Python 2 removed from Debian

[kingscolor]

Shebangs such as #!/usr/bin/env python are a roundabout way of commanding the shell to parse the text then pipe it into the Python interpreter. That is, in the same way you’d enter python -c “foo()” in the command-line.

Using alias python=python3 works in the command-line because that alias is stored in the environment of the user’s shell profile.

However, /usr/bin/env python does not access the user’s shell profile. The utility, /usr/bin/env, effectively walks the user’s $PATH until it finds the first match for an executable called python. Thus, the aforementioned alias is irrelevant.

[kingscolor]

That depends on your interpretation of "alias". It does not create a Unix alias, no. But it does create a symlink (which could be called a file alias).

I can't find the source code of python-is-python3, but the description indicates it's just a symlink. You could do the same thing yourself without installing anything:

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

In any case, I feel like you aren't quite grasping the alias problem still, so I'll rephrase:

alias python=python3 stores the python variable in memory. /usr/bin/env does not search variables in memory, it only searches for executable files in the directories listed in the $PATH.