{"id":8678,"date":"2015-02-12T15:49:01","date_gmt":"2015-02-12T15:49:01","guid":{"rendered":"https:\/\/wp.okra.host\/article\/changing-python-versions\/"},"modified":"2021-03-07T14:44:25","modified_gmt":"2021-03-07T13:44:25","slug":"changing-python-versions","status":"publish","type":"ht_kb","link":"https:\/\/kb.okra.host\/de\/article\/changing-python-versions\/","title":{"rendered":"\u00c4ndern von Python-Versionen"},"content":{"rendered":"

<\/del>Overview<\/h3>\n

Recent platforms (v6+<\/a>) support multiple Python interpreters from the shell using pyenv<\/a>. pyenv allows seamless switching between available Python versions, and manages version-specific\u00a0package installations<\/a>\u00a0too.<\/p>\n

Basics<\/h2>\n

All commands are done from the terminal<\/a>.<\/p>\n

Listing versions<\/h3>\n

To get a current list of available Python interpreters, use pyenv versions<\/code>:<\/p>\n

\n
\n
<\/div>\n
[myadmin@sol ~]$ pyenv versions
\nsystem
\n2.7.8
\n* 3.3.5 (set by \/home\/myadmin\/.python-version)
\n3.4.1<\/code><\/div>\n<\/div>\n<\/figure>\n
<\/div>\n

Of importance is\u00a0system<\/code>, which is the default system interpreter. Without pyenv installed,\u00a0system<\/code> is the default interpreter used.\u00a02.7.8<\/code>,\u00a03.3.5<\/code>, and\u00a03.4.1<\/code>\u00a0are additional interpreters installed in addition to the system version under \/.socket\/python\/versions<\/code>.<\/p>\n

Selecting\u00a0a version<\/h3>\n

A version may be\u00a0defined per-directory and inherited recursively within all child directories. Versions are defined for a directory, and its child directories, by pyenv local<\/code>:<\/p>\n

\n
\n
<\/div>\n
[myadmin@sol ~]$ pyenv version
\n3.3.5 (set by \/home\/myadmin\/.python-version)
\n[myadmin@sol ~]$ cd \/var\/www\/
\n[myadmin@sol www]$ pyenv local system
\n[myadmin@sol www]$ pyenv version
\nsystem (set by \/var\/www\/.python-version)
\n[myadmin@sol www]$ cd ~\/test
\n[myadmin@sol ~\/test]$ pyenv version
\n3.3.5 (set by \/home\/myadmin\/.python-version)<\/code><\/div>\n<\/div>\n<\/figure>\n
<\/div>\n

Two versions of Python are defined, one in \/home\/myadmin<\/code> and another in \/var\/www<\/code>. Versions are controlled by a file called .python-version<\/code>\u00a0and pyenv will ascend each parent directory until it reaches root (\/<\/code>) or .python-version<\/code>\u00a0is found. If no control file is found, the current Python interpreter will be used.<\/p>\n

You can then delegate a single Python version to handle a collection of projects by locating projects under an additional directory and defining a Python version in its parent directory:<\/p>\n

www       <-- NO .python-version\r\n|-- proj3 <-- .python-version (3.3.5)\r\n| |-- mydjango  (3.3.5)\r\n| `-- flask     (3.3.5)\r\n`-- proj2 <-- .python-version (2.7.5)\r\n  |-- newdjango (2.7.5)\r\n  `-- mezzanine (2.7.5)<\/pre>\n
Control files are located under \/var\/www\/proj3<\/code> and \/var\/www\/proj2<\/code> allowing projects located under these directories to use Python 3 or Python 2 respectively.<\/p>\n
Package locations<\/h3>\n
Packages are installed\u00a0using pip<\/a>\u00a0as normal. Packages are located under \/usr\/local\/lib\/python\/<MAJOR><\/em>.<MINOR><\/em>.<PATCH><\/em><\/code> using semantic versioning<\/a>\u00a0to avoid conflict. Any binaries installed under \/usr\/local\/bin, however, will conflict with each other if those binaries, bundled in different versions of the package, differ. pyenv-virtualenv<\/a> +\u00a0virtualenv<\/a> is recommended to isolate packages of different versions that rely on the same Python interpreter.<\/p>\n
Caveats<\/h3>\n
Shebangs<\/h4>\n
Shell or FastCGI scripts typically begin with #!\/usr\/bin\/python<\/code>. This is the system version that will bypass pyenv initialization. Change this line to #!\/usr\/bin\/env python<\/code> to use python found in the shell path.<\/p>\n
See also<\/h2>\n