Python3: HTTP Error 302 while using urllib

Question

I want to read the value of different stocks from websites. Therefore I wrote this tiny script, which reads the page source and then parses out the value:

stock_reader.py

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

from re import search
from urllib import request


def main():
    links = [
        [
            'CSG',
            'UBS',
        ],
        [
            'http://www.tradegate.de/orderbuch.php?isin=CH0012138530',
            'http://www.tradegate.de/orderbuch.php?isin=CH0244767585',
        ],
    ]

    for i in in range(len(links[0])):
        url = links[1][i]
        htmltext = request.urlopen(url).read().decode('utf-8')
        source = htmltext.splitlines()
        for line in source:
            if 'id="bid"' in line:
                m = search('\d+.\d+', line)
                print('{}'.format(m.string[m.start():m.end()]))


if __name__ == '__main__':
    main()

sometimes it works but sometimes this error gets raised:

error message

Traceback (most recent call last):
  File "./aktien_reader.py", line 39, in <module>
    main()
  File "./aktien_reader.py", line 30, in main
    htmltext = request.urlopen(url).read().decode('utf-8')
  File "/usr/lib/python3.3/urllib/request.py", line 160, in urlopen
    return opener.open(url, data, timeout)
  File "/usr/lib/python3.3/urllib/request.py", line 479, in open
    response = meth(req, response)
  File "/usr/lib/python3.3/urllib/request.py", line 591, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python3.3/urllib/request.py", line 511, in error
    result = self._call_chain(*args)
  File "/usr/lib/python3.3/urllib/request.py", line 451, in _call_chain
    result = func(*args)
  File "/usr/lib/python3.3/urllib/request.py", line 696, in http_error_302
    return self.parent.open(new, timeout=req.timeout)
  File "/usr/lib/python3.3/urllib/request.py", line 479, in open
    response = meth(req, response)
  File "/usr/lib/python3.3/urllib/request.py", line 591, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python3.3/urllib/request.py", line 511, in error
    result = self._call_chain(*args)
  File "/usr/lib/python3.3/urllib/request.py", line 451, in _call_chain
    result = func(*args)
  File "/usr/lib/python3.3/urllib/request.py", line 696, in http_error_302
    return self.parent.open(new, timeout=req.timeout)
  File "/usr/lib/python3.3/urllib/request.py", line 479, in open
    response = meth(req, response)
  File "/usr/lib/python3.3/urllib/request.py", line 591, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python3.3/urllib/request.py", line 511, in error
    result = self._call_chain(*args)
  File "/usr/lib/python3.3/urllib/request.py", line 451, in _call_chain
    result = func(*args)
  File "/usr/lib/python3.3/urllib/request.py", line 696, in http_error_302
    return self.parent.open(new, timeout=req.timeout)
  File "/usr/lib/python3.3/urllib/request.py", line 479, in open
    response = meth(req, response)
  File "/usr/lib/python3.3/urllib/request.py", line 591, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python3.3/urllib/request.py", line 511, in error
    result = self._call_chain(*args)
  File "/usr/lib/python3.3/urllib/request.py", line 451, in _call_chain
    result = func(*args)
  File "/usr/lib/python3.3/urllib/request.py", line 696, in http_error_302
    return self.parent.open(new, timeout=req.timeout)
  File "/usr/lib/python3.3/urllib/request.py", line 479, in open
    response = meth(req, response)
  File "/usr/lib/python3.3/urllib/request.py", line 591, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python3.3/urllib/request.py", line 511, in error
    result = self._call_chain(*args)
  File "/usr/lib/python3.3/urllib/request.py", line 451, in _call_chain
    result = func(*args)
  File "/usr/lib/python3.3/urllib/request.py", line 686, in http_error_302
    self.inf_msg + msg, headers, fp)
urllib.error.HTTPError: HTTP Error 302: The HTTP server returned a redirect error that would lead to an infinite loop.
The last 30x error message was:
Found

My question is: why is it happening and how can I avoid it?

Gamosepalous

Answer 1

This happens probably because the destination site uses cookies and redirect you in case you don't send cookies.

What you can use is something like that :

from http.cookiejar import CookieJar

url = "http://www.tradegate.de/orderbuch.php?isin=CH0012138530"

req = urllib.request.Request(url, None, {'Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8','Accept-Charset': 'ISO-8859-1,utf-8;q=0.7,*;q=0.3','Accept-Encoding': 'gzip, deflate, sdch','Accept-Language': 'en-US,en;q=0.8','Connection': 'keep-alive'})

cj = CookieJar()
opener = urllib.request.build_opener(urllib.request.HTTPCookieProcessor(cj))
response = opener.open(req)
response.read()

This way, you support Cookies and website will allow you to get the page :-)

Another way would be to use the requests package which is really simplest to use. In your case, it would lead to :

import requests

url = "http://www.tradegate.de/orderbuch.php?isin=CH0012138530"
r = requests.get(url, headers={'Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8'}, timeout=15)
print(r.content)

Interlocutory

Answer 2

This answer is a simplification of the one by Cédric J. You don't really need to import CookieJar or set various Accept headers if you don't want to. You should however generally set a timeout. It is tested with Python 3.7. I would typically remember to use a new opener for each random URL that I want cookies for.

from urllib.request import build_opener, HTTPCookieProcessor, Request
url = 'https://www.cell.com/cell-metabolism/fulltext/S1550-4131(18)30630-2'
opener = build_opener(HTTPCookieProcessor())

Without a Request object:

response = opener.open(url, timeout=30)
content = response.read()

With a Request object:

request = Request(url)
response = opener.open(request, timeout=30)
content = response.read()

Waterborne

Answer 3

HTTP Status code 302 it's a kind of a redirect, it will have a header with a new URL for access (Not necessary a working URL..)

Location: http://www.example.com/x/y/

This is quite often used to block bots who make to many requests in too of a short time frame. So not an coding problem.

Chilton