python更新后yum问题

How to switch between Python versions on Fedora Linux

Currently, the default python version on Fedora Linux is Python 2. Later
Fedora Linux release 22 will ship with the Python 3 as a default
version. In this config you will learn how to switch between python
versions on Fedora Linux. Let's start by listing all Python versions
available on your Fedora system:

# ls /usr/bin/python*
/usr/bin/python   /usr/bin/python2.7         /usr/bin/python3    /usr/bin/python3.4m           /usr/bin/python-coverage
/usr/bin/python2  /usr/bin/python2-coverage  /usr/bin/python3.4  /usr/bin/python3-mako-render

Now, check your default python version:

# python -V
Python 2.7.8

To change python version on per user basis simply create a new alias in you .bashrc located under your home directory:

$ alias python='/usr/bin/python3.4'
$ . ~/.bashrc
$ python --version
Python 3.4.2

To change python version globally first check whether python alternative version is already registered by alternatives command:

# alternatives --list | grep -i python

No output means not alternative python version is configured yet. Register the two above listed python version with alternative command.

# alternatives --install /usr/bin/python python /usr/bin/python3.4 2
# alternatives --install /usr/bin/python python /usr/bin/python2.7 1

The above commands will instruct alternatives command to create relevant symbolic links to be used anytime a python command is executed. We have also given python3.4 a higher priority 2 which means, if no python alternative is selected the python3.4 will be used as default. After execution of the above commands your python version should change to python3.4 due to its higher priority.

# python -V
Python 3.4.1

To switch between above alternative python version is now simple as:

# alternatives --config python

There are 2 programs which provide 'python'.

  Selection    Command
-----------------------------------------------
*+ 1           /usr/bin/python3.4
   2           /usr/bin/python2.7

Enter to keep the current selection[+], or type selection number: 2
[root@localhost fedora]# python -V
Python 2.7.8

1. Appendix

Known problems with Fedora Linux and Python 3 version: Error message:

# yum search package
  File "/usr/bin/yum", line 30
    except KeyboardInterrupt, e:
                            ^
SyntaxError: invalid syntax

If you set python3 globally on your system change the yumshebang to python2:

# vi /usr/bin/yum
FROM:
#!/usr/bin/python
TO:
#!/usr/bin/python2.7

Similarly: Error message:

Downloading packages:
  File "/usr/libexec/urlgrabber-ext-down", line 28
    except OSError, e:
                  ^
SyntaxError: invalid syntax
Exiting on user cancel

The issue is also related to Python 3 set as a default global version. To fix this error update /usr/libexec/urlgrabber-ext-down script:

# vi /usr/libexec/urlgrabber-ext-down
FROM:
#!/usr/bin/python
TO:
#!/usr/bin/python2.7