我不知道为什么当 virtualenv 处于活动状态时， pip freeze 仍然显示所有模块，而它认为不显示。我究竟做错了什么？
我尝试了两种方法来创建虚拟环境：
virtualenv my-virtualenv
virtualenv --no-site-packages my-virtualenv
然后激活它
source my-virtualenv/bin/activate
两者都显示所有包（当他们认为不显示时）。运行命令pip freeze我得到：
(my-virtualenv)$ pip freeze
DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7.
actionlib==1.11.9
angles==1.9.11
bondpy==1.7.19
camera-calibration==1.12.20
camera-calibration-parsers==1.11.12
catkin==0.7.6
cv-bridge==1.12.4
diagnostic-analysis==1.9.2
diagnostic-common-diagnostics==1.9.2
diagnostic-updater==1.9.2
dynamic-reconfigure==1.5.48
gazebo-plugins==2.5.13
gazebo-ros==2.5.13
...
rosnode==1.12.7
rosparam==1.12.7
rospy==1.12.7 …