Mismatch between sys.executable and sys.version in Python

Question

There are two Python interpreters installed:

[user@localhost ~]$ /usr/bin/python -V && /usr/local/bin/python -V
Python 2.4.3
Python 2.7.6

Sudo changes PATH for every command it runs as follows:

[user@localhost ~]$ env | grep PATH && sudo env | grep PATH
PATH=/usr/kerberos/bin:/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/home/user/bin
PATH=/usr/bin:/bin

I run a test script:

[user@localhost ~]$ cat what_python.py
#!/usr/bin/env python

import sys
print sys.executable
print sys.version
[user@localhost ~]$ sudo python what_python.py
/usr/bin/python
2.7.6 (default, Feb 27 2014, 17:05:07) 
[GCC 4.1.2 20080704 (Red Hat 4.1.2-54)]

and get path to Python 2.4.3 in sys.executable and version 2.7.6 reported in sys.version. Clearly sys.executable and sys.version do not match. Taking into account how sudo modifies PATH I can understand the value of sys.executable. However, why does sys.version report version 2.7.6 and not version 2.4.3, which would match usr/bin/python path reported by sys.executable?

This is a follow-up to my question Sudo changes PATH, yet executes the same binary

Answer 1

Both @Graeme

The fact that python may be unable to retrieve this suggests that it is doing its own PATH search (…)

and @twalberg

(…) it looks like sys.executable searches the current PATH instead of resolving argv[0] (or maybe because argv[0] is simpy python in this case...), (…)

were basically right. I was reluctant to believe that Python does something so simple (silly?) as using PATH to locate itself but this is true.

Python's sys module is implemented in Python/sysmodule.c file. As of version 2.7.6, sys.executable is set at line 1422 like this:

 SET_SYS_FROM_STRING("executable",
                     PyString_FromString(Py_GetProgramFullPath()));

Py_GetProgramFullPath() function is defined in file Modules/getpath.c starting from line 701:

char *
Py_GetProgramFullPath(void)
{
    if (!module_search_path)
        calculate_path();
    return progpath;
}

Function calcuate_path() is defined in the same file and contains the following comment:

/* If there is no slash in the argv0 path, then we have to
 * assume python is on the user's $PATH, since there's no
 * other way to find a directory to start the search from.  If
 * $PATH isn't exported, you lose.
 */

As can be seen in my case, one loses also when the first Python on exported $PATH is different than the Python being run.

More information on the process of calculating placement of interpreter's executable can be found at the top of getpath.c file:

Before any searches are done, the location of the executable is determined. If argv[0] has one or more slashes in it, it is used unchanged. Otherwise, it must have been invoked from the shell's path, so we search $PATH for the named executable and use that. If the executable was not found on $PATH (or there was no $PATH environment variable), the original argv[0] string is used.

Next, the executable location is examined to see if it is a symbolic link. If so, the link is chased (correctly interpreting a relative pathname if one is found) and the directory of the link target is used.

Let's make a couple of tests to verify the above:

If argv[0] has one or more slashes in it, it is used unchanged.

[user@localhost ~]$ sudo /usr/local/bin/python what_python.py
/usr/local/bin/python
2.7.6 (default, Feb 27 2014, 17:05:07) 
[GCC 4.1.2 20080704 (Red Hat 4.1.2-54)]

Ok.

If the executable was not found on $PATH (or there was no $PATH environment variable), the original argv[0] string is used.

[user@localhost ~]$ sudo PATH= python what_python.py
<empty line>
2.7.6 (default, Feb 27 2014, 17:05:07) 
[GCC 4.1.2 20080704 (Red Hat 4.1.2-54)]

Wrong. In this case statement from sys module's documentation is true – If Python is unable to retrieve the real path to its executable, sys.executable will be an empty string or None. .

Let's see if adding location of python's binary back to the PATH (after sudo had removed it) fixes the problem:

[user@localhost ~]$ sudo PATH=$PATH python what_python.py
/usr/local/bin/python
2.7.6 (default, Feb 27 2014, 17:05:07) 
[GCC 4.1.2 20080704 (Red Hat 4.1.2-54)]

It does.

Related:

• Python issue 7774 – sys.executable: wrong location if zeroth command argument is modified.
• Python issue 10835 – sys.executable default and altinstall
• python-dev mailing list thread – towards a stricter definition of sys.executable
• Stackoverflow question – how to find the location of the executable in C

Answer 2

I think /usr/local/bin/python is the executable that is running. The version string is almost certainly compiled into python, so it is very unlikely to be wrong. Looking at the documentation for sys.executable:

sys.executable

A string giving the absolute path of the executable binary for the Python interpreter, on systems where this makes sense. If Python is unable to retrieve the real path to its executable, sys.executable will be an empty string or None.

The fact that python may be unable to retrieve this suggests that it is doing its own PATH search using the PATH sudo has set (which as per my answer to the previous question is not the same as the one it used to find the executable).

The only way to be certain here is to dig through the python implementation, but generally I would say that the version string is more likely to be the one you can trust. On the other hand though, sudo uses execve to execute the command (at least according to the man page). You must specify the full path of the executable to execve (some of the exec variations do their own PATH search, this one doesn't). Therefore it should be a no brainer for python to fill in sys.executable.

I don't know if there is any way to get the actual argv[0] for the python interpreter (sys.argv[0] is always the name of the script or -c), but this would be interesting to see. If it is /usr/local/bin/python, this would be a bug in python.

I think the best thing to do is just to set secure_path in /etc/sudoers, hopefully then you will get some consistency.

Update

Actually execve takes on argument for the executable path and then an argv array, so argv[0] is not necessarily /usr/local/bin/python. You can still find it out though by making a script like:

import time
time.sleep(60)

Then run it and get ps to give you the full arguments:

sudo python sleep.py &
ps -o args= -C python

Also to be sure which python is being run, you can do:

sudo ls -l /proc/PID/exe

while the program is running.