Do not let powerline trigger loading wrong python

Used python version is controlled by `g:powerline_pycmd`. User configuration now
has top priority: if `g:powerline_pyeval` is set powerline will not try to use
`pyeval()` emulation in old Vim versions.

Closes #937 as WONTFIX

docs/source/overview.rst

@@ -127,6 +127,25 @@ hand: ``powerline`` is installed and run just like any other plugin using
 
     call vam#ActivateAddons(['powerline'])
 
+.. note::
+    If you use supplied :file:`powerline.vim` file to load powerline there are 
+    additional configuration variables available: ``g:powerline_pycmd`` and 
+    ``g:powerline_pyeval``. First sets command used to load powerline: expected 
+    values are ``"py"`` and ``"py3"``. Second sets function used in statusline, 
+    expected values are ``"pyeval"`` and ``"py3eval"``.
+
+    If ``g:powerline_pycmd`` is set to the one of the expected values then 
+    ``g:powerline_pyeval`` will be set accordingly. If it is set to some other 
+    value then you must also set ``g:powerline_pyeval``. Powerline will not 
+    check that Vim is compiled with Python support if you set 
+    ``g:powerline_pycmd`` to an unexpected value.
+
+    These values are to be used to specify the only Python that is to be loaded 
+    if you have both versions: Vim may disable loading one python version if 
+    other was already loaded. They should also be used if you have two python 
+    versions able to load simultaneously, but with powerline installed only for 
+    python-3 version.
+
 Shell prompts
 -------------
 

powerline/bindings/vim/plugin/powerline.vim

@@ -3,7 +3,41 @@ if exists('g:powerline_loaded')
 endif
 let g:powerline_loaded = 1
 
-if !has('python') && !has('python3')
+if exists('g:powerline_pycmd')
+	let s:pycmd = substitute(g:powerline_pycmd, '\v\C^(py)%[thon](3?)$', '\1\2', '')
+	if s:pycmd is# 'py'
+		let s:has_python = has('python')
+		let s:pyeval = get(g:, 'powerline_pyeval', 'pyeval')
+	elseif s:pycmd is# 'py3'
+		let s:has_python = has('python3')
+		let s:pyeval = 'py3eval'
+		let s:pyeval = get(g:, 'powerline_pyeval', 'py3eval')
+	else
+		if !exists('g:powerline_pyeval')
+			echohl ErrorMsg
+				echomsg 'g:powerline_pycmd was set to an unknown values, but g:powerline_pyeval'
+				echomsg 'was not set. You should either set g:powerline_pycmd to "py3" or "py",'
+				echomsg 'specify g:powerline_pyeval explicitly or unset both and let powerline'
+				echomsg 'figure them out.'
+			echohl None
+			finish
+		endif
+		let s:pyeval = g:powerline_pyeval
+		let s:has_python = 1
+	endif
+elseif has('python')
+	let s:has_python = 1
+	let s:pycmd = 'py'
+	let s:pyeval = get(g:, 'powerline_pyeval', 'pyeval')
+elseif has('python3')
+	let s:has_python = 1
+	let s:pycmd = 'py3'
+	let s:pyeval = get(g:, 'powerline_pyeval', 'py3eval')
+else
+	let s:has_python = 0
+endif
+
+if !s:has_python
 	if !exists('g:powerline_no_python_error')
 		echohl ErrorMsg
 			echomsg 'You need vim compiled with Python 2.6, 2.7 or 3.2 and later support'
@@ -13,9 +47,7 @@ if !has('python') && !has('python3')
 	endif
 	finish
 endif
-
-let s:pycmd = substitute(get(g:, 'powerline_pycmd', has('python') ? 'py' : 'py3'), '\v^(py)%[thon](3?)$', '\1\2', '')
-let s:pyeval = get(g:, 'powerline_pyeval', s:pycmd.'eval')
+unlet s:has_python
 
 let s:import_cmd = 'from powerline.vim import setup as powerline_setup'
 try
@@ -81,9 +113,12 @@ finally
 	endif
 endtry
 
+let s:can_replace_pyeval = !exists('g:powerline_pyeval')
+
 execute s:pycmd 'import vim'
-execute s:pycmd 'powerline_setup(pyeval=vim.eval("s:pyeval"), pycmd=vim.eval("s:pycmd"))'
+execute s:pycmd 'powerline_setup(pyeval=vim.eval("s:pyeval"), pycmd=vim.eval("s:pycmd"), can_replace_pyeval=int(vim.eval("s:can_replace_pyeval")))'
 execute s:pycmd 'del powerline_setup'
 
+unlet s:can_replace_pyeval
 unlet s:pycmd
 unlet s:pyeval

powerline/vim.py

@@ -155,16 +155,17 @@ class VimPowerline(Powerline):
 													__main__.__dict__)))
 
 
-def setup(pyeval=None, pycmd=None):
+def setup(pyeval=None, pycmd=None, can_replace_pyeval=True):
 	import sys
 	import __main__
 	if not pyeval:
 		pyeval = 'pyeval' if sys.version_info < (3,) else 'py3eval'
+		can_replace_pyeval = True
 	if not pycmd:
 		pycmd = 'python' if sys.version_info < (3,) else 'python3'
 
 	# pyeval() and vim.bindeval were both introduced in one patch
-	if not hasattr(vim, 'bindeval'):
+	if not hasattr(vim, 'bindeval') and can_replace_pyeval:
 		vim.command(('''
 				function! PowerlinePyeval(e)
 					{pycmd} powerline.pyeval()