如果你使用的是Linux发行版，例如Ubantu，那么你的系统中可能已经安装好python了。可以使用python -v来测试一下：
ortonwu@ubuntu:~$ python -V
Python 3.5.2
当然，这是我安装了Python3之后显示的。如果你测试出来你的系统安装的是Python 2.7或是提示command not found，想升级或安装Python3的话，可以使用如下命令下载最新的Python:
sudo apt-get install python3
输入root密码后下载完成。输入python可以进入python的命令行交互：
ortonwu@ubuntu:/usr/bin$ python
Python 2.7.12 (default, Nov 19 2016, 06:48:10)
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>
这里显示的Python版本是2.7，而不是刚下载的3.5。这是因为python被软连接到python2.7了。可以在/usr/bin下查看python link文件：
ortonwu@ubuntu:~$ ll /usr/bin/ | grep ^l | grep python
lrwxrwxrwx 1 root root 26 Apr 1 20:11 dh_pypy -> ../share/dh-python/dh_pypy*
lrwxrwxrwx 1 root root 29 Apr 1 20:11 dh_python3 -> ../share/dh-python/dh_python3*
lrwxrwxrwx 1 root root 23 Nov 19 01:36 pdb2.7 -> ../lib/python2.7/pdb.py*
lrwxrwxrwx 1 root root 23 Nov 17 11:26 pdb3.5 -> ../lib/python3.5/pdb.py*
lrwxrwxrwx 1 root root 31 Apr 1 20:11 py3versions -> ../share/python3/py3versions.py*
lrwxrwxrwx 1 root root 26 Apr 1 20:11 pybuild -> ../share/dh-python/pybuild*
lrwxrwxrwx 1 root root 18 Apr 25 07:43 python -> /usr/bin/python3.5*
lrwxrwxrwx 1 root root 9 Apr 1 20:11 python2 -> python2.7*
lrwxrwxrwx 1 root root 9 Apr 1 20:11 python3 -> python3.5*
lrwxrwxrwx 1 root root 10 Apr 1 20:11 python3m -> python3.5m*
lrwxrwxrwx 1 root root 29 Apr 1 20:11 pyversions -> ../share/python/pyversions.py*
重新建立到python3.5的软连接即可：
ortonwu@ubuntu:/usr/bin$ sudo rm -rf python
ortonwu@ubuntu:/usr/bin$ sudo ln -s /usr/bin/python3.5 /usr/bin/python
这时使用python即可进入python3.5命令行交互界面：
ortonwu@ubuntu:/usr/bin$ python
Python 3.5.2 (default, Nov 17 2016, 17:05:23)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>