virtualenv是什麼？它是爲了方便Python獨立開發應運而生的一款集成管理工具，通俗的說就是在和系統本身的環境不衝突的前提下獨立出一塊隔離的空間，把它留給新項目的依賴環境——就是爲了解決不同版本、不同類型的Python依賴衝突而誕生的！
virtualenv簡介和安裝
virtualenv是Github上的一個項目，按照它的原話就是『Python虛擬環境的構建者』，這是它的項目地址：Virtual Python Environment builder
這裏簡單介紹下它的工作原理：virtualenv把運行Python程序必須的基本環境，包括二進制Python自身、Python標準庫、pip安裝器，以及至關重要的site-packages目錄，全部拷貝一份到一個完全隔離的目錄下，這個目錄就是你的獨立開環境
在項目首頁最下面有安裝教程的超鏈接：Installation，你可以簡單敲pip命令進行安裝，這是最簡便的方法
pip install virtualenv另外還用第二種方法：把項目整個當下來，然後手動安裝
# Use virtualenv globally
$ curl -O https://pypi.python.org/packages/source/v/virtualenv/virtualenv-X.X.tar.gz
$ tar xvfz virtualenv-X.X.tar.gz
$ cd virtualenv-X.X
$ [sudo] python setup.py install
# Use virtualenv locally
$ curl -O https://pypi.python.org/packages/source/v/virtualenv/virtualenv-X.X.tar.gz
$ tar xvfz virtualenv-X.X.tar.gz
$ cd virtualenv-X.X
$ python virtualenv.py myVE事實上你需要全局安裝的Python套件只有兩個，即pip和virtualenv，如果 把virtualenv包括的pip算進去，那其實只要安裝一個virtualenv已足矣
使用virtualenv創建獨立項目
在安裝好virtualenv後，就可以創建隔離環境的Python項目了，下面以demo這個項目爲例
1. 創建項目根目錄，命名爲demo並進入該目錄
➜ ~ mkdir demo
➜ ~ cd demo
➜ demo 2. 使用virtualenv命令，建議加上–no-site-packages選項
➜ demo virtualenv --no-site-packages venv
Using base prefix '/Library/Frameworks/Python.framework/Versions/3.6'
New python executable in /Users/macbookpro/demo/venv/bin/python3.6
Also creating executable in /Users/macbookpro/demo/venv/bin/python
Installing setuptools, pip, wheel...done.3. 如果想要手動指定Python版本，請使用–python=$PYTHON_VERSION選項
➜ demo virtualenv --python=python2.7 --no-site-packages venv1
Running virtualenv with interpreter /usr/local/bin/python2.7
New python executable in /Users/macbookpro/demo/venv1/bin/python2.7
Also creating executable in /Users/macbookpro/demo/venv1/bin/python
Installing setuptools, pip, wheel...done.4. 你甚至可以指定系統沒有安裝的Python版本，比如我這裏的Python3.7，但是絕對不要想當然的填寫Pyhton版本號否則會報錯“doesn’t exist”
➜ demo virtualenv --python=python3.7 --no-site-packages venv2
Running virtualenv with interpreter /usr/local/bin/python3.7
Using base prefix '/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7'
/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/virtualenv.py:1041: DeprecationWarning: the imp module is deprecated in favour of importlib; see the module's documentation for alternative uses
import imp
New python executable in /Users/macbookpro/demo/venv2/bin/python3.7
Also creating executable in /Users/macbookpro/demo/venv2/bin/python
Installing setuptools, pip, wheel...done.
➜ demo virtualenv --python=python4.0 --no-site-packages venv3
The path python4.0 (from --python=python4.0) does not exist5. 在新建的venv目錄下，重點關注bin目錄，它包括了pip管理器和python對應版本的解釋器Pythonx.x.
➜ venv ls
bin include lib pip-selfcheck.json
➜ venv cd bin
➜ bin ls
activate easy_install pip3.6 python3.6
activate.csh easy_install-3.6 python wheel
activate.fish pip python-config
activate_this.py pip3 python3➜ demo venv/bin/pip install requests
Collecting requests
Downloading https://files.pythonhosted.org/packages/65/47/7e02164a2a3db50ed6d8a6ab1d6d60b69c4c3fdf57a284257925dfc12bda/requests-2.19.1-py2.py3-none-any.whl (91kB)
100% |████████████████████████████████| 92kB 138kB/s
Collecting idna<2.8,>=2.5 (from requests)
Downloading https://files.pythonhosted.org/packages/4b/2a/0276479a4b3caeb8a8c1af2f8e4355746a97fab05a372e4a2c6a6b876165/idna-2.7-py2.py3-none-any.whl (58kB)
100% |████████████████████████████████| 61kB 52kB/s
Collecting urllib3<1.24,>=1.21.1 (from requests)
Downloading https://files.pythonhosted.org/packages/bd/c9/6fdd990019071a4a32a5e7cb78a1d92c53851ef4f56f62a3486e6a7d8ffb/urllib3-1.23-py2.py3-none-any.whl (133kB)
100% |████████████████████████████████| 143kB 29kB/s
Collecting chardet<3.1.0,>=3.0.2 (from requests)
Downloading https://files.pythonhosted.org/packages/bc/a9/01ffebfb562e4274b6487b4bb1ddec7ca55ec7510b22e4c51f14098443b8/chardet-3.0.4-py2.py3-none-any.whl (133kB)
100% |████████████████████████████████| 143kB 53kB/s
Collecting certifi>=2017.4.17 (from requests)
Downloading https://files.pythonhosted.org/packages/7c/e6/92ad559b7192d846975fc916b65f667c7b8c3a32bea7372340bfe9a15fa5/certifi-2018.4.16-py2.py3-none-any.whl (150kB)
100% |████████████████████████████████| 153kB 41kB/s
Installing collected packages: idna, urllib3, chardet, certifi, requests
Successfully installed certifi-2018.4.16 chardet-3.0.4 idna-2.7 requests-2.19.1 urllib3-1.236. 用virtual內置pip安裝的包被放在lib目錄下
➜ demo venv/bin/pip show requests
Name: requests
Version: 2.19.1
Summary: Python HTTP for Humans.
Home-page: http://python-requests.org
Author: Kenneth Reitz
Author-email: me@kennethreitz.org
License: Apache 2.0
Location: /Users/macbookpro/demo/venv/lib/python3.6/site-packages
Requires: idna, certifi, chardet, urllib3
Required-by:7. 爲了減少打字，可激活activate文件，這樣原來的venv/bin/pip就要寫pip，代替了系統原來的pip命令，不用時激活deactivate命令即可
➜ demo source venv/bin/activate
(venv) ➜ demo pip list
Package Version
---------- ---------
certifi 2018.4.16
chardet 3.0.4
idna 2.7
pip 10.0.1
requests 2.19.1
setuptools 40.0.0
urllib3 1.23
wheel 0.31.1
(venv) ➜ demo pip uninstall requests
Uninstalling requests-2.19.1:
Would remove:
/Users/macbookpro/demo/venv/lib/python3.6/site-packages/requests-2.19.1.dist-info/*
/Users/macbookpro/demo/venv/lib/python3.6/site-packages/requests/*
Proceed (y/n)? y
Successfully uninstalled requests-2.19.1
(venv) ➜ demo pip list
Package Version
---------- ---------
certifi 2018.4.16
chardet 3.0.4
idna 2.7
pip 10.0.1
setuptools 40.0.0
urllib3 1.23
wheel 0.31.1
(venv) ➜ demo
(venv) ➜ demo deactivate
➜ demo 8. pip和virtualenv合作使用起來很爽，pip使用-r參數指定requirement文件
pip install -r requirements.txtok，這樣基本的隔離Python開發環境就被創建出來啦～關於virtualenv暫就講到這裏，還有些比較方便的功能有待大家共同探索^ ^
參考
ಥ_ಥ A non-magical introduction to Pip and Virtualenv for Python beginners
ಥ_ಥ Requirements Files
備註1：Python2.x和3.x的site-packages目錄
Python2.7：/Library/Python/2.7/site-packages
Python3.6：/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages
備註2：使用問題
事實上在工程中已經不用virtuakenv了，我們有更好的替代品virtualenvwrapper，它封裝了virtualenv的所有功能並有利於集中管理，不過不必擔心二者的基本思想是一樣的，變化的無非是一些命令罷了