runtime: Add script for bootstrapping the python host

runtime/plugin/python_setup.vim

@@ -0,0 +1,44 @@
+" Nvim plugin for loading python extensions via an external interpreter
+if exists("did_python_setup") || &cp
+  finish
+endif
+let did_python_setup = 1
+
+
+let s:get_version =
+  \ ' -c "import sys; sys.stdout.write(str(sys.version_info.major))"'
+
+" To load the python host a python 2 executable must be available
+if exists('python_interpreter')
+      \ && executable(g:python_interpreter)
+      \ && system(g:python_interpreter.s:get_version) == "2"
+  let s:python_interpreter = g:python_interpreter
+elseif executable('python') &&
+      \ system('python'.s:get_version) == "2"
+  let s:python_interpreter = 'python'
+elseif executable('python2') &&
+      \ system('python2'.s:get_version) == "2"
+  " In some distros, python3 is the default python
+  let s:python_interpreter = 'python2'
+else
+  finish
+endif
+
+" Execute python, import neovim and print a string. If import_result matches
+" the printed string, we can probably start the host
+let s:import_result = substitute(system(
+      \ s:python_interpreter .' -c "import neovim; print \"ok\""'),
+      \ '^[\s\n]*\(ok\)[\s\n]*$', '\1', '')
+if s:import_result != 'ok'
+  finish
+endif
+
+let s:pyhost_id = api_spawn(s:python_interpreter,
+      \ ['-c', 'import neovim; neovim.start_host()'])
+" Evaluate an expression in the script host as an additional sanity check, and
+" to block until all providers have been registered(or else some plugins loaded
+" by the user's vimrc would not get has('python') == 1
+if send_call(s:pyhost_id, 'python_eval', '"o" + "k"') != 'ok' || !has('python')
+  " Something went wrong
+  api_close(s:pyhost_id)
+endif