Comment 4 for bug 1702467

apt-get, apt and aptitude work all just fine and produce no errors nor warnings.

update-notifier/apt-check is, indeed, responsible for the error message.

    gioele:~$ /usr/lib/update-notifier/apt-check
    Error in sitecustomize; set PYTHONVERBOSE for traceback:
    AttributeError: module 'sys' has no attribute 'setdefaultencoding'
    0;0gioele:~$ echo $?
    0

Please note the missing newline between the end of the output of apt-check and the prompt.

    gioele:~$ which python
    /usr/bin/python
    gioele:~$ readlink -f /usr/bin/python
    /usr/bin/python2.7

Thank you Sebastien for the triaging.