I installed Python 3.6 from J Fernyhough's PPA by doing

sudo add-apt-repository ppa:jonathonf/python-3.6
sudo apt-get update
sudo apt-get install python3.6

I made a string, using the new literal string interpolation, but I supplied an invalid format specifier. I not only got the expected "ValueError: Invalid format specifier", but also the unexpected "ModuleNotFoundError: No module named 'apt_pkg'".


$ python3.6
Python 3.6.0 (default, Dec 29 2016, 21:40:36) 
[GCC 5.4.1 20161202] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> value = 4 * 20
>>> f'the value is {value:%A}'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid format specifier
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 63, in apport_excepthook
    from apport.fileutils import likely_packaged, get_recent_crashes
  File "/usr/lib/python3/dist-packages/apport/__init__.py", line 5, in <module>
    from apport.report import Report
  File "/usr/lib/python3/dist-packages/apport/report.py", line 30, in <module>
    import apport.fileutils
  File "/usr/lib/python3/dist-packages/apport/fileutils.py", line 23, in <module>
    from apport.packaging_impl import impl as packaging
  File "/usr/lib/python3/dist-packages/apport/packaging_impl.py", line 23, in <module>
    import apt
  File "/usr/lib/python3/dist-packages/apt/__init__.py", line 23, in <module>
    import apt_pkg
ModuleNotFoundError: No module named 'apt_pkg'

Original exception was:
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid format specifier

It seems to be vendor's issue not CPython itself. This same issue also happens in Ubuntu 16.10's Python 3.6. Raise any exception can cause this:

Python 3.6.0b2 (default, Oct 11 2016, 05:27:10) 
[GCC 6.2.0 20161005] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> raise Exception
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Exception
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 63, in apport_excepthook
    from apport.fileutils import likely_packaged, get_recent_crashes
  File "/usr/lib/python3/dist-packages/apport/__init__.py", line 5, in <module>
    from apport.report import Report
  File "/usr/lib/python3/dist-packages/apport/report.py", line 30, in <module>
    import apport.fileutils
  File "/usr/lib/python3/dist-packages/apport/fileutils.py", line 23, in <module>
    from apport.packaging_impl import impl as packaging
  File "/usr/lib/python3/dist-packages/apport/packaging_impl.py", line 23, in <module>
    import apt
  File "/usr/lib/python3/dist-packages/apt/__init__.py", line 23, in <module>
    import apt_pkg
ModuleNotFoundError: No module named 'apt_pkg'

Original exception was:
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
Exception
>>> 

Also see https://bugs.launchpad.net/ubuntu/+source/python3.6/+bug/1631367.

Yes, this appears to be the vendor's failure reporting infrastructure that is failing.  Why they'd want a report for every traceback at the interactive prompt is beyond me, but that appears to be what they are trying to do.

On Jan 18, 2017, at 10:56 AM, Jeroen Van Goey wrote:

>sudo add-apt-repository ppa:jonathonf/python-3.6
>sudo apt-get update
>sudo apt-get install python3.6
>
>I made a string, using the new literal string interpolation, but I supplied
>an invalid format specifier. I not only got the expected "ValueError: Invalid
>format specifier", but also the unexpected "ModuleNotFoundError: No module
>named 'apt_pkg'".

Please understand that installing Python 3.6 from a random PPA does *not*
provide full support for this version of the interpreter.  Python 3.6 is not
yet a supported version in any version of Ubuntu (which I'm assuming your
using), although we are working on it for 17.04.

Very often, you can install a new Python 3 interpreter package and many things
will work because the Ubuntu infrastructure shares pure-Python modules across
all installed Python 3's.  Technically speaking, they will all have
/usr/lib/python3/dist-packages on their sys.path so any third party
pure-Python module built for a support version of Python 3 will be importable
by any (package-built) installed version of Python 3.

But that 1) is a long way from saying that those third-party modules will
work; 2) does *not* include any packages containing C extension modules, which
must be rebuilt for the specific interpreter version.

Supporting a new version of Python is a long process, for which we are just
starting.  Please engage with ubuntu-devel@ubuntu.com for details.

Ubuntu does install a standard exception handler so that when Python
applications and such crash, we can gather crash statistics, so that we can
devote resources to fixing common problems and regressions.  apport (which you
see in the traceback) is that crash reporting infrastructure.  apport calls
apt_pkg, which is an (C++) extension module and thus won't have been built for
the version of Python 3.6 you installed from that PPA, unless of course the
PPA owner (who I don't know) has also done an archive-wide Python 3 rebuild.
Since I'm in the process of setting that up, and I know it's quite a bit of
work, I doubt that's been done for this rather random PPA.

The ubuntu-devel mailing list is a better place to discuss the ongoing work to
bring Python 3.6 as a supported version on Ubuntu.

Thanks Barry for your extensive explanation!