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Diffstat (limited to 'Doc')
| -rw-r--r-- | Doc/reference/datamodel.rst | 32 |
1 files changed, 32 insertions, 0 deletions
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst index 2bce677..c2c3241 100644 --- a/Doc/reference/datamodel.rst +++ b/Doc/reference/datamodel.rst @@ -1541,6 +1541,38 @@ explored including logging, interface checking, automatic delegation, automatic property creation, proxies, frameworks, and automatic resource locking/synchronization. +Here is an example of a metaclass that uses an :class:`OrderedDict` to +remember the order that class members were defined:: + + class OrderedClass(type): + + @classmethod + def __prepare__(metacls, name, bases, **kwds): + return collections.OrderedDict() + + def __new__(cls, name, bases, classdict): + result = type.__new__(cls, name, bases, dict(classdict)) + result.members = tuple(classdict) + return result + + class A(metaclass=OrderedClass): + def one(self): pass + def two(self): pass + def three(self): pass + def four(self): pass + + >>> A.members + ('__module__', 'one', 'two', 'three', 'four') + +When the class definition for *A* get executed, the first step is calling the +metaclass's :meth:`__prepare__` method which returns an empty +:class:`collections.OrderedDict`. That mapping records the methods and +attributes of *A* as they are defined within the body of the class statement. +Once those definitions are executed, the ordered dict is fully populated, and +then the metaclass's :meth:`__new__ ` method gets invoked. That method builds +the new type and saves the keys for the ordered dictionary in an attribute +called *members*. + .. _callable-types: |