Issue python#27064: The py.exe launcher now defaults to Python 3.

The Windows launcher ``py.exe`` no longer prefers an installed
Python 2 version over Python 3 by default when used interactively.

Doc/using/windows.rst

@@ -418,6 +418,8 @@ Getting started
 From the command-line
 ^^^^^^^^^^^^^^^^^^^^^
 
+.. versionchanged:: 3.6
+
 System-wide installations of Python 3.3 and later will put the launcher on your
 :envvar:`PATH`. The launcher is compatible with all available versions of
 Python, so it does not matter which version is installed. To check that the
@@ -427,25 +429,26 @@ launcher is available, execute the following command in Command Prompt:
 
   py
 
-You should find that the latest version of Python 2.x you have installed is
+You should find that the latest version of Python you have installed is
 started - it can be exited as normal, and any additional command-line
 arguments specified will be sent directly to Python.
 
-If you have multiple versions of Python 2.x installed (e.g., 2.6 and 2.7) you
-will have noticed that Python 2.7 was started - to launch Python 2.6, try the
+If you have multiple versions of Python installed (e.g., 2.7 and 3.6) you
+will have noticed that Python 3.6 was started - to launch Python 2.7, try the
 command:
 
 ::
 
-  py -2.6
+  py -2.7
 
-If you have a Python 3.x installed, try the command:
+If you want the latest version of Python 2.x you have installed, try the
+command:
 
 ::
 
-  py -3
+  py -2
 
-You should find the latest version of Python 3.x starts.
+You should find the latest version of Python 2.x starts.
 
 If you see the following error, you do not have the launcher installed:
 
@@ -500,6 +503,11 @@ version qualifier.  Assuming you have Python 2.6 installed, try changing the
 first line to ``#! python2.6`` and you should find the 2.6 version
 information printed.
 
+Note that unlike interactive use, a bare "python" will use the latest
+version of Python 2.x that you have installed.  This is for backward
+compatibility and for compatibility with Unix, where the command ``python``
+typically refers to Python 2.
+
 From file associations
 ^^^^^^^^^^^^^^^^^^^^^^
 

Doc/whatsnew/3.6.rst

@@ -62,8 +62,17 @@ Summary -- Release highlights
 .. This section singles out the most important changes in Python 3.6.
    Brevity is key.
 
+New syntax features:
+
 * PEP 498: :ref:`Formatted string literals <whatsnew-fstrings>`
 
+Windows improvements:
+
+* The ``py.exe`` launcher, when used interactively, no longer prefers
+  Python 2 over Python 3 when the user doesn't specify a version (via
+  command line arguments or a config file).  Handling of shebang lines
+  remains unchanged - "python" refers to Python 2 in that case.
+
 .. PEP-sized items next.
 
 .. _pep-4XX:

Misc/NEWS

@@ -32,6 +32,13 @@ Tests
 - Issue #25285: Tools/buildbot/test.bat script now uses -j1 by default to run
   each test file in fresh child process.
 
+Windows
+-------
+
+- Issue #27064: The py.exe launcher now defaults to Python 3.
+  The Windows launcher ``py.exe`` no longer prefers an installed
+  Python 2 version over Python 3 by default when used interactively.
+
 
 What's New in Python 3.6.0 alpha 1?
 ===================================

PC/launcher.c

@@ -465,7 +465,7 @@ get_configured_value(wchar_t * key)
 }
 
 static INSTALLED_PYTHON *
-locate_python(wchar_t * wanted_ver)
+locate_python(wchar_t * wanted_ver, BOOL from_shebang)
 {
     static wchar_t config_key [] = { L"pythonX" };
     static wchar_t * last_char = &config_key[sizeof(config_key) /
@@ -497,10 +497,17 @@ locate_python(wchar_t * wanted_ver)
         configured_value = get_configured_value(config_key);
         if (configured_value)
             result = find_python_by_version(configured_value);
+        /* Not found a value yet - try by major version.
+         * If we're looking for an interpreter specified in a shebang line,
+         * we want to try Python 2 first, then Python 3 (for Unix and backward
+         * compatibility). If we're being called interactively, assume the user
+         * wants the latest version available, so try Python 3 first, then
+         * Python 2.
+         */
         if (result == NULL)
-            result = find_python_by_version(L"2");
+            result = find_python_by_version(from_shebang ? L"2" : L"3");
         if (result == NULL)
-            result = find_python_by_version(L"3");
+            result = find_python_by_version(from_shebang ? L"3" : L"2");
         debug(L"search for default Python found ");
         if (result) {
             debug(L"version %ls at '%ls'\n",
@@ -1094,7 +1101,7 @@ find_by_magic(unsigned short magic)
 
     for (mp = magic_values; mp->min; mp++) {
         if ((magic >= mp->min) && (magic <= mp->max)) {
-            result = locate_python(mp->version);
+            result = locate_python(mp->version, FALSE);
             if (result != NULL)
                 break;
         }
@@ -1279,7 +1286,7 @@ specification: '%ls'.\nIn the first line of the script, 'python' needs to be \
 followed by a valid version specifier.\nPlease check the documentation.",
                                   command);
                         /* TODO could call validate_version(command) */
-                        ip = locate_python(command);
+                        ip = locate_python(command, TRUE);
                         if (ip == NULL) {
                             error(RC_NO_PYTHON, L"Requested Python version \
 (%ls) is not installed", command);
@@ -1485,7 +1492,7 @@ process(int argc, wchar_t ** argv)
         plen = wcslen(p);
         valid = (*p == L'-') && validate_version(&p[1]);
         if (valid) {
-            ip = locate_python(&p[1]);
+            ip = locate_python(&p[1], FALSE);
             if (ip == NULL)
                 error(RC_NO_PYTHON, L"Requested Python version (%ls) not \
 installed", &p[1]);
@@ -1512,7 +1519,7 @@ installed", &p[1]);
 
         /* If we didn't find one, look for the default Python */
         if (executable == NULL) {
-            ip = locate_python(L"");
+            ip = locate_python(L"", FALSE);
             if (ip == NULL)
                 error(RC_NO_PYTHON, L"Can't find a default Python.");
             executable = ip->executable;