Detect the python version based on the py launcher

pre_commit/languages/python.py

@@ -9,6 +9,7 @@ from pre_commit.envcontext import UNSET
 from pre_commit.envcontext import Var
 from pre_commit.languages import helpers
 from pre_commit.parse_shebang import find_executable
+from pre_commit.util import CalledProcessError
 from pre_commit.util import clean_path_on_failure
 from pre_commit.util import cmd_output
 from pre_commit.xargs import xargs
@@ -40,6 +41,17 @@ def in_env(repo_cmd_runner, language_version):
         yield
 
 
+def _find_by_py_launcher(version):  # pragma: no cover (windows only)
+    if version.startswith('python'):
+        try:
+            return cmd_output(
+                'py', '-{}'.format(version[len('python'):]),
+                '-c', 'import sys; print(sys.executable)',
+            )[1].strip()
+        except CalledProcessError:
+            pass
+
+
 def _get_default_version():  # pragma: no cover (platform dependent)
     def _norm(path):
         _, exe = os.path.split(path.lower())
@@ -66,6 +78,9 @@ def _get_default_version():  # pragma: no cover (platform dependent)
     if find_executable(exe):
         return exe
 
+    if _find_by_py_launcher(exe):
+        return exe
+
     # Give a best-effort try for windows
     if os.path.exists(r'C:\{}\python.exe'.format(exe.replace('.', ''))):
         return exe
@@ -99,6 +114,10 @@ def norm_version(version):
         if version_exec and version_exec != version:
             return version_exec
 
+        version_exec = _find_by_py_launcher(version)
+        if version_exec:
+            return version_exec
+
         # If it is in the form pythonx.x search in the default
         # place on windows
         if version.startswith('python'):