问Ubuntu18.04.2安装python2.7，未满足依赖性

EN

bo@ubuntu18:~$ sudo apt install python
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help resolve the situation:

The following packages have unmet dependencies:
 python : PreDepends: python-minimal (= 2.7.15~rc1-1) but it is not going to be installed
          Depends: python2.7 (>= 2.7.15~rc1-1~) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

The following packages have unmet dependencies:
 python2.7-minimal : Depends: libpython2.7-minimal (= 2.7.15~rc1-1) but 2.7.15~rc1-1ubuntu0.1 is to be installed
                     Recommends: python2.7 but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

bo@ubuntu18:~$ apt-cache policy python libpython2.7-minimal
python:
  Installed: (none)
  Candidate: 2.7.15~rc1-1
  Version table:
     2.7.15~rc1-1 500
        500 http://mirror.clibre.uqam.ca/ubuntu bionic/main amd64 Packages
libpython2.7-minimal:
  Installed: 2.7.15~rc1-1ubuntu0.1
  Candidate: 2.7.15~rc1-1ubuntu0.1
  Version table:
 *** 2.7.15~rc1-1ubuntu0.1 100
        100 /var/lib/dpkg/status
     2.7.15~rc1-1 500
        500 http://mirror.clibre.uqam.ca/ubuntu bionic/main amd64 Packages

bo@ubuntu18:~$ apt-cache policy python libpython2.7-minimal
python:
  Installed: 2.7.15~rc1-1
  Candidate: 2.7.15~rc1-1
  Version table:
 *** 2.7.15~rc1-1 500
        500 http://ca.archive.ubuntu.com/ubuntu bionic/main amd64 Packages
        100 /var/lib/dpkg/status
libpython2.7-minimal:
  Installed: 2.7.15-4ubuntu4~18.04.1
  Candidate: 2.7.15-4ubuntu4~18.04.1
  Version table:
 *** 2.7.15-4ubuntu4~18.04.1 500
        500 http://ca.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        500 http://ca.archive.ubuntu.com/ubuntu bionic-security/main amd64 Packages
        100 /var/lib/dpkg/status
     2.7.15~rc1-1 500
        500 http://ca.archive.ubuntu.com/ubuntu bionic/main amd64 Packages