Add v:python3_version support

This variable indicates the version of Python3 that Vim was built
against (PY_VERSION_HEX), and will be useful to check whether the Python
library you are loading in dynamically actually fits it. When built with
stable ABI, it will be the limited ABI version instead
(`Py_LIMITED_API`), which indicates the minimum version of Python 3 the
user should have, rather than the exact match. When stable ABI is used,
we won't be exposing PY_VERSION_HEX in this var because it just doesn't
seem necessary to do so (the whole point of stable ABI is the promise
that it will work across versions), and I don't want to confuse the user
with too many variables.

Also, cleaned up some documentation, and added help tags.

runtime/doc/eval.txt

@@ -2397,8 +2397,22 @@ v:progpath	Contains the command with which Vim was invoked, in a form
 
 				    *v:python3_version* *python3-version-variable*
 v:python3_version
-		TODO ychin: This needs to be implemented
-		Read-only.
+		Version of Python 3 that Vim was built against.  When
+		Python is loaded dynamically (|python-dynamic|), this version
+		should exactly match the Python library up to the minor
+		version (e.g.  3.10.2 and 3.10.3 are compatible as the minor
+		version is "10", whereas 3.9.4 and 3.10.3 are not compatible).
+		When |python-stable-abi| is used, this will be the minimum Python
+		version that you can use instead. (e.g. if v:python3_version
+		indicates 3.9, you can use 3.9, 3.10, or anything above).
+
+		This number is encoded as a hex number following Python ABI
+		versioning conventions.  Do the following to have a
+		human-readable full version in hex: >
+			echo printf("%08X", v:python3_version)
+<		You can obtain only the minor version by doing: >
+			echo and(v:python3_version>>16,0xff)
+<		Read-only.
 
 					*v:register* *register-variable*
 v:register	The name of the register in effect for the current normal mode

runtime/doc/if_pyth.txt

@@ -769,19 +769,19 @@ Unix ~
 The 'pythondll' or 'pythonthreedll' option can be used to specify the Python
 shared library file instead of DYNAMIC_PYTHON_DLL or DYNAMIC_PYTHON3_DLL file
 what were specified at compile time.  The version of the shared library must
-match the Python 2.x or Python 3 version Vim was compiled with unless using
-|python3-stable-abi|.
+match the Python 2.x or Python 3 version (|v:python3_version|) Vim was
+compiled with unless using |python3-stable-abi|.
 
 
 Stable ABI and mixing Python versions ~
 			*python-stable* *python-stable-abi* *python3-stable-abi*
-Unless Vim was compiled with Stable ABI (only available for Python 3), the
+If Vim was not compiled with Stable ABI (only available for Python 3), the
 version of the Python shared library must match the version that Vim was
 compiled with.  Otherwise, mixing versions could result in unexpected crashes
 and failures.  With Stable ABI, this restriction is relaxed, and any Python 3
 library with version of at least |v:python3_version| will work.  See
-|has-python| for how to check if Stable ABI is supported, or see
-|+python3/dyn-stable|.
+|has-python| for how to check if Stable ABI is supported, or see if version
+output includes |+python3/dyn-stable|.
 
 ==============================================================================
 10. Python 3						*python3*

runtime/doc/tags

@@ -1434,6 +1434,7 @@ $quote	eval.txt	/*$quote*
 +python/dyn	various.txt	/*+python\/dyn*
 +python3	various.txt	/*+python3*
 +python3/dyn	various.txt	/*+python3\/dyn*
++python3/dyn-stable	various.txt	/*+python3\/dyn-stable*
 +quickfix	various.txt	/*+quickfix*
 +reltime	various.txt	/*+reltime*
 +rightleft	various.txt	/*+rightleft*
@@ -9289,6 +9290,8 @@ python-path_hook	if_pyth.txt	/*python-path_hook*
 python-pyeval	if_pyth.txt	/*python-pyeval*
 python-range	if_pyth.txt	/*python-range*
 python-special-path	if_pyth.txt	/*python-special-path*
+python-stable	if_pyth.txt	/*python-stable*
+python-stable-abi	if_pyth.txt	/*python-stable-abi*
 python-strwidth	if_pyth.txt	/*python-strwidth*
 python-tabpage	if_pyth.txt	/*python-tabpage*
 python-tabpages	if_pyth.txt	/*python-tabpages*
@@ -9301,6 +9304,8 @@ python.vim	syntax.txt	/*python.vim*
 python2-directory	if_pyth.txt	/*python2-directory*
 python3	if_pyth.txt	/*python3*
 python3-directory	if_pyth.txt	/*python3-directory*
+python3-stable-abi	if_pyth.txt	/*python3-stable-abi*
+python3-version-variable	eval.txt	/*python3-version-variable*
 python_x	if_pyth.txt	/*python_x*
 python_x-special-comments	if_pyth.txt	/*python_x-special-comments*
 pythonx	if_pyth.txt	/*pythonx*
@@ -10622,6 +10627,7 @@ v:prevcount	eval.txt	/*v:prevcount*
 v:profiling	eval.txt	/*v:profiling*
 v:progname	eval.txt	/*v:progname*
 v:progpath	eval.txt	/*v:progpath*
+v:python3_version	eval.txt	/*v:python3_version*
 v:register	eval.txt	/*v:register*
 v:scrollstart	eval.txt	/*v:scrollstart*
 v:searchforward	eval.txt	/*v:searchforward*

src/evalvars.c

@@ -157,6 +157,7 @@ static struct vimvar
     {VV_NAME("sizeoflong",	 VAR_NUMBER), NULL, VV_RO},
     {VV_NAME("sizeofpointer",	 VAR_NUMBER), NULL, VV_RO},
     {VV_NAME("maxcol",		 VAR_NUMBER), NULL, VV_RO},
+    {VV_NAME("python3_version",	 VAR_NUMBER), NULL, VV_RO},
 };
 
 // shorthand
@@ -264,6 +265,10 @@ evalvars_init(void)
 
     set_vim_var_dict(VV_COLORNAMES, dict_alloc());
 
+#ifdef FEAT_PYTHON3
+    set_vim_var_nr(VV_PYTHON3_VERSION, python3_version());
+#endif
+
     // Default for v:register is not 0 but '"'.  This is adjusted once the
     // clipboard has been setup by calling reset_reg_var().
     set_reg_var(0);

src/if_python3.c

@@ -2039,4 +2039,14 @@ do_py3eval(char_u *str, typval_T *rettv)
 set_ref_in_python3(int copyID)
 {
     return set_ref_in_py(copyID);
+}
+
+    int
+python3_version()
+{
+#ifdef USE_LIMITED_API
+    return Py_LIMITED_API;
+#else
+    return PY_VERSION_HEX;
+#endif
 }

src/proto/if_python3.pro

@@ -10,4 +10,5 @@ void python3_window_free(win_T *win);
 void python3_tabpage_free(tabpage_T *tab);
 void do_py3eval(char_u *str, typval_T *rettv);
 int set_ref_in_python3(int copyID);
+int python3_version();
 /* vim: set ft=c : */

src/vim.h

@@ -2102,7 +2102,8 @@ typedef int sock_T;
 #define VV_SIZEOFLONG	103
 #define VV_SIZEOFPOINTER 104
 #define VV_MAXCOL	105
-#define VV_LEN		106	// number of v: vars
+#define VV_PYTHON3_VERSION 106
+#define VV_LEN		107	// number of v: vars
 
 // used for v_number in VAR_BOOL and VAR_SPECIAL
 #define VVAL_FALSE	0L	// VAR_BOOL