前言
centos7 自带有 python，但是却是 python2 版本的 python，如果你想安装个python3怎么办呢？
如果直接删除python2的话，可能会引起其他的问题，因为有些东西是依赖python2的，最好的解决办法是python3和python2共存，新安装一个python3的环境。
python2
先找到系统的python安装在哪个目录，查看对应版本号和相关安装包
cd / 先回到根目录
whereis python 查看python所在目录/usr/bin
cd /usr/bin 切到python目录
ll python* 查看python开头的相关文件详情
[root@VM-0-4-centos ~]# cd /
[root@VM-0-4-centos /]# whereis python
python: /usr/bin/python /usr/bin/python2.7-config /usr/bin/python2.7 /usr/lib/python2.7 /usr/lib64/python2.7 /etc/python /usr/include/python2.7 /usr/share/man/man1/python.1.gz
[root@VM-0-4-centos /]# cd /usr/bin
[root@VM-0-4-centos bin]# ll python*
lrwxrwxrwx 1 root root 7 Jun 11 14:55 python -> python2
lrwxrwxrwx 1 root root 9 Jun 11 14:55 python2 -> python2.7
-rwxr-xr-x 1 root root 7144 Apr 2 2020 python2.7
-rwxr-xr-x 1 root root 1835 Apr 2 2020 python2.7-config
lrwxrwxrwx 1 root root 16 Jun 11 14:55 python2-config -> python2.7-config
lrwxrwxrwx 1 root root 28 Oct 11 18:30 python3 -> /root/python38/bin/python3.8
lrwxrwxrwx 1 root root 14 Jun 11 14:55 python-config -> python2-config
[root@VM-0-4-centos bin]#
我这时已经安装好python3,所以会显示出来
从查看的结果可以看到python指向的是python2 ，python2指向的是python2.7。那也就是在控制台输入python、python2、python2.7都是运行python2.7
如果我们安装一个python3.8的环境，让python3指向3.8， python2指向python2.7, 那就可以完美的共存了
yum安装依赖
先安装相关依赖
yum -y install zlib-devel bzip2-devel openssl-devel ncurses-devel sqlite-devel readline-devel tk-devel gdbm-devel db4-devel libpcap-devel xz-devel mysql-devel
安装python3
从python安装包的历史版本中https://www.python.org/ftp/python/，找到需要的安装包，比如我这里选3.8.6版本
在Centos 7系统里面新建一个目录，用于存放下载的python3安装包，比如： /root/python38
[root@VM-0-4-centos bin]# mkdir /root/python38
cd到 root/pyrhon36目录，用wget下载3.8.6安装包
[root@VM-0-4-centos bin]# wget [https://www.python.org/ftp/python/3.8.6/Python-3.8.6.tgz
[root@VM-0-4-centos bin]# cd /root/python38
[root@VM-0-4-centos python38]# wget https://www.python.org/ftp/python/3.8.6/Python-3.8.6.tgz
--2020-10-11 18:08:12-- https://www.python.org/ftp/python/3.8.6/Python-3.8.6.tgz
Resolving www.python.org (www.python.org)... 151.101.108.223, 2a04:4e42:1a::223
Connecting to www.python.org (www.python.org)|151.101.108.223|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 23010188 (22M) [application/octet-stream]
Saving to: ‘Python-3.8.6.tgz’
100%[===============================================================================>] 23,010,188 14.2MB/s in 1.5s
2020-10-11 18:10:14 (14.2 MB/s) - ‘Python-3.8.6.tgz’ saved [23010188/23010188]
[root@VM-0-4-centos python38]#等待下载完成之后会在当前目录下出现一个tgz包，tar命令解压这个包到当前目录就可以
[root@VM-0-4-centos python38]# tar -xvf Python-3.8.6.tgz
[root@VM-0-4-centos python38]# ll
total 22476
drwxr-xr-x 17 501 501 4096 Dec 24 11:01 Python-3.8.6
-rw-r--r-- 1 root root 23010188 Dec 24 11:01 Python-3.8.6.tgz解压完之后需要编译Python-3.8.6包下的文件，先cd过去执行完这句命令之后，把python的安装目录指定一下，这样的话，里面的一些bin目录、lib目录就都会存放在这个目录下面。
如果不指定这个安装目录的话，最后python的安装文件将分散到linux的默认目录，不在一块。我们指定安装目录，以后卸载的话直接删除目录就可以干净卸载了。
[root@VM-0-4-centos python38]# cd Python-3.8.6
[root@VM-0-4-centos Python-3.8.6]# ./configure --prefix=/root/python38在当前目录root/pyrhon38/Python-3.8.6执行make，执行完之后，接着输入make install
[root@VM-0-4-centos Python-3.8.6]## make
[root@VM-0-4-centos Python-3.8.6]## make install耐心等待，时间会有点长。
添加软链接
由于系统默认的python是指向python2，这里执行把新安装的python3.8指向给/usr/bin/python3就可以了
[root@VM-0-4-centos bin]# ln -s /root/python38/bin/python3.8 /usr/bin/python3
[root@VM-0-4-centos bin]# python3 -V
Python 3.8.6
[root@VM-0-4-centos bin]# python3
Python 3.8.6 (default, Oct 11 2020, 18:28:25)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-39)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>> exit()
[root@VM-0-4-centos bin]#
输入python3 -V可以看到版本号，这样系统默认的就是python3.8.6版本了，如果想用python2.7版本，直接输入python2就可以了
pip环境
python3安装完成之后，如果想用pip安装一些第三方包，系统会默认安装到python2的环境里面。
添加pip3的软链接，这样输入pip3 install 就能把安装包安装到python3的环境了。
[root@VM-0-4-centos bin]# ln -s /root/python38/bin/pip3 /usr/bin/pip3
[root@VM-0-4-centos bin]# pip3 -V
pip 20.2.1 from /root/python38/lib/python3.8/site-packages/pip (python 3.8)
[root@VM-0-4-centos bin]# pip2 -V
pip 20.1.1 from /usr/lib/python2.7/site-packages/pip (python 2.7)
[root@VM-0-4-centos bin]# ll pip*
-rwxr-xr-x 1 root root 219 Jun 11 14:56 pip
-rwxr-xr-x 1 root root 219 Jun 11 14:56 pip2
-rwxr-xr-x 1 root root 219 Jun 11 14:56 pip2.7
lrwxrwxrwx 1 root root 23 Oct 11 18:37 pip3 -> /root/python38/bin/pip3
[root@VM-0-4-centos bin]#如果要安装python3的第三方包，那就输入pip3 install xx包名
[root@VM-0-4-centos bin]# pip3 install requests
Looking in indexes: https://mirrors.cloud.tencent.com/pypi/simple
Collecting requests
Downloading https://mirrors.cloud.tencent.com/pypi/packages/45/1e/0c169c6a5381e241ba7404532c16a21d86ab872c9bed8bdcd4c423954103/requests-2.24.0-py2.py3-none-any.whl (61 kB)
|████████████████████████████████| 61 kB 202 kB/s
Collecting idna<3,>=2.5
Downloading https://mirrors.cloud.tencent.com/pypi/packages/a2/38/928ddce2273eaa564f6f50de919327bf3a00f091b5baba8dfa9460f3a8a8/idna-2.10-py2.py3-none-any.whl (58 kB)
|████████████████████████████████| 58 kB 632 kB/s
Collecting certifi>=2017.4.17
Downloading https://mirrors.cloud.tencent.com/pypi/packages/5e/c4/6c4fe722df5343c33226f0b4e0bb042e4dc13483228b4718baf286f86d87/certifi-2020.6.20-py2.py3-none-any.whl (156 kB)
|████████████████████████████████| 156 kB 411 kB/s
Collecting chardet<4,>=3.0.2
Downloading https://mirrors.cloud.tencent.com/pypi/packages/bc/a9/01ffebfb562e4274b6487b4bb1ddec7ca55ec7510b22e4c51f14098443b8/chardet-3.0.4-py2.py3-none-any.whl (133 kB)
|████████████████████████████████| 133 kB 407 kB/s
Collecting urllib3!=1.25.0,!=1.25.1,<1.26,>=1.21.1
Downloading https://mirrors.cloud.tencent.com/pypi/packages/9f/f0/a391d1463ebb1b233795cabfc0ef38d3db4442339de68f847026199e69d7/urllib3-1.25.10-py2.py3-none-any.whl (127 kB)
|████████████████████████████████| 127 kB 798 kB/s
Installing collected packages: idna, certifi, chardet, urllib3, requests
Successfully installed certifi-2020.6.20 chardet-3.0.4 idna-2.10 requests-2.24.0 urllib3-1.25.10
WARNING: You are using pip version 20.2.1; however, version 20.2.3 is available.
You should consider upgrading via the '/root/python38/bin/python3.8 -m pip install --upgrade pip' command.
[root@VM-0-4-centos bin]# python3
Python 3.8.6 (default, Oct 11 2020, 18:28:25)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-39)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import requests
>>> exit()
[root@VM-0-4-centos bin]#
我安装的第三库都在/root/python38/lib/python3.8/site-packages/文件夹下，跟python3在一起，不需要直接就可以整个文件夹删除
