Пример 3.39. listDirectory

def listDirectory(directory, fileExtList):                                         
    """возвращает список объектов с метаинформацией для всех файлов с
указанным расширением"""                       
    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__]):       
        "оределяет класс, предназначеный для обработки файла, по расширению"      
        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]                              

	listDirectory is the main attraction of this entire module. It takes a directory (like c:\music\_singles\ in my case) and a list of interesting file extensions (like ['.mp3']), and it returns a list of class instances that act like dictionaries that contain metadata about each interesting file in that directory. And it does it in just a few straightforward lines of code.
	As we saw in the previous section, this line of code gets a list of the full pathnames of all the files in directory that have an interesting file extension (as specified by fileExtList).
	Old-school Pascal programmers may be familiar with them, but most people give me a blank stare when I tell them that Python supports nested functions -- literally, a function within a function. The nested function getFileInfoClass can only be called from the function in which it is defined, listDirectory. As with any other function, you don't need an interface declaration or anything fancy; just define the function and code it.
	Now that you've seen the os module, this line should make more sense. It gets the extension of the file (os.path.splitext(filename)[1]), forces it to uppercase (.upper()), slices off the dot ([1:]), and constructs a class name out of it with string formatting. So c:\music\ap\mahadeva.mp3 becomes .mp3 becomes .MP3 becomes MP3 becomes MP3FileInfo.
	Having constructed the name of the handler class that would handle this file, we check to see if that handler class actually exists in this module. If it does, we return the class, otherwise we return the base class FileInfo. This is a very important point: this function returns a class. Not an instance of a class, but the class itself.
	For each file in our “interesting files” list (fileList), we call getFileInfoClass with the filename (f). Calling getFileInfoClass(f) returns a class; we don't know exactly which class, but we don't care. We then create an instance of this class (whatever it is) and pass the filename (f again), to the __init__ method. As we saw earlier in this chapter, the __init__ method of FileInfo sets self["name"], which triggers __setitem__, which is overridden in the descendant (MP3FileInfo) to parse the file appropriately to pull out the file's metadata. We do all that for each interesting file and return a list of the resulting instances.