导言 本文是针对《Python大学实用教程》 和《跟老齐学Python：轻松入门》 两本书的基础内容之后的提升。
通常，在不同的项目中，会用到不同的Python版本——如果有必要的话，也包括一些库的版本，并非总是最新的就是最适合的或者最好的。对此，解决之道就是创建虚拟环境。
一个项目创建一个虚拟环境，在每个虚拟环境之中，就相当于一个“裸机”，里面所需要的东西任你根据当前的需要自由配置，并且不会影响到其他项目。特别是在诸如Linux系统上，有时候安装某些东西还需要root权限，此时虚拟环境的好处就更明显了。
创建虚拟环境 不论是Python2，还是Python3中，都可以创建虚拟环境——虽然现在广泛使用Python3，但也有项目在Python2上跑着呢。只是两个版本中创建方法稍有区别。
在Python3中创建虚拟环境 下面的方法是Python3中推荐的方法，务必掌握。
1 $ python3 -m venv python3venv
命令行上的-m参数是告诉解释器运行venv模块，此模块是Python3核心发行版的一部分。python3venv是虚拟环境的目录名称。执行之后就会创建python3venv，虚拟环境就在这个目录中。
如果对venv好奇，可以用下面的方式来瞧一瞧。
1 2 3 4 5 6 7 $ python3 Python 3.6 .4 (v3.6 .4 :d48ecebad5, Dec 18 2017 , 21 :07 :28 ) [GCC 4.2 .1 (Apple Inc. build 5666 ) (dot 3 )] on darwin Type "help" , "copyright" , "credits" or "license" for more information. >>> import venv>>> print(venv)<module 'venv' from '/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/venv/__init__.py' >
venv模块的代码在/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/venv/__init__.py，查看此文件（执行cat /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/venv/__init__.py即可看到），并到最底部，会看到如下内容：
1 2 3 4 5 6 7 8 if __name__ == '__main__' : rc = 1 try : main() rc = 0 except Exception as e: print('Error: %s' % e, file=sys.stderr) sys.exit(rc)
如果要查看命令行的参数，可以：
1 2 3 4 5 $ python3 -m venv usage: venv [-h] [--system-site-packages] [--symlinks | --copies] [--clear] [--upgrade] [--without-pip] [--prompt PROMPT] ENV_DIR [ENV_DIR ...] venv: error: the following arguments are required: ENV_DIR
在Python3.6及更低版本中创建虚拟环境 现在不推荐这种方法，但是，也介绍一下。因为这个方法主要用于Python3.6及以下版本中。
Python3.6及以下版本有一个名为pyvenv的可执行文件，它本质上是venv模块的二进制包装器。如果按按照下面的方法操作，就可以看到命令行的参数。
1 2 3 4 5 6 $ pyvenv WARNING: the pyenv script is deprecated in favour of `python3.6 -m venv` usage: venv [-h] [--system-site-packages] [--symlinks | --copies] [--clear] [--upgrade] [--without-pip] [--prompt PROMPT] ENV_DIR [ENV_DIR ...] venv: error: the following arguments are required: ENV_DIR
使用它创建虚拟目录的方法是：
1 2 $ pyvenv python3venv WARNING: the pyenv script is deprecated in favour of `python3.6 -m venv`
但是，会告诉我们，这个东西弃用了，后面要使用前面推荐的方法。
在Python2.x中创建虚拟环境 在Python2.x上，用virtualenv模块创建虚拟环境。在不同的操作系统中，对virtualenv可能有不同的处理方式，比如有的默认就安装了这个模块，有的没有安装，这就需要你先安装上。
检查一下它是不是在你的机器上了。
1 2 $ which virtualenv /Library/Frameworks/Python.framework/Versions/2.7 /bin/virtualenv
创建一个Python2.x的虚拟目录，其语法类似于前面的方法：
1 2 3 $ virtualenv python2venv New python executable in /Users/james/python2venv/bin/python Installing setuptools, pip, wheel...done.
检查虚拟环境 已经创建了两个虚拟环境的目录，分别是：
1 2 3 4 5 6 $ ls python3venv python2venv/ python2venv/: bin include lib python3venv: bin include lib pyvenv.cfg
这两个目录的却别在于，python3venv中能够看到pyvenv.cfg文件，如果你有兴趣了解有关此文件的更多信息，可以在PEP-0405 查看相关的背景知识。
激活虚拟环境 要使用虚拟环境，必须先激活。
注意，虚拟环境激活前后，shell的提示符会发生变化。原来是这样的：
1 2 $ echo $PATH /Library/Frameworks/Python.framework/Versions/3.6 /bin:/Library/Frameworks/Python.framework/Versions/2.7 /bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin
激活虚拟环境之后（执行source python3venv/bin/activate指令后）显示(python3venv) $。
1 2 $ source python3venv/bin/activate (python3venv) $
检查路径后，虚拟环境将优先于前面做同样操作是呈现的路径。
1 2 (python3venv) $ echo $PATH /Users/james/python3venv/bin:/Library/Frameworks/Python.framework/Versions/3.6 /bin:/Library/Frameworks/Python.framework/Versions/2.7 /bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin
执行python 如果我们现在执行python或pip，由于路径优先（前面的操作已经显示，python3.6优先执行了虚拟环境中），来自虚拟环境命令将优先执行。如果用下面的方式查看，会发现它们都在相应的虚拟环境中。
1 2 3 4 (python3venv) $ which python /Users/james/python3venv/bin/python (python3venv) $ which pip /Users/james/python3venv/bin/pip
比如执行python，本来本地机器上安装两个版本的Python，如果不用虚拟环境，python之后默认会执行Python2，要执行Python3，必须是python3。但是，现在不同了，因为激活了Python3.6的虚拟环境，当执行python命令后，会首先在指定的虚拟环境中搜索。并且，进入到Python3之后，查看sys.path，当前虚拟环境目录也被添加到Python搜索路径中了。
1 2 3 4 5 6 7 (python3venv) $ python Python 3.6 .4 (v3.6 .4 :d48ecebad5, Dec 18 2017 , 21 :07 :28 ) [GCC 4.2 .1 (Apple Inc. build 5666 ) (dot 3 )] on darwin Type "help" , "copyright" , "credits" or "license" for more information. >>> import sys>>> print(sys.path)['' , '/Library/Frameworks/Python.framework/Versions/3.6/lib/python36.zip' , '/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6' , '/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/lib-dynload' , '/Users/james/python3venv/lib/python3.6/site-packages' ]
用pip安装软件包 激活虚拟环境后，使用pip会将模块或者第三方包直接安装到虚拟环境。例如，如果我们安装了名为arrow的模块——专门解读日期、时间的模块，比传统的datatime模块使用更方便。
1 2 3 4 5 6 7 8 9 10 11 12 (python3venv) $ pip install arrow Collecting arrow Downloading https://files.pythonhosted.org/packages/f4/7 f/0360628 ba40bb93c10cd89cd289b6a8e9ea87b2db884b8edf32c80ee1c73/arrow-0.13 .1 -py2.py3-none-any.whl Collecting python-dateutil (from arrow) Downloading https://files.pythonhosted.org/packages/41 /17 /c62faccbfbd163c7f57f3844689e3a78bae1f403648a6afb1d0866d87fbb/python_dateutil-2.8 .0 -py2.py3-none-any.whl (226 kB) 100 % |████████████████████████████████| 235 kB 3.6 MB/s Collecting six>=1.5 (from python-dateutil->arrow) Downloading https://files.pythonhosted.org/packages/73 /fb/00 a976f728d0d1fecfe898238ce23f502a721c0ac0ecfedb80e0d88c64e9/six-1.12 .0 -py2.py3-none-any.whl Installing collected packages: six, python-dateutil, arrow Successfully installed arrow-0.13 .1 python-dateutil-2.8 .0 six-1.12 .0 You are using pip version 9.0 .1 , however version 19.0 .3 is available. You should consider upgrading via the 'pip install --upgrade pip' command.
上面的安装过程显示，在安装了arrow的过程中，还安装了six和python-dateutil两个依赖模块。我们可以再次确认arrow模块在虚拟环境中。
1 2 3 4 5 6 7 (python3venv) $ python Python 3.6 .4 (v3.6 .4 :d48ecebad5, Dec 18 2017 , 21 :07 :28 ) [GCC 4.2 .1 (Apple Inc. build 5666 ) (dot 3 )] on darwin Type "help" , "copyright" , "credits" or "license" for more information. >>> import arrow>>> print(arrow)<module 'arrow' from '/Users/james/python3venv/lib/python3.6/site-packages/arrow/__init__.py' >
升级pip 在上面用pip安装的时候，最后有警公告，提示要升级pip，如果看到这个提示，就要必须升级，为什么？请阅读：
必须升级pip，为什么？！
可以按以下方式升级虚拟环境中的pip。
1 2 3 4 5 6 7 8 9 10 11 12 (python3venv) $ pip install --upgrade pip Cache entry deserialization failed, entry ignored Collecting pip Using cached https://files.pythonhosted.org/packages/d8/f3/413 bab4ff08e1fc4828dfc59996d721917df8e8583ea85385d51125dceff/pip-19.0 .3 -py2.py3-none-any.whl Installing collected packages: pip Found existing installation: pip 9.0 .1 Uninstalling pip-9.0 .1 : Successfully uninstalled pip-9.0 .1 Successfully installed pip-19.0 .3 (python3venv) $ pip --version pip 19.0 .3 from /Users/james/python3venv/lib/python3.6 /site-packages/pip (python 3.6 )
（注意： 上面显示，不一定是你阅读到本文的时候pip的最新版本。）
集成环境需求 为了让下面的示例更能说明问题，在安装flask——一个用于web开发的框架。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 (python3venv) $ pip install flask Collecting flask Downloading https://files.pythonhosted.org/packages/7 f/e7/08578774 ed4536d3242b14dacb4696386634607af824ea997202cd0edb4b/Flask-1.0 .2 -py2.py3-none-any.whl (91 kB) 100 % |████████████████████████████████| 92 kB 2.4 MB/s Collecting Jinja2>=2.10 (from flask) Downloading https://files.pythonhosted.org/packages/7 f/ff/ae64bacdfc95f27a016a7bed8e8686763ba4d277a78ca76f32659220a731/Jinja2-2.10 -py2.py3-none-any.whl (126 kB) 100 % |████████████████████████████████| 133 kB 7.6 MB/s Collecting Werkzeug>=0.14 (from flask) Downloading https://files.pythonhosted.org/packages/20 /c4/12e3 e56473e52375aa29c4764e70d1b8f3efa6682bef8d0aae04fe335243/Werkzeug-0.14 .1 -py2.py3-none-any.whl (322 kB) 100 % |████████████████████████████████| 327 kB 13.0 MB/s Collecting itsdangerous>=0.24 (from flask) Downloading https://files.pythonhosted.org/packages/76 /ae/44 b03b253d6fade317f32c24d100b3b35c2239807046a4c953c7b89fa49e/itsdangerous-1.1 .0 -py2.py3-none-any.whl Collecting click>=5.1 (from flask) Downloading https://files.pythonhosted.org/packages/fa/37 /45185 cb5abbc30d7257104c434fe0b07e5a195a6847506c074527aa599ec/Click-7.0 -py2.py3-none-any.whl (81 kB) 100 % |████████████████████████████████| 81 kB 15.2 MB/s Collecting MarkupSafe>=0.23 (from Jinja2>=2.10->flask) Downloading https://files.pythonhosted.org/packages/f0/00 /a6aea33f5598b080b86d6b6d1214b51afe3ffa6100b902d5aa465080083f/MarkupSafe-1.1 .1 -cp36-cp36m-macosx_10_6_intel.whl Installing collected packages: MarkupSafe, Jinja2, Werkzeug, itsdangerous, click, flask Successfully installed Jinja2-2.10 MarkupSafe-1.1 .1 Werkzeug-0.14 .1 click-7.0 flask-1.0 .2 itsdangerous-1.1 .0
安装flask的同时，也会安装相关的依赖模块。这样，在虚拟环境中就有了比较多的模块，而且每个模块（或者库）都有一定的版本。
下面按照如下的指令执行，目的是要生成一个名为requirements.txt的文件，在这个文件中，将记录当前虚拟环境中所安装的模块及其版本（注意，requirements.txt文件目录，先下面代码这样指定，是为了后面使用方便。）。
1 2 3 4 5 6 7 8 9 10 (python3venv) $ pip freeze | tee /tmp/requirements.txt arrow==0.13 .1 Click==7.0 Flask==1.0 .2 itsdangerous==1.1 .0 Jinja2==2.10 MarkupSafe==1.1 .1 python-dateutil==2.8 .0 six==1.12 .0 Werkzeug==0.14 .1
有了这个文件，当我们需要重现这个虚拟环境中已经安装的各个模块是，就简单了。
重现虚拟环境配置 前面，曾经创建了一个Python2的虚拟环境，现在要将刚才在Python3的虚拟环境中安装的各个模块（即配置），移植到Python2的虚拟环境中——注意，这种做法并不提倡，但是，这里纯粹是为演示虚拟环境的重现移植。
首先，将现在的python3venv虚拟环境停用。
1 2 (python3venv) $ deactivate $
然后激活Python2的虚拟环境python2venv。
1 2 $ source python2venv/bin/activate (python2venv) $
升级pip，多数情况下，用pip install –upgrade pip即可，但是，如果遇到了意想不到的事情，可以用下面的方式尝试解决。
1 2 3 4 5 6 7 8 9 10 11 12 $ (python2venv) curl https://bootstrap.pypa.io/get-pip.py | python % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 100 1659k 100 1659k 0 0 2375k 0 --:--:-- --:--:-- --:--:-- 2377k DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7. Collecting pip Using cached https://files.pythonhosted.org/packages/d8/f3/413bab4ff08e1fc4828dfc59996d721917df8e8583ea85385d51125dceff/pip-19.0.3-py2.py3-none-any.whl Installing collected packages: pip Found existing installation: pip 9.0.1 Uninstalling pip-9.0.1: Successfully uninstalled pip-9.0.1 Successfully installed pip-19.0.3
现在，要将python3venv中的配置，在当前虚拟环境中重现，方法如下：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 $ (python2venv) pip install -r /tmp/requirements.txt DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7. Collecting arrow==0.13.1 (from -r /tmp/requirements.txt (line 1)) Using cached https://files.pythonhosted.org/packages/f4/7f/0360628ba40bb93c10cd89cd289b6a8e9ea87b2db884b8edf32c80ee1c73/arrow-0.13.1-py2.py3-none-any.whl Collecting Click==7.0 (from -r /tmp/requirements.txt (line 2)) Using cached https://files.pythonhosted.org/packages/fa/37/45185cb5abbc30d7257104c434fe0b07e5a195a6847506c074527aa599ec/Click-7.0-py2.py3-none-any.whl Collecting Flask==1.0.2 (from -r /tmp/requirements.txt (line 3)) Using cached https://files.pythonhosted.org/packages/7f/e7/08578774ed4536d3242b14dacb4696386634607af824ea997202cd0edb4b/Flask-1.0.2-py2.py3-none-any.whl Collecting itsdangerous==1.1.0 (from -r /tmp/requirements.txt (line 4)) Using cached https://files.pythonhosted.org/packages/76/ae/44b03b253d6fade317f32c24d100b3b35c2239807046a4c953c7b89fa49e/itsdangerous-1.1.0-py2.py3-none-any.whl Collecting Jinja2==2.10 (from -r /tmp/requirements.txt (line 5)) Using cached https://files.pythonhosted.org/packages/7f/ff/ae64bacdfc95f27a016a7bed8e8686763ba4d277a78ca76f32659220a731/Jinja2-2.10-py2.py3-none-any.whl Collecting MarkupSafe==1.1.1 (from -r /tmp/requirements.txt (line 6)) Downloading https://files.pythonhosted.org/packages/6d/d2/0ccd2c0e2cd93b35e765d9b3205cd6602e6b202b522fc7997531353715b3/MarkupSafe-1.1.1-cp27-cp27m-macosx_10_6_intel.whl Collecting python-dateutil==2.8.0 (from -r /tmp/requirements.txt (line 7)) Using cached https://files.pythonhosted.org/packages/41/17/c62faccbfbd163c7f57f3844689e3a78bae1f403648a6afb1d0866d87fbb/python_dateutil-2.8.0-py2.py3-none-any.whl Collecting six==1.12.0 (from -r /tmp/requirements.txt (line 8)) Using cached https://files.pythonhosted.org/packages/73/fb/00a976f728d0d1fecfe898238ce23f502a721c0ac0ecfedb80e0d88c64e9/six-1.12.0-py2.py3-none-any.whl Collecting Werkzeug==0.14.1 (from -r /tmp/requirements.txt (line 9)) Using cached https://files.pythonhosted.org/packages/20/c4/12e3e56473e52375aa29c4764e70d1b8f3efa6682bef8d0aae04fe335243/Werkzeug-0.14.1-py2.py3-none-any.whl Collecting backports.functools-lru-cache>=1.2.1; python_version == "2.7" (from arrow==0.13.1->-r /tmp/requirements.txt (line 1)) Downloading https://files.pythonhosted.org/packages/03/8e/2424c0e65c4a066e28f539364deee49b6451f8fcd4f718fefa50cc3dcf48/backports.functools_lru_cache-1.5-py2.py3-none-any.whl Installing collected packages: six, python-dateutil, backports.functools-lru-cache, arrow, Click, MarkupSafe, Jinja2, itsdangerous, Werkzeug, Flask Successfully installed Click-7.0 Flask-1.0.2 Jinja2-2.10 MarkupSafe-1.1.1 Werkzeug-0.14.1 arrow-0.13.1 backports.functools-lru-cache-1.5 itsdangerous-1.1.0 python-dateutil-2.8.0 six-1.12.0
虽然现在是一个Python2的虚拟环境，但是也按照requirement.txt文件中的记录，配置了与Python3虚拟环境相同的各个模块，并且还根据Python2的特定要求，增加了应有的依赖backports.functools-lru-cache==1.5 。于是当前虚拟环境中的配置如下：
1 2 3 4 5 6 7 8 9 10 11 12 (python2venv) $ pip freeze DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7. arrow==0.13.1 backports.functools-lru-cache==1.5 Click==7.0 Flask==1.0.2 itsdangerous==1.1.0 Jinja2==2.10 MarkupSafe==1.1.1 python-dateutil==2.8.0 six==1.12.0 Werkzeug==0.14.1
不激活的情况下，使用虚拟环境 一般情况下，按照上面所说的，激活虚拟环境，然后开始使用，这已经很方便了。但是，在某些情况下，或许有不激活虚拟环境的需求。
为此，可以这么做。比如要执行虚拟环境python3venv中的Python3，可以在没有激活任何虚拟环境的情况下，直接执行python3venv/bin/python运行python3venv中的Python。
在下面的操作中，首先要从python2venv的虚拟环境中退出。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 (python2venv) $ deactivate $ python3venv/bin/python Python 3.6 .4 (v3.6 .4 :d48ecebad5, Dec 18 2017 , 21 :07 :28 ) [GCC 4.2 .1 (Apple Inc. build 5666 ) (dot 3 )] on darwin Type "help" , "copyright" , "credits" or "license" for more information. >>> import arrow>>> print(arrow)<module 'arrow' from '/Users/james/python3venv/lib/python3.6/site-packages/arrow/__init__.py' > >>> ^D$ python2venv/bin/python Python 2.7 .14 (v2.7 .14 :84471935 ed, Sep 16 2017 , 12 :01 :12 ) [GCC 4.2 .1 (Apple Inc. build 5666 ) (dot 3 )] on darwin Type "help" , "copyright" , "credits" or "license" for more information. >>> import arrow>>> print(arrow)<module 'arrow' from '/Users/james/python2venv/lib/python2.7/site-packages/arrow/__init__.pyc' > >>> ^D
在程序文件中，也可以指定所使用的虚拟环境，方法就是用#!发起指令，如下面的代码所示：
1 2 3 4 5 6 7 $ cat <<EOF > test.py > > > import arrow > utc = arrow.utcnow() > print(utc) > EOF
设置示例文件test.py可执行，然后执行，可以确认执行了虚拟环境中的模块。
1 2 3 $ chmod u+x test.py $ ./test.py 2019 -03 -03 T21:17 :22.632130 +00 :00
如此，即可根据特定需求执行程序了。
打包 如果在本地完成了开发，要将程序移植到别的机器上，通常要在目标机器上配置与本地一样的环境。当然，现在有一种容器化的方案，比如使用docker等。如果不那样做，按照上面的方法完成了虚拟环境的配置，并且在虚拟环境中完成了程序编写。那么，就可以将真个虚拟环境打包。当然，这里有一个前提，目标机器上必须安装了与虚拟环境中相同版本的Python。
以前面使用过的python3venv为例，用tar命令将目录python3venv打包。
1 $ tar cf python3venv.tar python3venv
为了演示需要，将当前的python3venv目录删除。
然后从python3venv.tar中将目录提取出来。
1 $ tar xf python3venv.tar
继续使用test.py测试一下，python3venv虚拟目录及其内部安装的各模块，都已经恢复。
1 2 $ ./test.py 2019-03-03T21:26:47.082092+00:00
用pip管理虚拟环境 pip是一个好工具，它为管理和维护虚拟环境及其依赖提供简便方法。比如，对于虚拟环境python3venv中的模块，如果要删除arrow和flask，如下操作所示，使用pip uninstall packagename即可，不过，安装时候所安装的那些依赖项会依然保留。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 (python3venv) $ pip uninstall arrow flask Uninstalling arrow-0.13.1: Would remove: /Users/james/python3venv/lib/python3.6/site-packages/arrow-0.13.1.dist-info/* /Users/james/python3venv/lib/python3.6/site-packages/arrow/* Proceed (y/n)? y Successfully uninstalled arrow-0.13.1 Uninstalling Flask-1.0.2: Would remove: /Users/james/python3venv/bin/flask /Users/james/python3venv/lib/python3.6/site-packages/Flask-1.0.2.dist-info/* /Users/james/python3venv/lib/python3.6/site-packages/flask/* Proceed (y/n)? y Successfully uninstalled Flask-1.0.2 (python3venv) $ pip freeze Click==7.0 itsdangerous==1.1.0 Jinja2==2.10 MarkupSafe==1.1.1 python-dateutil==2.8.0 six==1.12.0 Werkzeug==0.14.1
除了上面简单的方法之外，还可以通过pip-tools中的工具，实现对模块更灵活的管理。
首先，要退出当前的虚拟环境，并删除python3venv目录。然后，重新创建虚拟环境——从这就可以看出虚拟环境的优势，你可以任意删除和重建，每次重建之后它都是“干干净净”的。
进入到新建的虚拟环境之后，升级pip——这是常规套路。再安装pip-tools。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 (python3venv) $ deactivate $ rm -rf python3venv $ python3 -m venv python3venv $ source python3venv/bin/activate (python3venv) $ pip install --upgrade pip Cache entry deserialization failed, entry ignored Collecting pip Using cached https://files.pythonhosted.org/packages/d8/f3/413 bab4ff08e1fc4828dfc59996d721917df8e8583ea85385d51125dceff/pip-19.0 .3 -py2.py3-none-any.whl Installing collected packages: pip Found existing installation: pip 9.0 .1 Uninstalling pip-9.0 .1 : Successfully uninstalled pip-9.0 .1 Successfully installed pip-19.0 .3 (python3venv) $ pip install pip-tools Collecting pip-tools Downloading https://files.pythonhosted.org/packages/58 /7 a/f93b24807b7ac2d9d0bd6b8a886bcbe67eb39c1b1184b985dd5e0e2eca92/pip_tools-3.4 .0 -py2.py3-none-any.whl (43 kB) 100 % |████████████████████████████████| 51 kB 2.3 MB/s Collecting six (from pip-tools) Using cached https://files.pythonhosted.org/packages/73 /fb/00 a976f728d0d1fecfe898238ce23f502a721c0ac0ecfedb80e0d88c64e9/six-1.12 .0 -py2.py3-none-any.whl Collecting click>=6 (from pip-tools) Using cached https://files.pythonhosted.org/packages/fa/37 /45185 cb5abbc30d7257104c434fe0b07e5a195a6847506c074527aa599ec/Click-7.0 -py2.py3-none-any.whl Installing collected packages: six, click, pip-tools Successfully installed click-7.0 pip-tools-3.4 .0 six-1.12 .0
创建一个名为requirements.in的文件，在其中写上需要的模块及其版版本，比如arrow和flask，版本如下：
1 2 3 4 (python3venv) $ cat <<EOF > requirements.in > arrow==0.13 .1 > Flask==1.0 .2 > EOF
然后，使用pip-compile （是pip-tools中提供的），依据requirements.in创建requirements.txt文件
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 (python3venv) $ pip-compile --generate-hashes --output-file requirements.txt requirements.in arrow==0.13 .1 \ --hash=sha256:3397e5448952 e18e1295bf047014659effa5ae8da6a5371d37ff0ddc46fa6872 \ --hash=sha256:6 f54d9f016c0b7811fac9fb8c2c7fa7421d80c54dbdd75ffb12913c55db60b8a click==7.0 \ --hash=sha256:2335065e6395 b9e67ca716de5f7526736bfa6ceead690adf616d925bdc622b13 \ --hash=sha256:5 b94b49521f6456670fdb30cd82a4eca9412788a93fa6dd6df72c94d5a8ff2d7 \ flask==1.0 .2 \ --hash=sha256:2271 c0070dbcb5275fad4a82e29f23ab92682dc45f9dfbc22c02ba9b9322ce48 \ --hash=sha256:a080b744b7e345ccfcbc77954861cb05b3c63786e93f2b3875e0913d44b43f05 itsdangerous==1.1 .0 \ --hash=sha256:321 b033d07f2a4136d3ec762eac9f16a10ccd60f53c0c91af90217ace7ba1f19 \ --hash=sha256:b12271b2047cb23eeb98c8b5622e2e5c5e9abd9784a153e9d8ef9cb4dd09d749 \ jinja2==2.10 \ --hash=sha256:74 c935a1b8bb9a3947c50a54766a969d4846290e1e788ea44c1392163723c3bd \ --hash=sha256:f84be1bb0040caca4cea721fcbbbbd61f9be9464ca236387158b0feea01914a4 \ markupsafe==1.1 .1 \ --hash=sha256:00 bc623926325b26bb9605ae9eae8a215691f33cae5df11ca5424f06f2d1f473 \ --hash=sha256:09027 a7803a62ca78792ad89403b1b7a73a01c8cb65909cd876f7fcebd79b161 \ --hash=sha256:09 c4b7f37d6c648cb13f9230d847adf22f8171b1ccc4d5682398e77f40309235 \ --hash=sha256:1027 c282dad077d0bae18be6794e6b6b8c91d58ed8a8d89a89d59693b9131db5 \ --hash=sha256:24982 cc2533820871eba85ba648cd53d8623687ff11cbb805be4ff7b4c971aff \ --hash=sha256:29872e92839765 e546828bb7754a68c418d927cd064fd4708fab9fe9c8bb116b \ --hash=sha256:43 a55c2930bbc139570ac2452adf3d70cdbb3cfe5912c71cdce1c2c6bbd9c5d1 \ --hash=sha256:46 c99d2de99945ec5cb54f23c8cd5689f6d7177305ebff350a58ce5f8de1669e \ --hash=sha256:500 d4957e52ddc3351cabf489e79c91c17f6e0899158447047588650b5e69183 \ --hash=sha256:535 f6fc4d397c1563d08b88e485c3496cf5784e927af890fb3c3aac7f933ec66 \ --hash=sha256:62 fe6c95e3ec8a7fad637b7f3d372c15ec1caa01ab47926cfdf7a75b40e0eac1 \ --hash=sha256:6 dd73240d2af64df90aa7c4e7481e23825ea70af4b4922f8ede5b9e35f78a3b1 \ --hash=sha256:717 ba8fe3ae9cc0006d7c451f0bb265ee07739daf76355d06366154ee68d221e \ --hash=sha256:79855e1 c5b8da654cf486b830bd42c06e8780cea587384cf6545b7d9ac013a0b \ --hash=sha256:7 c1699dfe0cf8ff607dbdcc1e9b9af1755371f92a68f706051cc8c37d447c905 \ --hash=sha256:88e5 fcfb52ee7b911e8bb6d6aa2fd21fbecc674eadd44118a9cc3863f938e735 \ --hash=sha256:8 defac2f2ccd6805ebf65f5eeb132adcf2ab57aa11fdf4c0dd5169a004710e7d \ --hash=sha256:98 c7086708b163d425c67c7a91bad6e466bb99d797aa64f965e9d25c12111a5e \ --hash=sha256:9 add70b36c5666a2ed02b43b335fe19002ee5235efd4b8a89bfcf9005bebac0d \ --hash=sha256:9 bf40443012702a1d2070043cb6291650a0841ece432556f784f004937f0f32c \ --hash=sha256:ade5e387d2ad0d7ebf59146cc00c8044acbd863725f887353a10df825fc8ae21 \ --hash=sha256:b00c1de48212e4cc9603895652c5c410df699856a2853135b3967591e4beebc2 \ --hash=sha256:b1282f8c00509d99fef04d8ba936b156d419be841854fe901d8ae224c59f0be5 \ --hash=sha256:b2051432115498d3562c084a49bba65d97cf251f5a331c64a12ee7e04dacc51b \ --hash=sha256:ba59edeaa2fc6114428f1637ffff42da1e311e29382d81b339c1817d37ec93c6 \ --hash=sha256:c8716a48d94b06bb3b2524c2b77e055fb313aeb4ea620c8dd03a105574ba704f \ --hash=sha256:cd5df75523866410809ca100dc9681e301e3c27567cf498077e8551b6d20e42f \ --hash=sha256:e249096428b3ae81b08327a63a485ad0878de3fb939049038579ac0ef61e17e7 \ python-dateutil==2.8 .0 \ --hash=sha256:7e6584 c74aeed623791615e26efd690f29817a27c73085b78e4bad02493df2fb \ --hash=sha256:c89805f6f4d64db21ed966fda138f8a5ed7a4fdbc1a8ee329ce1b74e3c74da9e \ six==1.12 .0 \ --hash=sha256:3350809 f0555b11f552448330d0b52d5f24c91a322ea4a15ef22629740f3761c \ --hash=sha256:d16a0141ec1a18405cd4ce8b4613101da75da0e9a7aec5bdd4fa804d0e0eba73 \ werkzeug==0.14 .1 \ --hash=sha256:c3fd7a7d41976d9f44db327260e263132466836cef6f91512889ed60ad26557c \ --hash=sha256:d5da73735293558eb1651ee2fddc4d0dedcfa06538b8813a2e20011583c9e49b \
从上述信息中可以看到，每个依赖都有相应的哈希值，这样能够保证所安装的模块彼此之间没有冲突。
有了requirements.txt文件之后，就可以使用pip-sync（也是pip-tools中的一个工具）将虚拟环境中的配置与之同步。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 (python3venv) $ pip-sync Collecting arrow==0.13 .1 (from -r /var/folders/xk/p46swdnn3b56rbd01qwdjhdh0000gn/T/tmpfwkk4j83 (line 1 )) Using cached https://files.pythonhosted.org/packages/f4/7 f/0360628 ba40bb93c10cd89cd289b6a8e9ea87b2db884b8edf32c80ee1c73/arrow-0.13 .1 -py2.py3-none-any.whl Collecting flask==1.0 .2 (from -r /var/folders/xk/p46swdnn3b56rbd01qwdjhdh0000gn/T/tmpfwkk4j83 (line 4 )) Using cached https://files.pythonhosted.org/packages/7 f/e7/08578774 ed4536d3242b14dacb4696386634607af824ea997202cd0edb4b/Flask-1.0 .2 -py2.py3-none-any.whl Collecting itsdangerous==1.1 .0 (from -r /var/folders/xk/p46swdnn3b56rbd01qwdjhdh0000gn/T/tmpfwkk4j83 (line 7 )) Using cached https://files.pythonhosted.org/packages/76 /ae/44 b03b253d6fade317f32c24d100b3b35c2239807046a4c953c7b89fa49e/itsdangerous-1.1 .0 -py2.py3-none-any.whl Collecting jinja2==2.10 (from -r /var/folders/xk/p46swdnn3b56rbd01qwdjhdh0000gn/T/tmpfwkk4j83 (line 10 )) Using cached https://files.pythonhosted.org/packages/7 f/ff/ae64bacdfc95f27a016a7bed8e8686763ba4d277a78ca76f32659220a731/Jinja2-2.10 -py2.py3-none-any.whl Collecting markupsafe==1.1 .1 (from -r /var/folders/xk/p46swdnn3b56rbd01qwdjhdh0000gn/T/tmpfwkk4j83 (line 13 )) Using cached https://files.pythonhosted.org/packages/f0/00 /a6aea33f5598b080b86d6b6d1214b51afe3ffa6100b902d5aa465080083f/MarkupSafe-1.1 .1 -cp36-cp36m-macosx_10_6_intel.whl Collecting python-dateutil==2.8 .0 (from -r /var/folders/xk/p46swdnn3b56rbd01qwdjhdh0000gn/T/tmpfwkk4j83 (line 42 )) Using cached https://files.pythonhosted.org/packages/41 /17 /c62faccbfbd163c7f57f3844689e3a78bae1f403648a6afb1d0866d87fbb/python_dateutil-2.8 .0 -py2.py3-none-any.whl Collecting werkzeug==0.14 .1 (from -r /var/folders/xk/p46swdnn3b56rbd01qwdjhdh0000gn/T/tmpfwkk4j83 (line 45 )) Using cached https://files.pythonhosted.org/packages/20 /c4/12e3 e56473e52375aa29c4764e70d1b8f3efa6682bef8d0aae04fe335243/Werkzeug-0.14 .1 -py2.py3-none-any.whl Requirement already satisfied: click>=5.1 in ./python3venv/lib/python3.6/site-packages (from flask==1.0.2->-r /var/folders/xk/p46swdnn3b56rbd01qwdjhdh0000gn/T/tmpfwkk4j83 (line 4)) (7.0) Requirement already satisfied: six>=1.5 in ./python3venv/lib/python3.6/site-packages (from python-dateutil==2.8.0->-r /var/folders/xk/p46swdnn3b56rbd01qwdjhdh0000gn/T/tmpfwkk4j83 (line 42)) (1.12.0) Installing collected packages: python-dateutil, arrow, werkzeug, itsdangerous, markupsafe, jinja2, flask Successfully installed arrow-0.13 .1 flask-1.0 .2 itsdangerous-1.1 .0 jinja2-2.10 markupsafe-1.1 .1 python-dateutil-2.8 .0 werkzeug-0.14 .1 (python3venv) $ pip freeze arrow==0.13 .1 Click==7.0 Flask==1.0 .2 itsdangerous==1.1 .0 Jinja2==2.10 MarkupSafe==1.1 .1 pip-tools==3.4 .0 python-dateutil==2.8 .0 six==1.12 .0 Werkzeug==0.14 .1
如果删除一个模块，例如删除Flask，只需要更新requirements.in文件，如下所示。然后重复上面的过程，使用pip-sync，就可以从当前虚拟环境中删除Flask，并且，在安装的时候，随着一同安装上的各种依赖模块，也同时被移除。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 (python3venv) $ cat <<EOF > requirements.in > arrow==0.13 .1 > EOF (python3venv) $ pip-compile --generate-hashes --output-file requirements.txt requirements.in arrow==0.13 .1 \ --hash=sha256:3397e5448952 e18e1295bf047014659effa5ae8da6a5371d37ff0ddc46fa6872 \ --hash=sha256:6 f54d9f016c0b7811fac9fb8c2c7fa7421d80c54dbdd75ffb12913c55db60b8a python-dateutil==2.8 .0 \ --hash=sha256:7e6584 c74aeed623791615e26efd690f29817a27c73085b78e4bad02493df2fb \ --hash=sha256:c89805f6f4d64db21ed966fda138f8a5ed7a4fdbc1a8ee329ce1b74e3c74da9e \ six==1.12 .0 \ --hash=sha256:3350809 f0555b11f552448330d0b52d5f24c91a322ea4a15ef22629740f3761c \ --hash=sha256:d16a0141ec1a18405cd4ce8b4613101da75da0e9a7aec5bdd4fa804d0e0eba73 \ (python3venv) $ pip-sync Uninstalling Flask-1.0 .2 : Successfully uninstalled Flask-1.0 .2 Uninstalling itsdangerous-1.1 .0 : Successfully uninstalled itsdangerous-1.1 .0 Uninstalling Jinja2-2.10 : Successfully uninstalled Jinja2-2.10 Uninstalling MarkupSafe-1.1 .1 : Successfully uninstalled MarkupSafe-1.1 .1 Uninstalling Werkzeug-0.14 .1 : Successfully uninstalled Werkzeug-0.14 .1 (python3venv) $ pip freeze arrow==0.13 .1 Click==7.0 pip-tools==3.4 .0 python-dateutil==2.8 .0 six==1.12 .0
指定PyPi库 如果需要将虚拟环境与特定的PyPi库关联起来，即要求安装指定的PyP中的模块或者库，为此需要在虚拟环境的根目录下创建一个名为pip.conf的配置文件，在这个文件中声明安装依赖的属性。下面是一个pip.conf的例子，要根据需要修改hostname.spurin.com 。
1 2 3 4 5 [global ] disable-pip-version-check = True trusted-host = hostname.spurin.com index = https://hostname.spurin.com/repository/pypi-all/pypi index-url = https://hostname.spurin.com/repository/pypi-all/simple
创建虚拟环境中模块的轮子 在必须升级pip，为什么？！ .whl文件的作用，虚拟环境中安装的模块，也可以编译为.whil文件，如果在另外一台机器上再安装同样模块，只需要用pip安装此文件即可，从而让虚拟环境的移植更简单了。
具体操作流程如下所示。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 (python3venv) $ pip wheel --wheel-dir=/tmp/wheel-dir -r requirements.txt Collecting arrow==0.13.1 (from -r requirements.txt (line 7)) Using cached https://files.pythonhosted.org/packages/f4/7f/0360628ba40bb93c10cd89cd289b6a8e9ea87b2db884b8edf32c80ee1c73/arrow-0.13.1-py2.py3-none-any.whl Saved /private/tmp/wheel-dir/arrow-0.13.1-py2.py3-none-any.whl Collecting python-dateutil==2.8.0 (from -r requirements.txt (line 10)) Using cached https://files.pythonhosted.org/packages/41/17/c62faccbfbd163c7f57f3844689e3a78bae1f403648a6afb1d0866d87fbb/python_dateutil-2.8.0-py2.py3-none-any.whl Saved /private/tmp/wheel-dir/python_dateutil-2.8.0-py2.py3-none-any.whl Collecting six==1.12.0 (from -r requirements.txt (line 14)) Using cached https://files.pythonhosted.org/packages/73/fb/00a976f728d0d1fecfe898238ce23f502a721c0ac0ecfedb80e0d88c64e9/six-1.12.0-py2.py3-none-any.whl Saved /private/tmp/wheel-dir/six-1.12.0-py2.py3-none-any.whl (python3venv) $ ls /tmp/wheel-dir arrow-0.13.1-py2.py3-none-any.whl python_dateutil-2.8.0-py2.py3-none-any.whl six-1.12.0-py2.py3-none-any.whl (python3venv) $ pip uninstall arrow python_dateutil six Uninstalling arrow-0.13.1: Would remove: /Users/james/python3venv/lib/python3.6/site-packages/arrow-0.13.1.dist-info/* /Users/james/python3venv/lib/python3.6/site-packages/arrow/* Proceed (y/n)? y Successfully uninstalled arrow-0.13.1 Uninstalling python-dateutil-2.8.0: Would remove: /Users/james/python3venv/lib/python3.6/site-packages/dateutil/* /Users/james/python3venv/lib/python3.6/site-packages/python_dateutil-2.8.0.dist-info/* Proceed (y/n)? y Successfully uninstalled python-dateutil-2.8.0 Uninstalling six-1.12.0: Would remove: /Users/james/python3venv/lib/python3.6/site-packages/six-1.12.0.dist-info/* /Users/james/python3venv/lib/python3.6/site-packages/six.py Proceed (y/n)? y Successfully uninstalled six-1.12.0 (python3venv) $ pip install --find-links=/tmp/wheel-dir -r requirements.txt Looking in links: /tmp/wheel-dir Collecting arrow==0.13.1 (from -r requirements.txt (line 7)) Collecting python-dateutil==2.8.0 (from -r requirements.txt (line 10)) Collecting six==1.12.0 (from -r requirements.txt (line 14)) Installing collected packages: six, python-dateutil, arrow Successfully installed arrow-0.13.1 python-dateutil-2.8.0 six-1.12.0
结束语 创建虚拟环境的目的，就是隔离特定模块的版本，从而满足不同项目的特定需要。
另外，除了上述创建虚拟环境的方法之外，现在也流行着一个名为pipenv的模块，有兴趣的可以试试，代码仓库地址：https://github.com/pypa/pipenv 。当然，对它也是仁者见仁智者见智。
参考文献 [1]. https://spurin.com/2019/03/12/Python-Virtual-Environments/
