I have installed the latest Python for Win10 from Releases for Windows. Just typing py in the Command Prompt Window starts Python.

Microsoft Windows [Version 10.0.15048]
(c) 2017 Microsoft Corporation. All rights reserved.

C:\Users\sg7>py
Python 3.6.3 (v3.6.3:2c5fed8, Oct  3 2017, 18:11:49) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>`enter code here`

Testing:

>>> print("hello!")
hello!
>>>

Please be aware that in my case Python was installed in C:\Users\sg7\AppData\Local\Programs\Python\Python36> directory

C:\Users\sg7\AppData\Local\Programs\Python\Python36>dir
 Volume in drive C is Windows7_OS
 Volume Serial Number is 1226-12D1

 Directory of C:\Users\sg7\AppData\Local\Programs\Python\Python36

08/05/2018  07:38 AM    <DIR>          .
08/05/2018  07:38 AM    <DIR>          ..
12/18/2017  09:12 AM    <DIR>          DLLs
12/18/2017  09:12 AM    <DIR>          Doc
12/18/2017  09:12 AM    <DIR>          include
12/18/2017  09:12 AM    <DIR>          Lib
12/18/2017  09:12 AM    <DIR>          libs
10/03/2017  07:17 PM            30,334 LICENSE.txt
10/03/2017  07:17 PM           362,094 NEWS.txt
10/03/2017  07:15 PM           100,504 python.exe
10/03/2017  07:12 PM            58,520 python3.dll
10/03/2017  07:12 PM         3,610,776 python36.dll
10/03/2017  07:15 PM            98,968 pythonw.exe
08/05/2018  07:38 AM           196,096 Removescons.exe
08/05/2018  07:38 AM            26,563 scons-wininst.log
08/05/2018  07:38 AM    <DIR>          Scripts
12/18/2017  09:12 AM    <DIR>          tcl
12/18/2017  09:12 AM    <DIR>          Tools
06/09/2016  11:53 PM            87,888 vcruntime140.dll
               9 File(s)      4,571,743 bytes
              10 Dir(s)  20,228,898,816 bytes free

When I am at C:\Users\sg7> directory level python can be invoked by typing AppData\Local\Programs\Python\Python36\python

C:\Users\samg>AppData\Local\Programs\Python\Python36\python
Python 3.6.3 (v3.6.3:2c5fed8, Oct  3 2017, 18:11:49) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>>

Newer Python 3.7 will be installed at: C:\Users\YourUserNameHere\AppData\Local\Programs\Python\Python37

If you wish you can add to your path environment variable: %USERPROFILE%\AppData\Local\Programs\Python\Python36

One can run the python installer again and click Modify button.

Select py launcher checkbox.

Then click install button and wait for the installation completes.

After all of these, py command will come back to the terminal.

Whenever I run python in cmd I get this error 'python' is not recognized as an internal or external command, operable program or batch file.

You need to add python.exe to your Windows path variable(s). You can do this when installing Python with the official installer from python.org by selecting a custom installation and marking the correct option to add Python to your environment variables.

If you need to add the path to Python to your Windows path variable(s) manually (ex. because Python is already installed), look for the Windows Search and type env into it. Click the first link indicating you wish to edit your environment variables.

• If you are on Windows 7, add e.g. ;C:\path\to\python to the end of your Path variable under System variables, where python is your Python installation folder (only use PATH under your user variables if you want Python to be accessible by that user alone). If you click the wrong link in Windows Search, you may need to click the Environment Variables... button to get to the settings described.

• On Windows 10, just add C:\path\to\python to the end of the list (in a new field).

• Do not add python.exe to your path entry (i.e. C:\path\to\python\python.exe is wrong).

Python will work in my Jupyter Notebook, but if I need to pip install something I am unable to do so.

Adding Python to you environment variables through the official installer (above) should solve this. If not, you will need to perform the same (rough) steps above but add C:\path\to\python\Scripts to your path (Scripts is the folder where pip.exe lives).

In either case, reboot your PC before attempting to access python/pip from the command line.

This could be a duplicate answer, but the following steps has worked for me.

Note: This is in the case of Anaconda is installed instead of vanilla Python in Windows environment.

Under System variables:

1. Create a variable

PYTHONHOME = C:\Users\username\Anaconda3

2. Add the below 2 entries under PATH system variable:

%PYTHONHOME%

C:\Users\username\Anaconda3\Scripts

3. Close the command prompt if opened earlier and try any python command (Note: no need to restart the OS):

example: python --version
output: Python 3.7.1

Hope this was useful. Cheers!

As suggested in comments, you could create an alias as follows:

alias python='python3'

by adding it to the ~/.bashrc file at the end of this file, exiting and reloading it in the current terminal using the next command: . ~/.bashrc

Or using linking:

As you can see below, my python points to python2, python2 points to python2.7.

To achieve the same, use:

sudo ln -s /usr/bin/python2.7 /usr/bin/python2 
sudo ln -s /usr/bin/python2 /usr/bin/python

If you want python pointing to 3rd version, you could use the same, but the last command should be:

sudo ln -s /usr/bin/python3 /usr/bin/python

Example

$ whereis python
python: /usr/bin/python3.7 /usr/bin/python2.7-config /usr/bin/python2.7 /usr/bin/python3.7m-config /usr/bin/python3.7-config /usr/bin/python /usr/bin/python3.7m /usr/lib/python3.7 /usr/lib/python2.7 /usr/lib/python3.8 /etc/python3.7 /etc/python2.7 /etc/python /usr/local/lib/python3.7 /usr/local/lib/python2.7 /usr/include/python3.7 /usr/include/python2.7 /usr/include/python3.7m /usr/share/python /usr/share/man/man1/python.1.gz
user@lenovo:~$ ls -ailh /usr/bin/python*
1446954 lrwxrwxrwx 1 root root    7 жов 10 14:32 /usr/bin/python -> python2
1446952 lrwxrwxrwx 1 root root    9 жов 10 14:32 /usr/bin/python2 -> python2.7
1465834 -rwxr-xr-x 1 root root 3,6M лис  7 12:07 /usr/bin/python2.7
1447155 lrwxrwxrwx 1 root root   33 лис  7 12:07 /usr/bin/python2.7-config -> x86_64-linux-gnu-python2.7-config
1447156 lrwxrwxrwx 1 root root   16 жов 10 14:32 /usr/bin/python2-config -> python2.7-config
1442842 lrwxrwxrwx 1 root root    9 лют 12 00:23 /usr/bin/python3 -> python3.7
1449245 -rwxr-xr-x 2 root root 4,9M лис 20 11:21 /usr/bin/python3.7
1447339 lrwxrwxrwx 1 root root   33 лис 20 11:21 /usr/bin/python3.7-config -> x86_64-linux-gnu-python3.7-config
1449245 -rwxr-xr-x 2 root root 4,9M лис 20 11:21 /usr/bin/python3.7m
1447340 lrwxrwxrwx 1 root root   34 лис 20 11:21 /usr/bin/python3.7m-config -> x86_64-linux-gnu-python3.7m-config
1447341 lrwxrwxrwx 1 root root   16 жов  2 15:31 /usr/bin/python3-config -> python3.7-config
1442843 -rwxr-xr-x 1 root root  384 січ 30  2019 /usr/bin/python3-futurize
1442847 lrwxrwxrwx 1 root root   10 лют 12 00:23 /usr/bin/python3m -> python3.7m
1447342 lrwxrwxrwx 1 root root   17 жов  2 15:31 /usr/bin/python3m-config -> python3.7m-config
1442844 -rwxr-xr-x 1 root root  388 січ 30  2019 /usr/bin/python3-pasteurize
1447157 lrwxrwxrwx 1 root root   14 жов 10 14:32 /usr/bin/python-config -> python2-config
1455649 lrwxrwxrwx 1 root root   58 лип 10  2019 /usr/bin/pythontex -> ../share/texlive/texmf-dist/scripts/pythontex/pythontex.py
1450999 -rwxr-xr-x 1 root root  306 лип 10  2019 /usr/bin/pythontex3

For managing python3 versions, you can use python alternatives to create symbolic links:

$ sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8 1
$ sudo update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.11 2

And choose which one to use as using the command:

$ sudo update-alternatives --config python3

For managing python2 and python3 using update-alternatives, you could see in michael's answer.