1 files changed, 47 insertions, 0 deletions
diff --git a/pre_commit_hooks/forbid_new_submodules.py b/pre_commit_hooks/forbid_new_submodules.py
new file mode 100644
index 0000000..0275808
--- /dev/null
+++ b/pre_commit_hooks/forbid_new_submodules.py
@@ -0,0 +1,47 @@
+import argparse
+import os
+from typing import Optional
+from typing import Sequence
+
+from pre_commit_hooks.util import cmd_output
+
+
+def main(argv: Optional[Sequence[str]] = None) -> int:
+    parser = argparse.ArgumentParser()
+    parser.add_argument('filenames', nargs='*')
+    args = parser.parse_args(argv)
+
+    if (
+        'PRE_COMMIT_FROM_REF' in os.environ and
+        'PRE_COMMIT_TO_REF' in os.environ
+    ):
+        diff_arg = '...'.join((
+            os.environ['PRE_COMMIT_FROM_REF'],
+            os.environ['PRE_COMMIT_TO_REF'],
+        ))
+    else:
+        diff_arg = '--staged'
+    added_diff = cmd_output(
+        'git', 'diff', '--diff-filter=A', '--raw', diff_arg, '--',
+        *args.filenames,
+    )
+    retv = 0
+    for line in added_diff.splitlines():
+        metadata, filename = line.split('\t', 1)
+        new_mode = metadata.split(' ')[1]
+        if new_mode == '160000':
+            print(f'{filename}: new submodule introduced')
+            retv = 1
+
+    if retv:
+        print()
+        print('This commit introduces new submodules.')
+        print('Did you unintentionally `git add .`?')
+        print('To fix: git rm {thesubmodule}  # no trailing slash')
+        print('Also check .gitmodules')
+
+    return retv
+
+
+if __name__ == '__main__':
+    raise SystemExit(main())