1 files changed, 61 insertions, 0 deletions
diff --git a/pre_commit_hooks/check_docstring_first.py b/pre_commit_hooks/check_docstring_first.py
new file mode 100644
index 0000000..d55f08a
--- /dev/null
+++ b/pre_commit_hooks/check_docstring_first.py
@@ -0,0 +1,61 @@
+from __future__ import annotations
+
+import argparse
+import io
+import tokenize
+from tokenize import tokenize as tokenize_tokenize
+from typing import Sequence
+
+NON_CODE_TOKENS = frozenset((
+    tokenize.COMMENT, tokenize.ENDMARKER, tokenize.NEWLINE, tokenize.NL,
+    tokenize.ENCODING,
+))
+
+
+def check_docstring_first(src: bytes, filename: str = '<unknown>') -> int:
+    """Returns nonzero if the source has what looks like a docstring that is
+    not at the beginning of the source.
+
+    A string will be considered a docstring if it is a STRING token with a
+    col offset of 0.
+    """
+    found_docstring_line = None
+    found_code_line = None
+
+    tok_gen = tokenize_tokenize(io.BytesIO(src).readline)
+    for tok_type, _, (sline, scol), _, _ in tok_gen:
+        # Looks like a docstring!
+        if tok_type == tokenize.STRING and scol == 0:
+            if found_docstring_line is not None:
+                print(
+                    f'{filename}:{sline}: Multiple module docstrings '
+                    f'(first docstring on line {found_docstring_line}).',
+                )
+                return 1
+            elif found_code_line is not None:
+                print(
+                    f'{filename}:{sline}: Module docstring appears after code '
+                    f'(code seen on line {found_code_line}).',
+                )
+                return 1
+            else:
+                found_docstring_line = sline
+        elif tok_type not in NON_CODE_TOKENS and found_code_line is None:
+            found_code_line = sline
+
+    return 0
+
+
+def main(argv: Sequence[str] | None = None) -> int:
+    parser = argparse.ArgumentParser()
+    parser.add_argument('filenames', nargs='*')
+    args = parser.parse_args(argv)
+
+    retv = 0
+
+    for filename in args.filenames:
+        with open(filename, 'rb') as f:
+            contents = f.read()
+        retv |= check_docstring_first(contents, filename=filename)
+
+    return retv