How to use virtualenv in Vim ?

I added plugin virtualenv (https://github.com/jmcantrell/vim-virtualenv) into my
runtimepath.

I'm new to virtualenv. The Vim plugin virtualenv has a function for statusline
show the virutalenv name. I added it into my statusline option like this:
set statusline+=\ \%{virtualenv#statusline()}
And I also add this virtualenv segment into Powerline statusline.
call Pl#Theme#InsertSegment('virtualenv', 'after', 'hgrev')

I execute some commands:
$ virtualenv test
$ source bin/activate
$ python bin/activate_this.py
$ vim test.py
Then I do not see the virtualenv name in statusline, both original statusline
and Powerline statusline.

Does anybody knows what I should do ? Thanks very much.

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