update-manager fails to upgrade from quantal to raring No module named '_sysconfigdata_m'

This seems to be because I had python3 from a backport installed. It's not obvious why it wasn't upgraded, but it's not going to affect many people.

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Status changed to 'Confirmed' because the bug affects multiple users.

Not sure this is correct, seems to be a software updater/apt-get issue that caused partial upgrade?
Still unable to get apt running again, (or software updater)

I'm running into the same issue, and I also had python3.3 installed already from backports. What is the workaround to get _sysconfigdata_m installed correctly? The package installs it at
/usr/lib/python3.3/plat-x86_64-linux-gnu/_sysconfigdata_m.py
but python can't find it overthere. I can symlink or copy it to /usr/lib/python3.3/ or I can set $PYTHONPATH to also look in plat-x86_64-linux-gnu, so what is the system supposed to look like?

TL;DR
The issue for me lied in /etc/bash.bashrc

This line near the bottom:
python /usr/lib/command-not-found -- $1

should be:
/usr/lib/command-not-found -- $1

Long explanation follows:
I had the same issue. For me issue occurred whenever I mistyped a command.

/usr/lib/command-not-found is a python program.

I found that if I ran it like:
python3.3 /usr/lib/command-not-found bad-command
it ran fine, but
python /usr/lib/command-not-found bad-command

Resulted in:
Traceback (most recent call last):
  File "/usr/lib/python3.3/site.py", line 631, in <module>
    main()
  File "/usr/lib/python3.3/site.py", line 616, in main
    known_paths = addusersitepackages(known_paths)
  File "/usr/lib/python3.3/site.py", line 284, in addusersitepackages
    user_site = getusersitepackages()
  File "/usr/lib/python3.3/site.py", line 260, in getusersitepackages
    user_base = getuserbase() # this will also set USER_BASE
  File "/usr/lib/python3.3/site.py", line 250, in getuserbase
    USER_BASE = get_config_var('userbase')
  File "/usr/lib/python3.3/sysconfig.py", line 580, in get_config_var
    return get_config_vars().get(name)
  File "/usr/lib/python3.3/sysconfig.py", line 530, in get_config_vars
    _init_posix(_CONFIG_VARS)
  File "/usr/lib/python3.3/sysconfig.py", line 403, in _init_posix
    from _sysconfigdata import build_time_vars
  File "/usr/lib/python3.3/_sysconfigdata.py", line 6, in <module>
    from _sysconfigdata_m import *
ImportError: No module named '_sysconfigdata_m'

The issue lied in /etc/bash.bashrc

This line near the bottom:
python /usr/lib/command-not-found -- $1

 was invoking python 2.

Changing it to:
python3.3 /usr/lib/command-not-found -- $1

or just plain:
/usr/lib/command-not-found -- $1

fixed the problem for me :)

OK - so your python 2 is broken (which is the problem). You have solved your specific issue by pointing a specific command at python 3 instead. That command now works. Your python 2 is still broken.

An, you say it's python 2, but from the stack trace, it plainly isn't.

This problem is probably related to:
https://bugs.launchpad.net/command-not-found/+bug/1480388

In that case a simple solution is to change a line in /usr/lib/command-not-found from:

os.execvp("python3", [sys.argv[0]] + sys.argv)

to:

os.execvp("/usr/bin/python3", [sys.argv[0]] + sys.argv)

Status changed to 'Confirmed' because the bug affects multiple users.