Address more @asottile's review comments

pre_commit/languages/python.py

@@ -68,6 +68,8 @@ def _find_by_sys_executable():
 def _get_default_version():  # pragma: no cover (platform dependent)
     # First attempt from `sys.executable` (or the realpath)
     exe = _find_by_sys_executable()
+    if exe:
+        return exe
 
     # Next try the `pythonX.X` executable
     exe = 'python{}.{}'.format(*sys.version_info)