{article Dive into Python}{title} {text} {/article}
Now you've seen enough to understand the file handling code in the fileinfo.py sample code from teh previous chapter. This example shows how to safely open and read from a file and gracefully handle errors.
File Objects in MP3FileInfo
>>> try:
fsock = open(filename, "rb",
try:
fsock.seek(-128, 2)
tagdata = fsock.read(128)
finally:
if tagdata[:3] == 'TAG':
for tag, (start, end, parseFunc) in self.tagDataMap.items():
self[tag] = parseFunc(tagdata[start:end])
except IOError:
pass
Because opening and reading files is risky and may raise an exception, all of this code is wrapped in a try...except block. (Hey, isn't standardized indentation great? This is where you start to appreciate it.)
The open function may raise an IOError. (Maybe the file doesn't exist.)
The seek method may raise an IOError. (Maybe the file is smaller than 128 bytes.)
The read method may raise an IOError. (Maybe the disk has a bad sector, or it's on a network drive and the network just went down.)
This is new: a try...finally block. Once the file has been opened successfully by the open function, you want to make absolutely sure that you close it, even if an exception is raised by the seek or read methods. That's what a try...finally block is for: code in the finally block will always be executed, even if
something in the try block raises an exception. Think of it as code that gets executed on the way out, regardless of what happened before.
At last, you handle your IOError exception. This could be the IOError exception raised by the call to open, seek, or read. Here, you really don't care, because all you're going to do is ignore it silently and continue. (Remember, pass is a Python statement that does nothing.) That's perfectly legal; "handling" an exception can mean explicitly doing nothing. It still counts as handled, and processing will continue normally on the next line of code after the try...except block.
fileinfo.py
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<pre class="brush:py;">
"""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.
This program is part of "Dive Into Python", a free Python book for
experienced programmers. Visit http://diveintopython.org/ for the
latest version.
"""
__author__ = "Mark Pilgrim (
__version__ = "$Revision: 1.3 $"
__date__ = "$Date: 2004/05/05 21:57:19 $"
__copyright__ = "Copyright (c) 2001 Mark Pilgrim"
__license__ = "Python"
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 ("\n".join(["%s=%s" % (k, v) for k, v in info.items()]))
print
</pre>
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