If one of the hash functions isn't defined in _hashlib, the code suggests it should just be skipped

===
      # this one has no builtin implementation, don't define it
                pass
===

This doesn't happen however; due to ImportError not being caught the module decides the whole _hashlib module isn't available and tries to fall back to the older individual libraries.  You then get thrown an unrelated error about _md5 being unavailable

You can easily replicate this

---
$ python
Python 2.6.6 (r266:84292, Dec 26 2010, 22:31:48) 
[GCC 4.4.5] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> def foo():
...     raise ValueError
... 
>>> import _hashlib
>>> _hashlib.openssl_sha224 = foo
>>> import hashlib
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/hashlib.py", line 136, in <module>
    md5 = __get_builtin_constructor('md5')
  File "/usr/lib/python2.6/hashlib.py", line 63, in __get_builtin_constructor
    import _md5
ImportError: No module named _md5
>>> 
---

I think the solution is to catch the ImportError in __get_builtin_constructor and, if caught, consider the hash function unsupported

New changeset 572fbd9ca28f by Gregory P. Smith in branch '2.7':
Fixes Issue #12059: Properly handle missing hash functions even when
http://hg.python.org/cpython/rev/572fbd9ca28f

New changeset ad9249f35914 by Gregory P. Smith in branch '3.2':
Fixes Issue #12059: Properly handle missing hash functions even when
http://hg.python.org/cpython/rev/ad9249f35914

New changeset 298888fd46b0 by Gregory P. Smith in branch 'default':
Fixes Issue #12059: Properly handle missing hash functions even when
http://hg.python.org/cpython/rev/298888fd46b0