Improve vim plugin loading

Several of the plugin loading methods have been joined into one plugin
file that will be loaded by updating the runtimepath. More informative
error messages will be displayed if Python support is missing or if the
module import fails.

Note that this commit will break existing plugin loading, the new
method with updating the runtimepath will be required.

Closes #156.
Closes #181.

docs/source/overview.rst

@@ -83,26 +83,17 @@ Usage
 Vim statusline
 --------------
 
-Add the following line to your :file:`vimrc`:
+Add the following line to your :file:`vimrc`, where ``{path}`` is the 
+absolute path to your Powerline installation directory:
 
 .. code-block:: vim
 
-   python from powerline.bindings.vim import source_plugin; source_plugin()
+   set rtp+={path}/powerline/bindings/vim
 
-If you want to enable Python 3 support, substitute the ``python`` command 
-above with ``python3``. Note that this is somewhat experimental as some 
-segments don't have support for Python 3 yet.
-
-If Powerline is installed somewhere other than Python's site-packages 
-directories you'll either have to use a plugin manager like Vundle, or 
-source the vim plugin file with an absolute path to the plugin location.
-
-Add the following line to your :file:`vimrc`, where ``{path}`` is the path 
-to the main Powerline project directory:
-
-.. code-block:: vim
-
-   source {path}/powerline/bindings/vim/plugin/source_plugin.vim
+If you're using Vundle or Pathogen and don't want Powerline functionality in 
+any other applications, simply add Powerline as a bundle and point the path 
+above to the Powerline bundle directory, e.g.  
+``~/.vim/bundle/powerline/powerline/bindings/vim``.
 
 Shell prompts
 -------------
@@ -128,7 +119,7 @@ absolute path to your Powerline installation directory:
    . {path}/powerline/bindings/zsh/powerline.zsh
 
 Tmux statusline
-^^^^^^^^^^^^^^^
+---------------
 
 Add the following line to your :file:`tmux.conf`, where ``{path}`` is the 
 absolute path to your Powerline installation directory::
@@ -136,7 +127,7 @@ absolute path to your Powerline installation directory::
    source '{path}/powerline/bindings/tmux/powerline.conf'
 
 IPython prompt
-^^^^^^^^^^^^^^
+--------------
 
 For IPython<0.11 add the following lines to your 
 :file:`.ipython/ipy_user_conf.py`::

powerline/bindings/vim/__init__.py

@@ -2,19 +2,6 @@
 
 import vim
 
-
-def source_plugin():
-	import sys
-	import os
-	if sys.version_info[:2] == (3, 3):
-		vim.command('let g:powerline_pycmd = "python3"')
-		vim.command('let g:powerline_pyeval = "py3eval"')
-	else:
-		vim.command('let g:powerline_pycmd = "python"')
-		vim.command('let g:powerline_pyeval = "pyeval"')
-	fnameescape = vim_get_func('fnameescape')
-	vim.command('source ' + fnameescape(os.path.join(os.path.abspath(os.path.dirname(__file__)), 'powerline.vim')).decode('utf-8'))
-
 try:
 	_vim_globals = vim.bindeval('g:')
 

powerline/bindings/vim/plugin/powerline.vim

@@ -3,11 +3,39 @@ if exists('g:powerline_loaded')
 endif
 let g:powerline_loaded = 1
 
+function! s:CriticalError(message)
+	echohl ErrorMsg
+	echomsg a:message
+	echohl None
+endfunction
+
+if ! has('python') && ! has('python3')
+	call s:CriticalError('You need vim compiled with Python 2.7 or 3.3+ support
+		\ for Powerline to work. Please consult the documentation for more details.')
+	finish
+endif
+
 let s:powerline_pycmd = substitute(get(g:, 'powerline_pycmd', 'py'), '\v^(py)%[thon](3?)$', '\1\2', '')
 let s:powerline_pyeval = get(g:, 'powerline_pyeval', s:powerline_pycmd.'eval')
 
 exec s:powerline_pycmd 'import uuid'
-exec s:powerline_pycmd 'from powerline.core import Powerline'
+try
+	exec s:powerline_pycmd 'from powerline.core import Powerline'
+catch
+	" An error occured while importing the module, it could be installed
+	" outside of Python's module search paths. Update sys.path and try again.
+	exec s:powerline_pycmd 'import sys, vim'
+	exec s:powerline_pycmd 'sys.path.append(vim.eval(''expand("<sfile>:h:h:h:h:h")''))'
+	try
+		exec s:powerline_pycmd 'from powerline.core import Powerline'
+	catch
+		call s:CriticalError('An error occured while importing the Powerline package.
+			\ This could be caused by an invalid sys.path setting, or by an incompatible
+			\ Python version (Powerline requires Python 2.7 or 3.3+ to work). Please consult
+			\ the troubleshooting section in the documentation for possible solutions.')
+		finish
+	endtry
+endtry
 exec s:powerline_pycmd 'powerline = Powerline("vim")'
 
 if exists('*'. s:powerline_pyeval)

powerline/bindings/vim/plugin/source_plugin.vim

@@ -1,11 +0,0 @@
-if ! has('python') && ! has('python3')
-	echohl ErrorMsg
-	echomsg 'You need vim compiled with Python 3.3 or Python 2.7 support for Powerline to work. Please consult the documentation for more details.'
-	echohl None
-	finish
-endif
-
-python import sys, vim
-python sys.path.append(vim.eval('expand("<sfile>:h:h:h:h:h")'))
-
-source <sfile>:h:h/powerline.vim