An extra parentheses in a call to a nested function in Python

Question

I'm studying the module of Mark Pilgrim's Dive Into Python Book in chapter 6, and I'm kind of stuck with what this line of code return [getFileInfoClass(f)(f) for f in fileList] does. getFileInfo is a nested function and I was wondering what's the duplicate (f), the extra parentheses is for. I was hoping someone can help me out. Here's the complete function:

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]

Answer 1

getFileInfoClass returns a class; classes are then callable themselves -- calling a class by just a name returns an instance of it. The two pairs of parens in quick succession are just shorthand. It is effectively:

file_info_class = getFileInfoClass(f)
file_info_instance = file_info_class(f)

Generally, the two pairs of parens like that is probably a questionable decision, as it's not very readable, but I suppose in this case the writer deemed it reasonable because it allowed him or her to put everything inside a single list comprehension.

Answer 2

getFileInfoClass returns a class, so getFileInfoClass(f) is a class. When you take a class name and write parentheses after it, you call a constructor.

So, [getFileInfoClass(f)(f) for f in fileList] makes a list of FileInfo objects.