如何在不破坏apt的情况下更新Python 3的替代方案？

Question

前几天，我决定让python命令默认启动python3而不是python2。

所以我这么做了：

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更新替代品——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

但不久我就意识到，在安装和删除python包时，我已经打破了apt/aptitude，因为apt期待python2的出现。

事情就是这样。

$ 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)

最后，我猜它希望python2作为默认值，所以我按如下方式取消了更改：

$ 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) ...

因此，我不得不将其保留为默认的Python2，但我是在Python3中开发的，因此希望在运行python和空闲时，我的系统默认为Python3。

有谁能告诉我，我怎样才能做到这一点，而不打破apt？

我的系统是一个运行Ubuntu的树莓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"

Answer 1

替换

[bash:~] $ sudo update-alternatives --install /usr/bin/python python \
/usr/bin/python2.7 2

[bash:~] $ sudo update-alternatives --install /usr/bin/python python \
/usr/bin/python3.5 3

与

[bash:~] $ sudo update-alternatives --install /usr/local/bin/python python \
/usr/bin/python2.7 2

[bash:~] $ sudo update-alternatives --install /usr/local/bin/python python \
/usr/bin/python3.5 3

例如，安装到/usr/local/bin而不是/usr/bin。

并确保路径中的/usr/local/bin在/usr/bin之前。

即

[bash:~] $ echo $PATH
/usr/local/bin:/usr/bin:/bin

通过添加

export PATH=/usr/local/bin:$PATH

到~/.bashrc文件的末尾。通常建议使用自定义bin文件夹（如/usr/local/bin或/opt/<some install>/bin）作为PATH环境变量的前缀，以确保在默认系统文件夹之前找到自定义。