ipython: UserWarning: Attempting to work in a virtualenv. If you encounter problems, please install IPython inside the virtualenv.

/Users/netsu/.local/share/virtualenvs/csr-44kfYsMA/lib/python2.7/site-packages/IPython/core/interactiveshell.py:726: UserWarning: Attempting to work in a virtualenv. If you encounter problems, please install IPython inside the virtualenv.
  warn("Attempting to work in a virtualenv. If you encounter problems, please "
Python 2.7.14 (default, Nov 14 2017, 19:52:12) 
Type "copyright", "credits" or "license" for more information.

IPython 5.5.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: python.el: native completion setup loaded

In [2]: import os, sys, IPython

In [3]: print os.environ['VIRTUAL_ENV']
print sys.executable
print IPython.__file__
print sys.path
/Users/netsu/.virtualenvs/csr-44kfYsMA

In [4]: /Users/netsu/.local/share/virtualenvs/csr-44kfYsMA/bin/python2.7

In [5]: /Users/netsu/.local/share/virtualenvs/csr-44kfYsMA/lib/python2.7/site-packages/IPython/__init__.pyc

In [6]: ['/Users/netsu/.virtualenvs/csr-44kfYsMA/lib/python2.7/site-packages', '', '/Users/netsu/.local/share/virtualenvs/csr-44kfYsMA/bin', '/Users/netsu/.local/share/virtualenvs/csr-44kfYsMA/lib/python27.zip', '/Users/netsu/.local/share/virtualenvs/csr-44kfYsMA/lib/python2.7', '/Users/netsu/.local/share/virtualenvs/csr-44kfYsMA/lib/python2.7/plat-darwin', '/Users/netsu/.local/share/virtualenvs/csr-44kfYsMA/lib/python2.7/plat-mac', '/Users/netsu/.local/share/virtualenvs/csr-44kfYsMA/lib/python2.7/plat-mac/lib-scriptpackages', '/Users/netsu/.local/share/virtualenvs/csr-44kfYsMA/lib/python2.7/lib-tk', '/Users/netsu/.local/share/virtualenvs/csr-44kfYsMA/lib/python2.7/lib-old', '/Users/netsu/.local/share/virtualenvs/csr-44kfYsMA/lib/python2.7/lib-dynload', '/usr/local/Cellar/python/2.7.14/Frameworks/Python.framework/Versions/2.7/lib/python2.7', '/usr/local/Cellar/python/2.7.14/Frameworks/Python.framework/Versions/2.7/lib/python2.7/plat-darwin', '/usr/local/Cellar/python/2.7.14/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-tk', '/usr/local/Cellar/python/2.7.14/Frameworks/Python.framework/Versions/2.7/lib/python2.7/plat-mac', '/usr/local/Cellar/python/2.7.14/Frameworks/Python.framework/Versions/2.7/lib/python2.7/plat-mac/lib-scriptpackages', '/Users/netsu/.local/share/virtualenvs/csr-44kfYsMA/lib/python2.7/site-packages', '/Users/netsu/.local/share/virtualenvs/csr-44kfYsMA/lib/python2.7/site-packages/IPython/extensions', '/Users/netsu/.ipython']

What cause that warning?