Ubuntu16.04 switch python version

Python2.7 and python3.5 have been installed by default in ubuntu16.04

hanli@ubuntu:/usr/bin$ ls |grep python
dh_python2
dh_python3
python
python2
python2.7
python2.7-config
python2-config
python3
python3.5
python3.5m
python3m
python-config
x86_64-linux-gnu-python2.7-config
x86_64-linux-gnu-python-config

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How to switch?

1. It is now 2.7

hanli@ubuntu:/usr/bin$ python
Python2.7.12(default,Nov192016,06:48:10)

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2. Execute two commands

hanli@ubuntu:/usr/bin$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python2 100
update-alternatives: using /usr/bin/python2 to provide /usr/bin/python (python)in auto mode
hanli@ubuntu:/usr/bin$ sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 150
update-alternatives: using /usr/bin/python3 to provide /usr/bin/python (python)in auto mode

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3. Check it again has become 3.5

hanli@ubuntu:/usr/bin$ python
Python3.5.2(default,Nov172016,17:05:23)

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4. How to switch at will

hanli@ubuntu:/usr/bin$ sudo update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).

 Selection    Path              Priority   Status
------------------------------------------------------------
* 0   /usr/bin/python3   150       auto mode
 1   /usr/bin/python2   100       manual mode
 2   /usr/bin/python3   150       manual mode

Press <enter> to keep the current choice[*], or type selection number: 1
update-alternatives: using /usr/bin/python2 to provide /usr/bin/python (python) in manual mode

# Before the number*The sign means the current version, enter 1 to switch to 2.7. Check again as follows

hanli@ubuntu:/usr/bin$ sudo update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).

 Selection    Path              Priority   Status
------------------------------------------------------------
 0   /usr/bin/python3   150       auto mode
* 1   /usr/bin/python2   100       manual mode
 2   /usr/bin/python3   150       manual mode

Press <enter> to keep the current choice[*], or type selection number: