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第 5 章 对象和面向对象

• 5.1. 概览
• 5.2. 使用 from module import 导入模块
• 5.3. 类的定义
  • 5.3.1. 初始化并开始类编码
  • 5.3.2. 了解何时去使用 self 和 __init__
• 5.4. 类的实例化
  • 5.4.1. 垃圾回收
• 5.5. 探索 UserDict: 一个封装类
• 5.6. 专用类方法
  • 5.6.1. 获得和设置数据项
• 5.7. 高级专用类方法
• 5.8. 类属性介绍
• 5.9. 私有函数
• 5.10. 小结

这一章，和此后的许多章，均讨论了面向对象的 Python 程序设计。

5.1. 概览

下面是一个完整的，可运行的 Python 程序。请阅读模块、类和函数的 doc strings，可以大概了解这个程序所做的事情和工作情况。象平时一样，不用担心你不理解的东西，这就是本章其它部分将告诉你的内容。

例 5.1. fileinfo.py

如果您还没有下载本书附带的例子程序, 可以 下载本程序和其他例子程序。

"""Framework for getting filetype-specific metadata.
Instantiate appropriate class with filename.  Returned object acts like a
dictionary, with key-value pairs for each piece of metadata.
    import fileinfo
    info = fileinfo.MP3FileInfo("/music/ap/mahadeva.mp3")
    print "\\n".join(["%s=%s" % (k, v) for k, v in info.items()])
Or use listDirectory function to get info on all files in a directory.
    for info in fileinfo.listDirectory("/music/ap/", [".mp3"]):
        ...
Framework can be extended by adding classes for particular file types, e.g.
HTMLFileInfo, MPGFileInfo, DOCFileInfo.  Each class is completely responsible for
parsing its files appropriately; see MP3FileInfo for example.
"""
import os
import sys
from UserDict import UserDict
def stripnulls(data):
    "strip whitespace and nulls"
    return data.replace("\00", "").strip()
class FileInfo(UserDict):
    "store file metadata"
    def __init__(self, filename=None):
        UserDict.__init__(self)
        self["name"] = filename
class MP3FileInfo(FileInfo):
    "store ID3v1.0 MP3 tags"
    tagDataMap = {"title"   : (  3,  33, stripnulls),
                  "artist"  : ( 33,  63, stripnulls),
                  "album"   : ( 63,  93, stripnulls),
                  "year"    : ( 93,  97, stripnulls),
                  "comment" : ( 97, 126, stripnulls),
                  "genre"   : (127, 128, ord)}
    def __parse(self, filename):
        "parse ID3v1.0 tags from MP3 file"
        self.clear()
        try:                               
            fsock = open(filename, "rb", 0)
            try:                           
                fsock.seek(-128, 2)        
                tagdata = fsock.read(128)  
            finally:                       
                fsock.close()              
            if tagdata[:3] == "TAG":
                for tag, (start, end, parseFunc) in self.tagDataMap.items():
                    self[tag] = parseFunc(tagdata[start:end])               
        except IOError:                    
            pass                           
    def __setitem__(self, key, item):
        if key == "name" and item:
            self.__parse(item)
        FileInfo.__setitem__(self, key, item)
def listDirectory(directory, fileExtList):                                        
    "get list of file info objects for files of particular extensions"
    fileList = [os.path.normcase(f)
                for f in os.listdir(directory)]           
    fileList = [os.path.join(directory, f) 
               for f in fileList
                if os.path.splitext(f)[1] in fileExtList] 
    def getFileInfoClass(filename, module=sys.modules[FileInfo.__module__]):      
        "get file info class from filename extension"                             
        subclass = "%sFileInfo" % os.path.splitext(filename)[1].upper()[1:]       
        return hasattr(module, subclass) and getattr(module, subclass) or FileInfo
    return [getFileInfoClass(f)(f) for f in fileList]                             
if __name__ == "__main__":
    for info in listDirectory("/music/_singles/", [".mp3"]): 
        print "\n".join(["%s=%s" % (k, v) for k, v in info.items()])
        print

这个程序的输入要取决于你硬盘上的文件。为了得到有意义的输出，你应该修改目录路径指向你自已机器上的一个 MP3 文件目录。

下面就是从我的机器上得到的输出。你的输出将不一样，除非，由于某些令人吃惊的巧合，你与我有着共同的音乐品味。

album=
artist=Ghost in the Machine
title=A Time Long Forgotten (Concept
genre=31
name=/music/_singles/a_time_long_forgotten_con.mp3
year=1999
comment=http://mp3.com/ghostmachine
album=Rave Mix
artist=***DJ MARY-JANE***
title=HELLRAISER****Trance from Hell
genre=31
name=/music/_singles/hellraiser.mp3
year=2000
comment=http://mp3.com/DJMARYJANE
album=Rave Mix
artist=***DJ MARY-JANE***
title=KAIRO****THE BEST GOA
genre=31
name=/music/_singles/kairo.mp3
year=2000
comment=http://mp3.com/DJMARYJANE
album=Journeys
artist=Masters of Balance
title=Long Way Home
genre=31
name=/music/_singles/long_way_home1.mp3
year=2000
comment=http://mp3.com/MastersofBalan
album=
artist=The Cynic Project
title=Sidewinder
genre=18
name=/music/_singles/sidewinder.mp3
year=2000
comment=http://mp3.com/cynicproject
album=Digitosis@128k
artist=VXpanded
title=Spinning
genre=255
name=/music/_singles/spinning.mp3
year=2000
comment=http://mp3.com/artists/95/vxp