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.
+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.
When used from within a virtual environment, common installation tools such as
:pypi:`pip` will install Python packages into a virtual environment