Question

6.7. 小结

在 第 5 章 介绍的 fileinfo.py 程序现在应该完全理解了。

"""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

在研究下一章之前，确保你可以无困难地完成下面的事情:

• 使用 try...except 来捕捉异常
• 使用 try...finally 来保护额外的资源
• 读取 文件
• 在一个 for 循环 中一次赋多个值
• 使用 os 模块来满足你的跨平台文件操作的需要
• 动态地 实例化未知类型的类 通过将类看成对象并传入参数