问virtualenv中的'pip list‘出现断言错误。

EN

(virtenv)[user@xyz ~]$ pip list
beautifulsoup4 (4.2.1)
biopython (1.61)
distribute (0.6.35)
methylpy (0.1.0)
MySQL-python (1.2.4)
numpy (1.7.1)
pip (1.4)
py (1.4.15)
pytest (2.3.5)
PyUblas (2013.1)
Exception:
Traceback (most recent call last):
  File "/home/user/virtenv/lib/python2.7/site-packages/pip/basecommand.py", line 134, in main
    status = self.run(options, args)
  File "/home/user/virtenv/lib/python2.7/site-packages/pip/commands/list.py", line 80, in run
    self.run_listing(options)
  File "/home/user/virtenv/lib/python2.7/site-packages/pip/commands/list.py", line 127, in run_listing
    self.output_package_listing(installed_packages)
  File "/home/user/virtenv/lib/python2.7/site-packages/pip/commands/list.py", line 136, in output_package_listing
    if dist_is_editable(dist):
  File "/home/user/virtenv/lib/python2.7/site-packages/pip/util.py", line 347, in dist_is_editable
    req = FrozenRequirement.from_dist(dist, [])
  File "/home/user/virtenv/lib/python2.7/site-packages/pip/__init__.py", line 194, in from_dist
    assert len(specs) == 1 and specs[0][0] == '=='
AssertionError