有一天，我决定让 python 命令默认启动 python3 而不是 python2。
所以我这样做了：
$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python2.7 2
$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python3.5 3
$ sudo update-alternatives --config python
$ sudo update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/bin/python3.5 3 auto mode
1 /usr/bin/python2.7 2 manual mode
2 /usr/bin/python3.5 3 manual mode
Press <enter> to keep the current choice[*], or type selection number: 0
这一切都奏效了。伟大的！ :)
$ python -V
Python 3.5.2
但没过多久，我就意识到在安装和删除 python 包时我已经破坏了 apt/aptitude，因为 apt 期待 python2 它发生了。
这就是发生的事情。
$ sudo apt remove python-samba
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following package was automatically installed and is no longer required:
samba-libs
Use 'sudo apt autoremove' to remove it.
The following packages will be REMOVED:
python-samba
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
After this operation, 5,790 kB disk space will be freed.
Do you want to continue? [Y/n]
(Reading database ... 187285 files and directories currently installed.)
Removing python-samba (2:4.3.11+dfsg-0ubuntu0.16.04.5) ...
File "/usr/bin/pyclean", line 63
except (IOError, OSError), e:
^
SyntaxError: invalid syntax
dpkg: error processing package python-samba (--remove):
subprocess installed pre-removal script returned error exit status 1
Traceback (most recent call last):
File "/usr/bin/pycompile", line 35, in <module>
from debpython.version import SUPPORTED, debsorted, vrepr, \
File "/usr/share/python/debpython/version.py", line 24, in <module>
from ConfigParser import SafeConfigParser
ImportError: No module named 'ConfigParser'
dpkg: error while cleaning up:
subprocess installed post-installation script returned error exit status 1
Errors were encountered while processing:
python-samba
E: Sub-process /usr/bin/dpkg returned an error code (1)
最终我猜想它想要 python2 作为默认值，所以我取消了我的更改，如下所示：
$ sudo update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).
Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/bin/python3.5 3 auto mode
1 /usr/bin/python2.7 2 manual mode
2 /usr/bin/python3.5 3 manual mode
Press <enter> to keep the current choice[*], or type selection number: 1
$ python -V
Python 2.7.12
然后apt又工作了
$ sudo apt remove python-samba
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following package was automatically installed and is no longer required:
samba-libs
Use 'sudo apt autoremove' to remove it.
The following packages will be REMOVED:
python-samba
0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
1 not fully installed or removed.
After this operation, 5,790 kB disk space will be freed.
Do you want to continue? [Y/n]
(Reading database ... 187285 files and directories currently installed.)
Removing python-samba (2:4.3.11+dfsg-0ubuntu0.16.04.5) ...
所以我不得不将其保留为默认为 python 2，但我在 python 3 中开发，因此希望我的系统在运行 python 和空闲时默认为 python 3。
谁能告诉我如何在不破坏 apt 的情况下实现这一目标？
我的系统是运行 Ubuntu 的 Raspberry Pi 3B：
Linux mymachine 4.4.38-v7+ #938 SMP Thu Dec 15 15:22:21 GMT 2016 armv7l armv7l armv7l GNU/Linux
（实际上是arm v8）
$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=16.04
DISTRIB_CODENAME=xenial
DISTRIB_DESCRIPTION="Ubuntu 16.04.2 LTS"
