As you improve your basic Python skills, you're going to find out (usually the hard way) that Python 2.x and Python 3.x are not as compatible as you expected. That's why it's important to establish Python version before starting a new project or deploying it into new environment.
Quickest Way to Check Python Version
Simply use -V option to one of the Python binaries available on your system:
root@srv:~ # python -V
root@srv:~ # python3 -V
You'll get the same result if you use –version instead of -V:
root@srv:~ # python --version
root@srv:~ # python3 --version
Check Python Version By Running python Command
If you start Python in interpreter mode, it should report version information at the very start:
root@srv:~ # python
Python 2.7.16 (default, Oct 10 2019, 22:02:15)
[GCC 8.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
You'll need to press Ctrl+D (or Cmd+D in macOS) to stop Python.
Check Python Version using sys.version
Sometimes you need to check Python version programmatically, from within your Python script. Then your best bet is to use the sys module:
greys@srv:~ $ python -c "import sys; print(sys.version)"
2.7.16 (default, Oct 10 2019, 22:02:15)