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The following error appeared on some buildbots:
http://buildbot.python.org/all/builders/x86%20Gentoo%203.x/builds/4195/steps/test/logs/stdio
======================================================================
ERROR: test_ftp (test.test_urllib2net.OtherNetworkTests)
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Traceback (most recent call last):
 File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/urllib/request.py", line 2337, in retrfile
 self.ftp.cwd(file)
 File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/ftplib.py", line 622, in cwd
 return self.voidcmd(cmd)
 File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/ftplib.py", line 272, in voidcmd
 return self.voidresp()
 File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/ftplib.py", line 245, in voidresp
 resp = self.getresp()
 File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/ftplib.py", line 240, in getresp
 raise error_perm(resp)
ftplib.error_perm: 550 Failed to change directory.
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/test/test_urllib2net.py", line 112, in test_ftp
 self._test_urls(urls, self._extra_handlers())
 File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/test/test_urllib2net.py", line 218, in _test_urls
 f = urlopen(url, req, TIMEOUT)
 File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/test/test_urllib2net.py", line 33, in wrapped
 return _retry_thrice(func, exc, *args, **kwargs)
 File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/test/test_urllib2net.py", line 23, in _retry_thrice
 return func(*args, **kwargs)
 File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/urllib/request.py", line 462, in open
 response = self._open(req, data)
 File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/urllib/request.py", line 480, in _open
 '_open', req)
 File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/urllib/request.py", line 440, in _call_chain
 result = func(*args)
 File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/urllib/request.py", line 1464, in ftp_open
 fp, retrlen = fw.retrfile(file, type)
 File "/var/lib/buildslave/3.x.murray-gentoo/build/Lib/urllib/request.py", line 2339, in retrfile
 raise URLError('ftp error: %d' % reason) from reason
TypeError: %d format: a number is required, not error_perm

I cannot reproduce the issue on Ubuntu.
As for the second exception I think it's safe to just do:
- raise URLError('ftp error: %d' % reason) from reason
+ raise URLError('ftp error: %s' % reason) from reason
(will commit that later)

Bug in urllib/request.py.
format string formatted error type variable 
 2373 except ftplib.error_perm as reason:
 2374 raise URLError('ftp error: %d' % reason) from reason
variable reason here is a instance of class ftplib.error_perm.
We need to passed in a integer object. 
Patch supplied.

Fixed like this:
raise URLError('ftp error: %d' % int(str(reason)[:3])) from reason
I think this is the original author's intention. 
Actually, need to fix two places in urllib/request.py

That's not safe as a misbehaving FTP server might not send a response code at all (highly unlikely but still...).
Furthermore having the complete server response (response code + accompaining message) is a lot more helpful.

yes, the malformed server could do evil things. If we need to cover this situation, we need to some extra fixes in this file. 
Maybe the exception message look like this is the better one ?
 ftplib.error_perm: 550 Failed to change directory

I think "raise URLError('ftp error: %s' % reason) from reason" is just fine.

Yes, the simplest fix is just replace '%d' to '%s'.
Line 2362 and 2374 all need to modify.

Any progress for this issue?
I changed the title of the issue.

Giampaolo's suggestion LGTM.

This was changed to %r by Benjamin Peterson in 27e470952085.