command-not-found

command-not-found fails when user-installed python3 is in PATH

Bug #1480388 reported by Matthew Bergkoetter on 2015-07-31

This bug affects 3 people

Affects		Status	Importance	Assigned to	Milestone
	command-not-found	New	Undecided	Unassigned

Bug Description

I have a second installation of python3 (from Anaconda) and it's first in my user path:

$ which python3
/home/matt/Programs/anaconda3/bin/python3

This seems to be causing a problem with command-not-found. When I enter an invalid command in the terminal:

$ fake_command
Failed to import the site module
Traceback (most recent call last):
  File "/usr/lib/python3.4/site.py", line 586, in <module>
    main()
  File "/usr/lib/python3.4/site.py", line 572, in main
    known_paths = addusersitepackages(known_paths)
  File "/usr/lib/python3.4/site.py", line 287, in addusersitepackages
    user_site = getusersitepackages()
  File "/usr/lib/python3.4/site.py", line 263, in getusersitepackages
    user_base = getuserbase() # this will also set USER_BASE
  File "/usr/lib/python3.4/site.py", line 253, in getuserbase
    USER_BASE = get_config_var('userbase')
  File "/usr/lib/python3.4/sysconfig.py", line 597, in get_config_var
    return get_config_vars().get(name)
  File "/usr/lib/python3.4/sysconfig.py", line 540, in get_config_vars
    _init_posix(_CONFIG_VARS)
  File "/usr/lib/python3.4/sysconfig.py", line 412, in _init_posix
    from _sysconfigdata import build_time_vars
  File "/usr/lib/python3.4/_sysconfigdata.py", line 6, in <module>
    from _sysconfigdata_m import *
ImportError: No module named '_sysconfigdata_m'

I can get the expected behavior by removing the line in .bashrc that adds anaconda to the path:

$ fake_command
fake_command: command not found

The source of the problem appears to be a line in /usr/lib/command-not-found where it's trying to re-run itself in python3 if it's being run in python2 (which is currently what /usr/bin/python points to):

    if sys.version < '3':
        # We might end up being executed with Python 2 due to an old
        # /etc/bash.bashrc.
        import os
        if "COMMAND_NOT_FOUND_FORCE_PYTHON2" not in os.environ:
            os.execvp("python3", [sys.argv[0]] + sys.argv)

Notice that it calls the command 'python3', which will resolve to the user-installed version of python3 if it's first in the path. That won't work because the anaconda installation doesn't have the modules the script needs to run. I can make the command work by being explicit about which installation of python3 to use:

$ /usr/bin/python3 /usr/lib/command-not-found fake_command
fake_command: command not found

Instead of:

$ /home/matt/Programs/anaconda3/bin/python3 /usr/lib/command-not-found fake_command
Traceback (most recent call last):
File "/usr/lib/command-not-found", line 27, in <module>
from CommandNotFound.util import crash_guard
ImportError: No module named 'CommandNotFound'

Now, I don't know why the stack trace in the original error looks like it's coming from the system installation of python3.4, but I know that the user installation is the real wrench in the works, because the problem doesn't occur when that's not on the path. A simple solution is to make command-not-found be explicit about which installation it uses. Changing the line to:

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

solves the problem on my system. I suppose the original intent was to use the system installation anyway, since the script relies on modules that are included with ubuntu but not in the python standard library.