allow default language_version of system when homedir is /

pre_commit/languages/helpers.py

@@ -37,8 +37,14 @@ def exe_exists(exe: str) -> bool:
         common = None
 
     return (
-        not SHIMS_RE.search(found) and  # it is not in a /shims/ directory
-        common != homedir  # it is not in the home directory
+        # it is not in a /shims/ directory
+        not SHIMS_RE.search(found) and
+        (
+            # the homedir is / (docker, service user, etc.)
+            os.path.dirname(homedir) == homedir or
+            # the exe is not contained in the home directory
+            common != homedir
+        )
     )
 
 

tests/languages/helpers_test.py

@@ -55,6 +55,12 @@ def test_exe_exists_commonpath_raises_ValueError(find_exe_mck, homedir_mck):
         assert helpers.exe_exists('ruby') is True
 
 
+def test_exe_exists_true_when_homedir_is_slash(find_exe_mck):
+    find_exe_mck.return_value = os.path.normpath('/usr/bin/ruby')
+    with mock.patch.object(os.path, 'expanduser', return_value=os.sep):
+        assert helpers.exe_exists('ruby') is True
+
+
 def test_basic_get_default_version():
     assert helpers.basic_get_default_version() == C.DEFAULT