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Title: urllib2 maybe blocks too long with small chunks
Type: behavior Stage: resolved
Components: Library (Lib) Versions: Python 2.7
Status: closed Resolution: out of date
Dependencies: Superseder:
Assigned To: orsenthil Nosy List: Anrs.Hu, Jim.Jewett, ZackerySpytz, hongqn, martin.panter, orsenthil
Priority: normal Keywords:

Created on 2012-04-12 12:23 by Anrs.Hu, last changed 2022-04-11 14:57 by admin. This issue is now closed.

Messages (7)
msg158124 - (view) Author: Anrs Hu (Anrs.Hu) Date: 2012-04-12 12:23
If HTTP URL response's Transfer-Encoding is 'Chunked', then the urllib2.urlopen(URL).readline() will block until there're enough 8192 bytes, even though the first chunk is just a line. 

Every chunks should be processed as soon as posible, so the readline() behavior should read a line and return immediately, rather than read 8K data to buffer and look up a line from the buffer.
msg158125 - (view) Author: Senthil Kumaran (orsenthil) * (Python committer) Date: 2012-04-12 12:26
I am trying to this test this to determine the fault.
msg158225 - (view) Author: Jim Jewett (Jim.Jewett) * (Python triager) Date: 2012-04-13 19:06
It would be helpful to have a testcase, so that it will stay fixed.
msg158246 - (view) Author: Anrs Hu (Anrs.Hu) Date: 2012-04-14 01:55
Okay, there's a test case of

Server codes are following:
import web
class index(object):
    def GET(self):
        yield 'hello\n'
        yield 'world\n'

client is Python interpreter
>>> resp = urllib.urlopen(URL)
>>> resp.readline() # will be 'hello'
>>> resp.readline() # will be 'world'
>>> resp.readline() # huh, it's blocked, and we to agree with it.

>>> # but to use urllib2 will another behavor.
>>> urllib2.urlopen(URL).readline() # huh, it's blocked even if 'hello' and 'world' returned yet. Because urllib2 uses a 8KiB buffer on socket._fileobjece within, it read 8K data to buffer first.
msg165927 - (view) Author: Senthil Kumaran (orsenthil) * (Python committer) Date: 2012-07-20 13:59
I had a discussion with Anrs on this, and it went along these lines - 

I confused the buffering issue (encountered with streaming data) of urllib2 with chunked transfer encoding.

The flow will be blocked in the case at the socket level waiting for 8192 bytes. But this buffer size has been kept for buffered reading purposes of normal read scenarios.
However, in case of streaming data, this may be not the best way.

Here it is explained best -

The advise is to make the socket buffer size to 0.

import socket

socket._fileobject.default_bufsize = 0

Now, if we come to chunked transfer encoding, the chunked transfer encoding will behave as it is advertised, like sending one chunk at the time, but still having the readline limit set by MAXLINE in the For the chunked transfer encoding to be recognized the client will have to get a header "transfer-encoding: chunked" from the server and when it receives that header, it will follow the path reading MAXLINE at the time and then returning. For smaller chunks with a blocking behavior of the server ( like you illustrated), we may still need to adopt to turn off default_bufsize to 0 to ensure quick responses to fill the buffer.

At this moment, I think that the above thing could be documented in the urllib2 docs for the issue you had raised. Not sure, if any other approach would be suitable to handle this behavior.

Anrs (The original poster) also responded that they way he had to overcome this for a very small chunks is setting the socket file size to 0 locally.

>> resp =, urllib.urlencode(data))
>> resp =
>> resp.fp._rbufsize = 0
>> for line in iter(resp.readline, ''):
>>     yield line

I think, this could be documented in a certain fashion (like support for streaming without buffering or transfers for small data sizes without buffering).
msg239099 - (view) Author: Martin Panter (martin.panter) * (Python committer) Date: 2015-03-24 08:52
I can reproduce this with Python 2, but not with current Python 3, nor with v3.3.3. Probably doesn’t affect 3.2 either, but I haven’t tried.
msg370091 - (view) Author: Zackery Spytz (ZackerySpytz) * (Python triager) Date: 2020-05-27 15:10
Python 2 is EOL, so I think this issue should be closed.
Date User Action Args
2022-04-11 14:57:29adminsetgithub: 58767
2020-05-28 02:27:09benjamin.petersonsetstatus: open -> closed
resolution: out of date
stage: needs patch -> resolved
2020-05-27 15:10:15ZackerySpytzsetnosy: + ZackerySpytz
messages: + msg370091
2016-06-17 01:52:49martin.pantersetversions: - Python 3.2
2015-03-24 08:52:16martin.pantersetnosy: + martin.panter

messages: + msg239099
versions: - Python 3.3
2012-07-20 13:59:41orsenthilsetmessages: + msg165927
stage: needs patch
2012-04-16 06:10:57hongqnsetnosy: + hongqn
2012-04-14 01:55:38Anrs.Husetmessages: + msg158246
2012-04-13 19:07:36Jim.Jewettsettitle: urllib2 maybe blocks too long -> urllib2 maybe blocks too long with small chunks
2012-04-13 19:06:40Jim.Jewettsetnosy: + Jim.Jewett
messages: + msg158225
2012-04-12 12:26:47orsenthilsetversions: + Python 3.2, Python 3.3
nosy: + orsenthil

messages: + msg158125

assignee: orsenthil
2012-04-12 12:23:09Anrs.Hucreate