The Python symbolic link

Many Python scripts are written to be executed with /usr/bin/python, which they expect to give them the proper version of the Python interpreter. In a session at the Python Language Summit—held April 8 in conjunction with PyCon—the future of the program at that path was discussed. At this point, /usr/bin/python is a symbolic link that points to Python 2 in most cases, but there are arguments for pointing it elsewhere—or removing it entirely. Several different developers offered their ideas on what you should happen with that link in the future: move it, eliminate it, or something else entirely. Perhaps surprisingly, "something else entirely" won a straw poll at the end of the discussion.

Nick Coghlan was first up in the discussion. He noted that the symbolic link will be gone from Fedora soon. Prior to that, programs that wanted Python 2 would use either "/usr/bin/python" or "/usr/bin/python2", while those wanting Python 3 would always use "/usr/bin/python3". Enough progress has been made in the Fedora tools that most installation images will only have Python 3—and no symbolic link for /usr/bin/python will be installed. Installing the Python 2 package will create the symbolic link (and point it at Python 2) but that will not be the default.

Coghlan wondered whether the "upstream recommendation" about the symbolic link (in the form of PEP 394) should change. It currently recommends that "python" point to Python 2, but that will eventually need to change. By the time Python 3.6 is released in early 2017, the unqualified symbolic link will have been gone from Fedora for more than a year, Coghlan said. The question is whether Fedora (and others) will want to bring the symbolic link back in the Python 3.6 time frame, but point it to Python 3, rather than 2, which is his preferred solution. Over that year, anything that refers to the unqualified symbolic link will break in Fedora, which should force it to get fixed. So more conservative platforms could potentially upgrade directly from the link pointing to Python 2 to pointing at Python 3.

Up next was Matthias Klose, who does not think the symbolic link should be changed at all. He noted that distributions have been dealing with upgrades like Python 2 to 3 for a long time with a variety of mechanisms. For GCC, for example, Debian and Ubuntu manually handled a symbolic link for GCC 4.9. The "alternatives" mechanism is another possibility, but that makes more sense for things like choosing an editor than it does for choosing a version of Python. "Diversions", where the program gets renamed and replaced by a newer version, can also be used, but that is not done often, he said.

There is a parallel to switching the Python symbolic link in the switch of the /bin/sh link from Bash to dash. That change was made in Ubuntu in 2006 and in Debian in 2009 but there are still complaints about shell scripts that won't run on Ubuntu (because they contain Bash-isms). He showed that there are still unresolved bugs in the Debian bug tracker from the switch. That change was made more than eight years ago and problems are still trickling in, he said.

The /bin/sh program has a "concrete meaning" as a POSIX shell, but the Python symbolic link lacks that. Some distributions have already switched the link to Python 3, which has caused "breakage in the wild", Klose said. It will take years to track down all of the breakage and fix it, so it is just easier not to change the symbolic link at all. Programs that care should simply specify Python 2 or 3.

Barry Warsaw said that he was aligned with Klose. PEP 394 should clearly state that programs should be explicit and choose either python2 or python3. Coghlan asked, what about programs that don't care what version they get? Warsaw said that programs shipped by distributions should care, thus should be explicit.

There is a different problem for what users get when they sit down at the terminal and type "python", though. The Bash "command not found" functionality could be used to suggest python3 to users, Warsaw said. For distribution-supplied programs, though, the "#!/usr/bin/python" or "#!/usr/bin/env python" lines (also known as "shebang" lines) should be changed to be explicit on which version the program needs. If they don't care, they should "use Python 3".

Monty Taylor is "somewhere in between" Coghlan and Klose/Warsaw. He would like to see the symbolic link continue to exist until Python 2 fades away. That would come in something like five years, he said, not six months. He would like to not to have to care about Python 2.6, which is now past its end of life but, because Red Hat is still supporting it for certain RHEL releases, he still needs to support it for his packages. Those kinds of situations are likely to persist. Someday, there will be only one Python, but that is not true yet.

Thomas Wouters asked about PyPy. Does the symbolic link always mean CPython? No one seemed interested in switching the link in that direction, however.

The version a user gets when they type python is an important question that should be decoupled from the question about the shebang line, Glyph Lefkowitz said. Having an unqualified python on the shebang line should give a warning now, but for the command-line case, something different could be done. He suggested creating some sort of tool that gives users a menu of choices when they type "python". Warsaw suggested that some kind of configuration parameter could be used to govern whether users got the menu or a particular version of Python. That is what Apple does for programs of this sort, Lefkowitz said.

The various tutorials and other documentation typically just specify "python" for the command line (or the shebang line), so distributions will need to provide something there, one attendee noted. Users are likely to just want whatever the default is, which is Python 2 for now, but that will change.

Larry Hastings conducted a straw poll to see which of the four options was most popular. It was an informal poll that explicitly allowed people to vote more than once, but the outcome was interesting. Seven developers thought that python should point to Python 3 in the 3.6 time frame; 11 thought the symbolic link should not be changed; 19 thought it should be switched at the point where there is only one Python; and 27 agreed with Lefkowitz's idea of a new program that would get run when users type python.