Clarify metaclass docs and add example.

This commit is contained in:
Georg Brandl 2008-01-07 19:17:10 +00:00
parent 61d2886491
commit 3ccb49afed
1 changed files with 17 additions and 2 deletions

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@ -1177,7 +1177,8 @@ Basic customization
:meth:`__init__` method will not be invoked.
:meth:`__new__` is intended mainly to allow subclasses of immutable types (like
int, str, or tuple) to customize instance creation.
int, str, or tuple) to customize instance creation. It is also commonly
overridden in custom metaclasses in order to customize class creation.
.. method:: object.__init__(self[, ...])
@ -1651,7 +1652,7 @@ definition is read into a separate namespace and the value of class name is
bound to the result of ``type(name, bases, dict)``.
When the class definition is read, if *__metaclass__* is defined then the
callable assigned to it will be called instead of :func:`type`. The allows
callable assigned to it will be called instead of :func:`type`. This allows
classes or functions to be written which monitor or alter the class creation
process:
@ -1660,6 +1661,20 @@ process:
* Returning an instance of another class -- essentially performing the role of a
factory function.
These steps will have to be performed in the metaclass's :meth:`__new__` method
-- :meth:`type.__new__` can then be called from this method to create a class
with different properties. This example adds a new element to the class
dictionary before creating the class::
class metacls(type):
def __new__(mcs, name, bases, dict):
dict['foo'] = 'metacls was here'
return type.__new__(mcs, name, bases, dict)
You can of course also override other class methods (or add new methods); for
example defining a custom :meth:`__call__` method in the metaclass allows custom
behavior when the class is called, e.g. not always creating a new instance.
.. data:: __metaclass__