How do I get Vim to be able to run both python and python3 on a Linux system in the same session?

Question

On Linux systems, packaged Vim typically has only one of either python or python3 enabled. It is possible for both to be enabled (using python/dyn and python3/dyn), but during a session, only one can be used. This mailing list discussion said:

It depends on how the Python libraries are built. On Debian-based systems (for reasons I don't recall offhand), they're built such that RTLD_GLOBAL has to be used to get access to the symbols. This prevents loading both libpython2.x and libpython3.x in the same process.

What can be done to enable loading of both in the same session?

Options that I can see:

• Rebuild the Python{2,3} packages so that RTLD_GLOBAL (whatever that may be) isn't needed.
• Somehow get the library loaded earlier to be unloaded (?!) by Vim. (Is that even possible?)

For any particulars of the distro, assume, in increasing order of specificity:

• Debian-based
• Ubuntu
• Ubuntu 14.04
• Or, Arch Linux, if a Debian-based system is too complex.

Note that I have got Vim to build with dynamic loading support for both, so building Vim isn't a problem.

Answer 1

I'm the current Vim maintainer for Debian and the person quoted in the referenced mailing list discussion.

As you stated, this isn't a question about Vim. It's about building the software that Vim links against in a way that meets your needs. There's a more thorough discussion (at least for the Debian aspect) of the issue in a bug requesting Python3 to be enabled in Debian's Vim packaging.

This boils down to

• Debian's Python packaging doesn't link the Python extensions against the relevant libpython shared library. This is what causes Debian's Vim packaging to require using RTLD_GLOBAL when using dlopen() to dynamically load the Python language bindings.

• There aren't good ways to express the relationship between the Vim packages and the dynamically loaded libraries to ensure they're upgraded together when appropriate. Even if the first point were resolved, this issue would still prevent me from enabling dynamic loading of the Python support.

  The main point of dynamically loading the language support in Vim is to not require users to install libraries they won't use. This means the Vim packaging can't specify a hard dependency on a minimum version of a library.

  Therefore, if Vim is built against a newer version of a library that isn't backwards compatible with the older version and the user doesn't upgrade them together, Vim will crash. This isn't something I want the users of the packages to encounter.

I'd love to be able to re-enable (it was available for a short time back in 2010-2011) dynamic loading of language support, but the above issues have to be resolved first.

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As of version 2:7.4.2330-1, Debian's packaging has switched to using Python3 instead of Python2 for the Python bindings.

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As an alternative, the neovim package supports use of both Python2 and Python3 from the same nvim process, since the Python support is provided by external modules (the python-neovim and python3-neovim packages). Externalizing the Python code, rather than embedding it like Vim does, avoids the issue of dealing with how libpython is built.

Answer 2

Ubuntu 16.04 now has vim-*-py2 packages included in the repo. This means all Debian Vim users can migrate to Ubuntu if needed.

The erstwhile vim-* packages now provide +python3, and the binaries are named differently to avoid conflict:

• vim.nox for vim-nox and +python3
• vim.nox-py2 for vim-nox-py2 and +python

And so on.