patch 8.0.0251: not easy to select Python 2 or 3

Problem:    It is not so easy to write a script that works with both Python 2
            and Python 3, even when the Python code works with both.
Solution:   Add 'pyxversion', :pyx, etc. (Marc Weber, Ken Takata)

runtime/doc/eval.txt

@@ -2239,6 +2239,7 @@ printf({fmt}, {expr1}...)	String	format text
 pumvisible()			Number	whether popup menu is visible
 pyeval({expr})			any	evaluate |Python| expression
 py3eval({expr})			any	evaluate |python3| expression
+pyxeval({expr})			any	evaluate |python_x| expression
 range({expr} [, {max} [, {stride}]])
 				List	items from {expr} to {max}
 readfile({fname} [, {binary} [, {max}]])
@@ -6163,6 +6164,14 @@ pyeval({expr})						*pyeval()*
 		non-string keys result in error.
 		{only available when compiled with the |+python| feature}
 
+pyxeval({expr})						*pyxeval()*
+		Evaluate Python expression {expr} and return its result
+		converted to Vim data structures.
+		Uses Python 2 or 3, see |python_x| and 'pyxversion'.
+		See also: |pyeval()|, |py3eval()|
+		{only available when compiled with the |+python| or the
+		|+python3| feature}
+
 							*E726* *E727*
 range({expr} [, {max} [, {stride}]])				*range()*
 		Returns a |List| with Numbers:
@@ -8402,6 +8411,7 @@ printer			Compiled with |:hardcopy| support.
 profile			Compiled with |:profile| support.
 python			Compiled with Python 2.x interface. |has-python|
 python3			Compiled with Python 3.x interface. |has-python|
+pythonx			Compiled with |python_x| interface. |has-pythonx|
 qnx			QNX version of Vim.
 quickfix		Compiled with |quickfix| support.
 reltime			Compiled with |reltime()| support.

runtime/doc/if_pyth.txt

@@ -16,6 +16,7 @@ The Python Interface to Vim				*python* *Python*
 8. pyeval(), py3eval() Vim functions		|python-pyeval|
 9. Dynamic loading				|python-dynamic|
 10. Python 3					|python3|
+11. Python X					|python_x|
 
 {Vi does not have any of these commands}
 
@@ -711,6 +712,7 @@ vim.Function object				*python-Function*
 
 To facilitate bi-directional interface, you can use |pyeval()| and |py3eval()| 
 functions to evaluate Python expressions and pass their values to VimL.
+|pyxeval()| is also available.
 
 ==============================================================================
 9. Dynamic loading					*python-dynamic*
@@ -811,5 +813,69 @@ dynamically, these has() calls will try to load them.  If only one can be
 loaded at a time, just checking if Python 2 or 3 are available will prevent
 the other one from being available.
 
+==============================================================================
+11. Python X						*python_x* *pythonx*
+
+Because most python code can be written so that it works with python 2.6+ and
+python 3 the pyx* functions and commands have been writen.  They work exactly
+the same as the Python 2 and 3 variants, but select the Python version using
+the 'pyxversion' setting.
+
+You should set 'pyxversion' in your |.vimrc| to prefer Python 2 or Python 3
+for Python commands. If you change this setting at runtime you may risk that
+state of plugins (such as initialization) may be lost.
+
+If you want to use a module, you can put it in the {rtp}/pythonx directory.
+See |pythonx-directory|.
+
+							*:pyx* *:pythonx*
+The `:pyx` and `:pythonx` commands work similar to `:python`.  A simple check
+if the `:pyx` command is working: >
+	:pyx print("Hello")
+
+To see what version of Python is being used: >
+	:pyx import sys
+	:pyx print(sys.version)
+<
+					*:pyxfile* *python_x-special-comments*
+The `:pyxfile` command works similar to `:pyfile`.  However you can add one of
+these comments to force Vim using `:pyfile` or `:py3file`: >
+  #!/any string/python2		" Shebang. Must be the first line of the file.
+  #!/any string/python3		" Shebang. Must be the first line of the file.
+  # requires python 2.x		" Maximum lines depend on 'modelines'.
+  # requires python 3.x		" Maximum lines depend on 'modelines'.
+Unlike normal modelines, the bottom of the file is not checked.
+If none of them are found, the 'pyxversion' setting is used.
+							*W20* *W21*
+If Vim does not support the selected Python version a silent message will be
+printed.  Use `:messages` to read them.
+
+							*:pyxdo*
+The `:pyxdo` command works similar to `:pydo`.
+
+							*has-pythonx*
+You can test if pyx* commands are available with: >
+	if has('pythonx')
+	  echo 'pyx* commands are available. (Python ' . &pyx . ')'
+	endif
+
+When compiled with only one of |+python| or |+python3|, the has() returns 1.
+When compiled with both |+python| and |+python3|, the test depends on the
+'pyxversion' setting.  If 'pyxversion' is 0, it tests Python 3 first, and if
+it is not available then Python 2.  If 'pyxversion' is 2 or 3, it tests only
+Python 2 or 3 respectively.
+
+Note that for has('pythonx') to work it may try to dynamically load Python 3
+or 2.  This may have side effects, especially when Vim can only load one of
+the two.
+
+If a user prefers Python 2 and want to fallback to Python 3, he needs to set
+'pyxversion' explicitly in his |.vimrc|.  E.g.: >
+	if has('python')
+	  set pyx=2
+	elseif has('python3')
+	  set pyx=3
+	endif
+
 ==============================================================================
  vim:tw=78:ts=8:ft=help:norl:

runtime/doc/index.txt

@@ -1440,6 +1440,10 @@ tag	      command	      action ~
 |:python|	:py[thon]	execute Python command
 |:pydo|		:pyd[o]		execute Python command for each line
 |:pyfile|	:pyf[ile]	execute Python script file
+|:pyx|		:pyx		execute |python_x| command
+|:pythonx|	:pythonx	same as :pyx
+|:pyxdo|	:pyxd[o]	execute |python_x| command for each line
+|:pyxfile|	:pyxf[ile]	execute |python_x| script file
 |:quit|		:q[uit]		quit current window (when one window quit Vim)
 |:quitall|	:quita[ll]	quit Vim
 |:qall|		:qa[ll]		quit Vim

runtime/doc/options.txt

@@ -5786,6 +5786,34 @@ A jump table for the options with a short description can be found at |Q_op|.
 	Specifies the name of the Python 3 shared library. The default is
 	DYNAMIC_PYTHON3_DLL, which was specified at compile time.
 	Environment variables are expanded |:set_env|.
+	This option cannot be set from a |modeline| or in the |sandbox|, for
+	security reasons.
+
+						*'pyxversion'* *'pyx'*
+'pyxversion' 'pyx'	number	(default depends on the build)
+			global
+			{not in Vi}
+			{only available when compiled with the |+python| or
+			the |+python3| feature}
+	Specifies the python version used for pyx* functions and commands
+	|python_x|.  The default value is as follows:
+
+		Compiled with		     Default ~
+		|+python| and |+python3|	0
+		only |+python|			2
+		only |+python3|			3
+
+	Available values are 0, 2 and 3.
+	If 'pyxversion' is 0, it is set to 2 or 3 after the first execution of
+	any python2/3 commands or functions.  E.g. `:py` sets to 2, and `:py3`
+	sets to 3. `:pyx` sets it to 3 if Python 3 is available, otherwise sets
+	to 2 if Python 2 is available.
+	See also: |has-pythonx|
+
+	If Vim is compiled with only |+python| or |+python3| setting
+	'pyxversion' has no effect.  The pyx* functions and commands are
+	always the same as the compiled version.
+
 	This option cannot be set from a |modeline| or in the |sandbox|, for
 	security reasons.
 

runtime/doc/quickref.txt

@@ -835,6 +835,7 @@ Short explanation of each option:		*option-list*
 'pumheight'	  'ph'	    maximum height of the popup menu
 'pythondll'		    name of the Python 2 dynamic library
 'pythonthreedll'	    name of the Python 3 dynamic library
+'pyxversion'	  'pyx'	    Python version used for pyx* commands
 'quoteescape'	  'qe'	    escape characters used in a string
 'readonly'	  'ro'	    disallow writing the buffer
 'redrawtime'	  'rdt'     timeout for 'hlsearch' and |:match| highlighting

runtime/optwin.vim

@@ -923,7 +923,7 @@ if has("folding")
   call append("$", "foldmarker\tmarkers used when 'foldmethod' is \"marker\"")
   call append("$", "\t(local to window)")
   call <SID>OptionL("fmr")
-  call append("$", "foldnestmax\tmaximum fold depth for when 'foldmethod is \"indent\" or \"syntax\"")
+  call append("$", "foldnestmax\tmaximum fold depth for when 'foldmethod' is \"indent\" or \"syntax\"")
   call append("$", "\t(local to window)")
   call <SID>OptionL("fdn")
 endif
@@ -1324,6 +1324,10 @@ if exists("&perldll")
   call append("$", "perldll\tname of the Perl dynamic library")
   call <SID>OptionG("perldll", &perldll)
 endif
+if has('pythonx')
+  call append("$", "pyxversion\twhether to use Python 2 or 3")
+  call append("$", " \tset pyx=" . &wd)
+endif
 if exists("&pythondll")
   call append("$", "pythondll\tname of the Python 2 dynamic library")
   call <SID>OptionG("pythondll", &pythondll)