Titles should be in English (translated if quoting output).  I am closing because there is no indication that there is a bug in Python, as opposed to a misunderstanding of how to use it.

Extended help discussion is better conducted on python-list.  For one thing, discussion there is seen by many more people. 

Isa: traditionally, on *nix, 'python' ran Python2 and 'python3' has been used to run Python3.  Sometimes 'python2' has also been used.  Different Linux distributions distributions disagree about whether and when to make 'python' run Python3.  I believe that you can make this happen for your login, but it might disable other programs, especially things provided by Ubuntu.  So it is probably best if you write a shell script that makes the change temporarily.  Since this is going to be a common problem, please ask a question on python-list.  Suggestion:
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(Subject) Make 'python' run Python 3.8 on Ubuntu

I want to run a program on Ubuntu 20.04 that expects 'python' to run Python 3 instead of Python 2.  How can I do this without disabling other programs that expect the opposite?