The Python,dos window has been installed but prompts "Python is not an internal command or an external command, nor a running program"

Python is already installed, but running the Python command in a DOS window prompts "Python is not an internal command or an external command, nor a running program."
reason : A value is not added to path in the environment variable.
Workaround 1 (recommended): Open the environment variable and add your Python installation path to the path variable in the system variable, for example, I am the following path: D:\install\ python3.6, note the separation, the end does not need a semicolon, and then reboot the DOS window, do not need to restart the computer, and then run Python.

Solution 2: can also directly install Anaconda software https://www.anaconda.com/download/#_windows, with the python running environment, you can directly edit the compilation.