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OpenSSL provides a method, SSL_CTX_set_default_verify_paths(), for loading a default certificate store, which is used by many distributions.
In openSUSE, the default store is not a bundle, but a directory-based store, which is not supported at all by the SSL module in Python 2.7. A bug related to this was assigned to me here:
https://bugzilla.novell.com/show_bug.cgi?id=761501
I created patches for the Python 2.7.3 and 3.2.3 SSL modules that will load the distribution-specific store if ca_certs is omitted.

Here's the patch for Python 3.

Well, this is basically a change in behaviour, so it could only go in the default branch (3.3). But 3.3 already has SSLContext.set_default_verify_paths(), so it's not obvious why the patch is needed.
(furthermore, automatically selecting the system-wide certificates could be surprising. There may be applications where you don't want to trust them.)

By the way, I see that the original bug is against the python-requests library? Perhaps it would be better to advocate the use of load_verify_locations() there, since it's a more adequate place for a policy decision.
(that said, Python's own urllib.request could perhaps grow a similar option)

load_verify_locations() is not available in Python 2.x. It was added in 3.x. Also, there is no way to load a directory-based certificate store at all in Python 2.x, which is why the bug was opened.

Sorry, by our policy this change is a new feature and cannot go into stable versions.

Fair enough. What about a patch to handle a directory store passed through the ca_certs parameter? As it stands now, it's impossible to load the distribution-supplied cert store on openSUSE.

> What about a patch to handle a directory store passed through the
> ca_certs parameter? As it stands now, it's impossible to load the
> distribution-supplied cert store on openSUSE.
I'm afraid it would still be a new feature, unsuitable for a bugfix release. Other distros simply have both a directory-based cert store and a cert bundle. In Mageia I see both /etc/pki/tls/rootcerts/ (a directory-based cert store) and /etc/pki/tls/certs/ca-bundle.crt (a single file cert bundle). (yes, I hope they're synchronized :))
Generally, the only reason we would add a new feature in a bugfix release is if it's necessary to fix a security issue (such as the hash randomization feature). Here it's not necessary: you could simply ship a cert bundle in addition to the cert store. I suppose its generation is easily automated with a script.
(and, yes, the ssl module has long lacked important features; its history is a bit bumpy)
Again, for 3.3, a patch allowing urllib.request to call load_default_verify_locations() could be a good idea.

Something like this perhaps?
--- a/Lib/urllib/request.py Fri May 11 13:11:02 2012 -0400
+++ b/Lib/urllib/request.py Fri May 11 11:03:02 2012 -0700
@@ -135,16 +135,19 @@
 
 _opener = None
 def urlopen(url, data=None, timeout=socket._GLOBAL_DEFAULT_TIMEOUT,
- *, cafile=None, capath=None):
+ *, cafile=None, capath=None, cadefault=True):
 global _opener
 if cafile or capath:
 if not _have_ssl:
 raise ValueError('SSL support not available')
 context = ssl.SSLContext(ssl.PROTOCOL_SSLv23)
 context.options |= ssl.OP_NO_SSLv2
- if cafile or capath:
+ if cafile or capath or cadefault:
 context.verify_mode = ssl.CERT_REQUIRED
- context.load_verify_locations(cafile, capath)
+ if cafile or capath:
+ context.load_verify_locations(cafile, capath)
+ else:
+ context.load_default_verify_locations()
 check_hostname = True
 else:
 check_hostname = False

> Something like this perhaps?
For example, yes. Now we need to find a way of testing this...

Ok, here's a patch with a test and documentation updates.

> Ok, here's a patch with a test and documentation updates.
Ok, thanks! The only change I would make is that cadefault needs to be
False by default; particularly because some platforms don't have a
OpenSSL-compatible default CA store (Windows comes to mind) and all
HTTPS requests would then start failing.
You don't have to upload a new patch; I can change it when committing.

Oh, by the way, could you sign and send a contributor agreement? See http://www.python.org/psf/contrib/
(it is not a copyright assignment, just a formal licensing agreement)

New changeset f2ed5de1c568 by Antoine Pitrou in branch 'default':
Issue #14780: urllib.request.urlopen() now has a `cadefault` argument to use the default certificate store.
http://hg.python.org/cpython/rev/f2ed5de1c568 

Patch committed. I propose to close the issue, unless further enhancements are suggested.

Ok, perfect. I submitted a copy of the agreement.

The documentation of the default value "cadefault=False" was fixed in Issue 17977. A later change seems to have made this paramter redundant. Anyway I think this can be closed.