Use sys.executable if it matches the requested version

pre_commit/languages/python.py

@@ -89,8 +89,26 @@ def get_default_version():
         return get_default_version()
 
 
+def _sys_executable_matches(version):
+    if version == 'python':
+        return True
+    elif not version.startswith('python'):
+        return False
+
+    try:
+        info = tuple(int(p) for p in version[len('python'):].split('.'))
+    except ValueError:
+        return False
+
+    return sys.version_info[:len(info)] == info
+
+
 def norm_version(version):
     if os.name == 'nt':  # pragma: no cover (windows)
+        # first see if our current executable is appropriate
+        if _sys_executable_matches(version):
+            return sys.executable
+
         # Try looking up by name
         version_exec = find_executable(version)
         if version_exec and version_exec != version:

tests/languages/python_test.py

@@ -2,6 +2,10 @@ from __future__ import absolute_import
 from __future__ import unicode_literals
 
 import os.path
+import sys
+
+import mock
+import pytest
 
 from pre_commit.languages import python
 
@@ -16,3 +20,15 @@ def test_norm_version_expanduser():
         expected_path = home + '/.pyenv/versions/3.4.3/bin/python'
     result = python.norm_version(path)
     assert result == expected_path
+
+
+@pytest.mark.parametrize('v', ('python3.6', 'python3', 'python'))
+def test_sys_executable_matches(v):
+    with mock.patch.object(sys, 'version_info', (3, 6, 7)):
+        assert python._sys_executable_matches(v)
+
+
+@pytest.mark.parametrize('v', ('notpython', 'python3.x'))
+def test_sys_executable_matches_does_not_match(v):
+    with mock.patch.object(sys, 'version_info', (3, 6, 7)):
+        assert not python._sys_executable_matches(v)