Bugs item #1682241, was opened at 2007-03-16 17:00 Message generated for change (Comment added) made by lucas_malor You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1682241&group_id=5470
Please note that this message will contain a full copy of the comment thread, including the initial issue submission, for this request, not just the latest update. Category: Python Library Group: Python 2.5 Status: Open Resolution: None Priority: 5 Private: No Submitted By: Lucas Malor (lucas_malor) Assigned to: Nobody/Anonymous (nobody) Summary: Problems with urllib2 read() Initial Comment: urllib2 objects opened with urlopen() does not have the method seek() as file objects. So reading only some bytes from opened urls is pratically forbidden. An example: I tried to open an url and check if it's a gzip file. If IOError is raised I read the file (to do this I applied the #1675951 patch: https://sourceforge.net/tracker/index.php?func=detail&aid=1675951&group_id=5470&atid=305470 ) But after I tried to open the file as gzip, if it's not a gzip file the current position in the urllib object is on the second byte. So read() returns the data from the 3rd to the last byte. You can't check the header of the file before storing it on hd. Well, so what is urlopen() for? If I must store the file by url on hd and reload it, I can use urlretrieve() ... ---------------------------------------------------------------------- >Comment By: Lucas Malor (lucas_malor) Date: 2007-04-26 22:41 Message: Logged In: YES user_id=1403274 Originator: YES In my opinion it's not complicated, it's convoluted. I must use two object to handle one data stream. Furthermore it's a waste of resources. I must copy data to another object. Luckily in my script I download and handle only little files. But what if a python program must handle big files? If seek() can't be used (an except is raised), urllib could use a sequential access method. ---------------------------------------------------------------------- Comment By: Calvin Spealman (ironfroggy) Date: 2007-04-26 15:55 Message: Logged In: YES user_id=112166 Originator: NO I have to agree that this is not a bug. HTTP responses are strings, not random access files. Adding a seek would have disastrous performance penalties. If you think the work around is too complicated, I can't understand why. ---------------------------------------------------------------------- Comment By: Zacherates (maenpaa) Date: 2007-03-21 02:39 Message: Logged In: YES user_id=1421845 Originator: NO > I use the method you wrote, but this must be done manually, > and I don't know why. read() is a stream processing method, whereas seek() is a random access processing method. HTTP resources are in essence streams so they implement read() but not seek(). Trying to shoehorn a stream to act like a random access file has some rather important technical implications. For example: what happens when an HTTP resource is larger than available memory and we try to maintain a full featured seek() implementation? > so what is urlopen() for? Fetching a webpage or RSS feed and feeding it to a parser, for example. StringIO is a class that was designed to implement feature complete, random access, file-like object behavior that can be wrapped around a stream. StringIO can and should be used as an adapter for when you have a stream that you need random access to. This allows designers the freedom to simply implement a good read() implementation and let clients wrap the output in a StringIO if needed. If in your application you always want random access and you don't have to deal with large files: def my_urlopen(*args, **kwargs): return StringIO.StringIO(urllib2.urlopen(*args, **kwargs).read()) Python makes delegation trivially easy. In essence, urlfiles (the result of urllib2.urlopen()) and regular files (the result of open()) behave differently because they implement different interfaces. If you use the common interface (read), then you can treat them equally. If you use the specialized interface (seek, tell, etc.) you'll have trouble. The solution is wrap the general objects in a specialized object that implements the desired interface, StringIO. ---------------------------------------------------------------------- Comment By: Lucas Malor (lucas_malor) Date: 2007-03-20 09:59 Message: Logged In: YES user_id=1403274 Originator: YES > If you need to seek, you can wrap the file-like object in a > StringIO (which is what urllib would have to do internally > [...] ) I think it's really a bug, or at least a non-pythonic method. I use the method you wrote, but this must be done manually, and I don't know why. Actually without this "trick" you can't handle url and file objects together as they don't work in the same manner. I think it's not too complicated using the internal StringIO object in urllib class when I must seek() or use other file-like methods. > You can check the type of the response content before you try > to uncompress it via the Content-Encoding header of the > response It's not a generic solution (thanks anyway for suggested solutions :) ) ---------------------------------------------------------------------- Comment By: Zacherates (maenpaa) Date: 2007-03-20 03:43 Message: Logged In: YES user_id=1421845 Originator: NO I'd contend that this is not a bug: * If you need to seek, you can wrap the file-like object in a StringIO (which is what urllib would have to do internally, thus incurring the StringIO overhead for all clients, even those that don't need the functionality). * You can check the type of the response content before you try to uncompress it via the Content-Encoding header of the response. The meta-data is there for a reason. Check http://www.diveintopython.org/http_web_services/gzip_compression.html for a rather complete treatment of your use-case. ---------------------------------------------------------------------- You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=1682241&group_id=5470 _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
