How to do it...

1. Activate the virtual environment by running this:

$ source .venv/bin/activate

You'll notice that the prompt will display (.venv), showing that the virtual environment is active.

2. Notice that the Python interpreter used is the one inside the virtual environment, and not the general operative system one from step 3 of Getting ready. Checking the location within a virtual environment:

(.venv) $ which python
/root_dir/.venv/bin/python
(.venv) $ which pip
/root_dir/.venv/bin/pip

3. Upgrade the version of pip and check the version:

(.venv) $ pip install --upgrade pip
...
Successfully installed pip-10.0.1
(.venv) $ pip --version
pip 10.0.1 from /root_dir/.venv/lib/python3.6/site-packages/pip (python 3.6)

4. Get out of the environment and run pip to check the version, which will return the previous environment. Check the pip version and the Python interpreter to show the ones before activating the virtual environment ones, as shown in step 3 of the Getting ready section. Note that they are different pip versions!

(.venv) $ deactivate 
$ which python3
/usr/local/bin/python3
$ pip --version
pip 9.0.3 from /usr/local/lib/python3.6/site-packages/pip (python 3.6)