Everything you need to know about the switch to Python 3

About the transition

Kali Linux fully switched to Python 3. This means that any tool packaged by Kali that was using Python 2 has been either dropped or converted to use Python 3. Any tool converted to Python 3 contains only scripts with /usr/bin/python3 as their shebang.

Concerning the packages that come straight from Debian, they did the same on most packages, but there are a few exceptions where packages are allowed to continue to rely on Python 2. However those packages have been updated so that all their scripts use /usr/bin/python2 as their shebang and so that they depend on the python2 binary package (instead of the former python).

Thanks to those changes, Debian no longer need to provide /usr/bin/python and recent upgrades will effectively get rid of that symlink.

Unfortunately, when you download a Python script on the web, it will likely have /usr/bin/python as its shebang. If you try to execute it without fixing the shebang line, you will end up with an error like this one:

zsh: /home/kali/test.py: bad interpreter: /usr/bin/python: no such file or directory

In Debian, you can reinstate the /usr/bin/python symlink by installing:

  • python-is-python2 if you want to have it point to python2
  • python-is-python3 if you want to have it point to python3

Keeping backwards compatibility in Kali

Given the large number of users that would not know how to avoid the above error, we decided that Kali would continue to ship Python 2 by default (as long as Debian still provides it) and that /usr/bin/python would point to it. We are also keeping a few common external modules (like requests) so that random exploit scripts have a reasonable chance to run successfully.

Howewer, pip for Python2 (aka python-pip) is gone, /usr/bin/pip is the same as /usr/bin/pip3 and it will install modules for Python 3. See the FAQ below for more information.

This compatibility has been implemented by having kali-linux-headless recommend python2, python-is-python2 and offsec-awae-python2 so that they are installed by default while being removable by users that would like to get rid of them.

The make users aware of this situation, we are displaying a promiment message on login:

┏━(Message from Kali developers)
┃
┃ We have kept /usr/bin/python pointing to Python 2 for backwards
┃ compatibility. Learn how to change this and avoid this message:
┃ ⇒ https://www.kali.org/docs/general-use/python3-transition/
┃
┗━(Run “touch ~/.hushlogin” to hide this message)

The hope is that they will read this article and know how do deal with the various issues that they will encounter.

Frequently Asked Questions

I have downloaded a Python script, what should I do?

You need to inspect its shebang line. The shebang line is the first line of a script and it starts with #! followed by the path of the interpreter that will be used to execute the script.

If the interpreter is /usr/bin/python, you should read the documentation to see whether the script can run with Python 3. If yes, then you should update the shebang line to point to /usr/bin/python3. Otherwise you should update it to point to /usr/bin/python2.

Good shebang lines that can be kept:

  • #!/usr/bin/python3
  • #!/usr/bin/python2
  • #!/usr/bin/env python3
  • #!/usr/bin/env python2

Bad shebang lines that need to be updated:

  • #!/usr/bin/python
  • #!/usr/bin/env python

How can I get rid of the login message?

The message will only be displayed as long as /usr/bin/python points to the deprecated Python 2. Now that you know about this situation and that you know how to fix the shebang line of old scripts, you can safely get rid of /usr/bin/python:

$ sudo apt remove python-is-python2

Or you can decide to make it point to Python 3:

$ sudo apt install python-is-python3

Either of those actions will get rid of the login message.

Alternatively, if you want to keep /usr/bin/python pointing to python2 and you still want to disable the message, you can do this:

$ mkdir -p ~/.local/share/kali-motd
$ touch ~/.local/share/kali-motd/disable-old-python-warning

I have a Python 2 script that doesn't run, what should I do?

If your Python 2 script uses modules that are not among those that we ship in the offsec-awae-python2 compatibility package (see list here), then you can try pyenv to setup a fully isolated Python 2 environment where you can use pip to install supplementary modules. See our Using EoL Python versions on Kali article.

I want pip for Python 2, how can I get it back?

Try pyenv. See our Using EoL Python versions on Kali article.

I wrote a Python script, what should I do?

Be nice with end users:

  • document clearly whether your code runs with Python 3 or Python 2
  • use /usr/bin/python3 or /usr/bin/python2 as the shebang line, it's more expressive than /usr/bin/python and is more likely to have the desired result
  • update it for Python 3 compatibility if that's not the case yet

Updated on: 2020-Nov-10
Author: rhertzog