下了個ubuntu18.04，更新源後安裝pip3

apt install python3-pip

結果居然報錯了

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 to resolve the situation:

The following packages have unmet dependencies:
 python3-pip : Depends: python-pip-whl (= 8.1.1-2) but 8.1.1-2ubuntu0.4 is to be installed
               Recommends: python3-dev (>= 3.2) but it is not going to be installed
               Recommends: python3-wheel but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
 

之前安裝都沒出現過這個錯誤，只好按提示操作了，缺少依賴，於是

apt install python-pip-whl

但是安裝的時候又提示說缺少其它依賴了

百度了一下，有說

apt install aptitude

用這個會自動解決依賴問題，然而我試了一下

居然又出現新的依賴問題

實在沒轍之際，突然想到是不是軟體源的問題，我安裝完ubuntu後，將軟體源修改為阿里源了，但是修改完之後執行apt update和apt upgrade的時候卻又沒問題。
死馬當活馬醫，再次將軟體源改為系統預設的

改完後執行apt install python3-pip這次沒報錯，順利的安裝完pip3 了

雖然問題解決了，但是不懂原來阿里源的問題在哪，之前在ubuntu14.04和ubuntu16.04修改成阿里源後也沒出現過這個問題

附系統自帶的源

#deb cdrom:[Ubuntu 18.04.1 LTS _Bionic Beaver_ - Release amd64 (20180725)]/ bionic main restricted

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://us.archive.ubuntu.com/ubuntu/ bionic main restricted
# deb-src http://us.archive.ubuntu.com/ubuntu/ bionic main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://us.archive.ubuntu.com/ubuntu/ bionic-updates main restricted
# deb-src http://us.archive.ubuntu.com/ubuntu/ bionic-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://us.archive.ubuntu.com/ubuntu/ bionic universe
# deb-src http://us.archive.ubuntu.com/ubuntu/ bionic universe
deb http://us.archive.ubuntu.com/ubuntu/ bionic-updates universe
# deb-src http://us.archive.ubuntu.com/ubuntu/ bionic-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://us.archive.ubuntu.com/ubuntu/ bionic multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ bionic multiverse
deb http://us.archive.ubuntu.com/ubuntu/ bionic-updates multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ bionic-updates multiverse

## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://us.archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ bionic-backports main restricted universe multiverse

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu bionic partner
# deb-src http://archive.canonical.com/ubuntu bionic partner

deb http://security.ubuntu.com/ubuntu bionic-security main restricted
# deb-src http://security.ubuntu.com/ubuntu bionic-security main restricted
deb http://security.ubuntu.com/ubuntu bionic-security universe
# deb-src http://security.ubuntu.com/ubuntu bionic-security universe
deb http://security.ubuntu.com/ubuntu bionic-security multiverse
# deb-src http://security.ubuntu.com/ubuntu bionic-security multiverse