安装python

brew install python

安装成功提示:

Python has been installed as
  /usr/local/bin/python3

Unversioned symlinks `python`, `python-config`, `pip` etc. pointing to
`python3`, `python3-config`, `pip3` etc., respectively, have been installed into
  /usr/local/opt/python@3.9/libexec/bin

按照上面提示我们打开目录/usr/local/opt/python@3.9/libexec/bin
这个就是python3.9的目录

open /usr/local/opt/python@3.9/libexec/bin

会看到下面文件

这时系统里面同时有python2.7和新安装的python3
查看默认python版本

python -V 

得到默认版本:Python 2.7.16

执行python3 -V,得到版本3.9

python3 -V
Python 3.9.10

现在把默认执行python的时候,从python 2.7改成python3

让命令行每次输入python的时候都默认执行的是python3而不是python2.7

查找默认的python在哪里

which python

得到路径:/usr/bin/python

tdw@bogon ~ % which python
/usr/bin/python

查找python3的路径
which python3

tdw@bogon ~ % which python3
/usr/local/bin/python3

打开文件~/.bashrc

open ~/.bashrc 

在里面把默认的python重命名,把python2设置成python2.7的路径把 python设置成python3的路径

alias python2='/usr/bin/python'
alias python='/usr/local/bin/python3'

保存文件
运行.bashrc文件

source ~/.bashrc

现在再查看python和python2版本,发现默认的已经是python3了
python -V
Python 3.9.10
python2 -V
Python 2.7.16

添加python3的环境变量

打开文件 ~/.bash_profile

open ~/.bash_profile

添加环境变量的格式是这样:

export PATH="路径名:$PATH"

这个路径名就是上面我们安装完python3.9的时候提示的路径:/usr/local/opt/python@3.9/libexec/bin

Python has been installed as
  /usr/local/bin/python3

Unversioned symlinks `python`, `python-config`, `pip` etc. pointing to
`python3`, `python3-config`, `pip3` etc., respectively, have been installed into
  /usr/local/opt/python@3.9/libexec/bin

把这个路径拼接上上面的格式就成了这样:

export PATH="/usr/local/opt/python@3.9/libexec/bin:$PATH"

把这句话添加到~/.bash_profile
文件中,可以看到里面还有其他的export PATH,这些都是其他的路径,如下图:

然后保存文件
运行文件

source ~/.bash_profile