草稿：Python virtualenvs

# Virtual Environments ("virtualenvs") keep
# your project dependencies separated.
# They help you avoid version conflicts
# between packages and different versions
# of the Python runtime.

# Before creating & activating a virtualenv:
# `python` and `pip` map to the system
# version of the Python interpreter
# (e.g. Python 2.7)
$ which python
/usr/local/bin/python

# Let's create a fresh virtualenv using
# another version of Python (Python 3):
$ python3 -m venv ./venv

# A virtualenv is just a "Python
# environment in a folder":
$ ls ./venv
bin      include    lib      pyvenv.cfg

# Activating a virtualenv configures the
# current shell session to use the python
# (and pip) commands from the virtualenv
# folder instead of the global environment:
$ source ./venv/bin/activate

# Note how activating a virtualenv modifies
# your shell prompt with a little note
# showing the name of the virtualenv folder:
(venv) $ echo "wee!"

# With an active virtualenv, the `python`
# command maps to the interpreter binary
# *inside the active virtualenv*:
(venv) $ which python
/Users/dan/my-project/venv/bin/python3

# Installing new libraries and frameworks
# with `pip` now installs them *into the
# virtualenv sandbox*, leaving your global
# environment (and any other virtualenvs)
# completely unmodified:
(venv) $ pip install requests

# To get back to the global Python
# environment, run the following command:
(venv) $ deactivate

# (See how the prompt changed back
# to "normal" again?)
$ echo "yay!"

# Deactivating the virtualenv flipped the
# `python` and `pip` commands back to
# the global environment:
$ which python
/usr/local/bin/python