each with their own independent set of Python packages installed in
their :mod:`site` directories.
A virtual environment is created on top of an existing
-Python installation, known as the virtual environment's "base" Python, and may
-optionally be isolated from the packages in the base environment,
-so only those explicitly installed in the virtual environment are available.
-See :ref:`sys-path-init-virtual-environments` and :mod:`site`'s
+Python installation, known as the virtual environment's "base" Python, and by
+default is isolated from the packages in the base environment,
+so that only those explicitly installed in the virtual environment are
+available. See :ref:`sys-path-init-virtual-environments` and :mod:`site`'s
:ref:`virtual environments documentation <site-virtual-environments-configuration>`
for more information.