tex +++ b/Doc/tut/tut.tex @@ -4329,8 +4329,7 @@ class DerivedClassName(Base1, Base2, Base3): <statement-N> \end{verbatim} -The only rule necessary to explain the semantics is the resolution -rule used for class attribute references. This is depth-first, +For old-style classes, the only rule is depth-first, left-to-right. Thus, if an attribute is not found in \class{DerivedClassName}, it is searched in \class{Base1}, then (recursively) in the base classes of \class{Base1}, and only if it is @@ -4345,16 +4344,26 @@ a name conflict with an attribute of \class{Base2}. The depth-first rule makes no differences between direct and inherited attributes of \class{Base1}.) -It is clear that indiscriminate use of multiple inheritance is a -maintenance nightmare, given the reliance in Python on conventions to -avoid accidental name conflicts. A well-known problem with multiple -inheritance is a class derived from two classes that happen to have a -common base class. While it is easy enough to figure out what happens -in this case (the instance will have a single copy of ``instance -variables'' or data attributes used by the common base class), it is -not clear that these semantics are in any way useful. +For new-style classes, the method resolution order changes dynamically +to support cooperative calls to \function{super()}. This approach +is known in some other multiple-inheritance languages as call-next-method +and is more powerful than the super call found in single-inheritance languages. + +With new-style classes, dynamic ordering is necessary because all +cases of multiple inheritance exhibit one or more diamond relationships +(where one at least one of the parent classes can be accessed through +multiple paths from the bottommost class). For example, all new-style +classes inherit from \class{object}, so any case of multiple inheritance +provides more than one path to reach \class{object}. To keep the +base classes from being accessed more than once, the dynamic algorithm +linearizes the search order in a way that preserves the left-to-right +ordering specified in each class, that calls each parent only once, and +that is monotonic (meaning that a class can be subclassed without affecting +the precedence order of its parents). Taken together, these properties +make it possible to design reliable and extensible classes with +multiple inheritance. For more detail, see +\url{http://www.python.org/download/releases/2.3/mro/}. -%% XXX Add rules for new-style MRO? \section{Private Variables \label{private}} |