What can meta class do for you? @PyCon Taiwan 2012

What CanMeta ClassDo For You? hychenPyCon Taiwan 2012, 6/10, Taiwan

Who am I陳信屹 a.k.a hychen OPython Developer since 2006本來是學 Perl, 然後學不會，所以就一路寫 Python 了Debian MaintainerMaintain 2 Python module package MSoftware Engineer in Canonical Ltd.http://hychen.wuweig.orghttp://about.me/hychen

Python 2New-StyleClass Only

OutlineWhat is metaclass?Example Singleton Countable Class Verification Logged Auto Delegation ...Conclusion

Type → Class → Object 型別產生類別產生物件

Type → Class XXXXX型別產生類別 XXXXX

Objects are instances of classes. 物件是類別的實例 Classes are instance of types. 類別是型別的實例

所有類別繼承 object 類別object class is base class of all classes. 所有類別是 type 的實例 type is metaclass of all classes.

Subclassing a classclass MyClass(object): name = None sex = None Same>>> MyClass = type.__new__(type, MyClass, (), {name:None, sex:None})>>> type.__init__(MyClass, MyClass, (), {name:None, sex:None}>>> MyClass.__class__>>> <type type>>>> MyClass.__bases__>>> (<type object>,)

Class is an instance of a meta class if..Class has __metaclass__, orClasss ancetor class has__metaclass__type is default metaclass

subclassing a typeclass MyType(type): def __new__(mbs, name, bases, _dict): return type.__new__(mbs, name, bases, _dict) def __init__(cls, name, bases, _dict): return type.__init__(cls, name, bases, _dict)

MyClass = Mytype()MyType() is MyType.__call__() is …MyClass = type.__new__(MyType....)type.__init__(MyClass....)

應用For Example.....

壹Singleton

Before using metaclass

After using metclass

Only one instance please use get_instance function to get the instance 136815764 please use get_instance function to get the instance 136815764 please use get_instance function to get the instance 136815764

The Magic

貳Countable

Class code

RunI have 0 booksI have 4 books

The Magic

參Class Verification

Class code

>>> from cls_verification import JokeInPTT>>> ERROR: Can not find HaHaPoint in JokeInPTT 同樣的，這場 Talk 也找不到笑點 ...

The Magic

肆Logged

Class code

Run info:Enter func:get info:Got return value:I am get method info:Exit func:get

The Magic

Logger function

伍Delegation

Class Code

Run the data is [1, 2, 3, 4, 5] girl args

無量大數 > 10^68Composed ClassAssociationAuto Initial AttributesMore and more..

ConclusionGood Less work, play more Inheritance → composition Metaclass focus on trait of a classBad Too Magic, it is not easy to understand You colleague may complain you

Q: When should I useMetaClass?A: Never (as long as youreasking this question anyway) ~ Shal ab h Chat ur v e d i

ReferenceArticle, Python types and objects, ShalabhChaturvediSlide, Meta Class Programming in Python, Juan-Manuel GimenoSlide, Metaclass In Python, Guy WienerMetaclass programming in Python, IBMDeveloperWorld

If your colleague complain your code, My friend Thinker Li suggests a solution....# This is a meta class,# you are not supposed to read it,# so skip it please.class Singleton(type): …. Just a kidding....

Thanks謝謝聆聽

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What can meta class do for you? @PyCon Taiwan 2012

by HsinYi Chen

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