无法使用virt env为django项目安装依赖项

Question

我在为django项目安装依赖项时遇到问题：

(my_project123_env)$ python manage.py runserver
Traceback (most recent call last):
  File "manage.py", line 8, in <module>
    from django.core.management import execute_from_command_line
  File "/Users/alex/.virtualenvs/my_project123_env/lib/python2.7/site-packages/django/core/management/__init__.py", line 1, in <module>
    import collections
ImportError: No module named collections

(my_project123_env)silveranbook:my_project123 alex$ pip install collections
Traceback (most recent call last):
  File "/Users/alex/.virtualenvs/my_project123_env/bin/pip", line 7, in <module>
    from pip import main
  File "/Users/alex/.virtualenvs/my_project123_env/lib/python2.7/site-packages/pip/__init__.py", line 2, in <module>
    from __future__ import absolute_import
ImportError: No module named __future__

(my_project123_env)silveranbook:my_project123 alex$ pip install __future__
Traceback (most recent call last):
  File "/Users/alex/.virtualenvs/my_project123_env/bin/pip", line 7, in <module>
    from pip import main
  File "/Users/alex/.virtualenvs/my_project123_env/lib/python2.7/site-packages/pip/__init__.py", line 2, in <module>
    from __future__ import absolute_import
ImportError: No module named __future__

(my_project123_env)silveranbook:my_project123 alex$ pip install future
Traceback (most recent call last):
  File "/Users/alex/.virtualenvs/my_project123_env/bin/pip", line 7, in <module>
    from pip import main
  File "/Users/alex/.virtualenvs/my_project123_env/lib/python2.7/site-packages/pip/__init__.py", line 2, in <module>
    from __future__ import absolute_import
ImportError: No module named __future__

我该如何解决？