From c17bfd8af643e7959ef255562e577001c0184a37 Mon Sep 17 00:00:00 2001
From: Matias Bordese <mbordese@gmail.com>
Date: Sun, 13 Sep 2026 12:11:28 -0300
Subject: [PATCH] Raise UnidiffParseError on target header without source
 (#148)

---
 tests/test_parser.py | 10 ++++++++++
 unidiff/patch.py     | 10 ++++++++++
 2 files changed, 20 insertions(+)

diff --git a/tests/test_parser.py b/tests/test_parser.py
index 432d9fb..e2b3a33 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -270,6 +270,16 @@ def test_parse_malformed_diff_shorter_than_expected(self):
         with open(utf8_file, 'r') as diff_file:
             self.assertRaises(UnidiffParseError, PatchSet, diff_file)
 
+    def test_parse_target_without_source(self):
+        """Parse a target file header with no preceding source header."""
+        # regression test for issue #148: this used to raise UnboundLocalError
+        for diff in ('+++ b/file\n',
+                     '+++ b/file\n@@ -0,0 +1 @@\n+a\n',
+                     'some preamble\n+++ b/file\n'):
+            with self.assertRaises(UnidiffParseError) as cm:
+                PatchSet(diff)
+            self.assertIn('Target without source', str(cm.exception))
+
     def test_from_filename_with_cr_in_diff_text_files(self):
         """Parse git diff text files that contain CR"""
         utf8_file = os.path.join(self.samples_dir, 'samples/git_cr.diff')
diff --git a/unidiff/patch.py b/unidiff/patch.py
index 3cb87f0..e0af12b 100644
--- a/unidiff/patch.py
+++ b/unidiff/patch.py
@@ -462,6 +462,9 @@ def _parse(self, diff: Iterable, encoding: Optional[str],
                metadata_only: bool) -> None:
         current_file = None
         patch_info = None
+        # pending source file header details, consumed by the target header
+        source_file = None
+        source_timestamp = None
 
         diff_lines = enumerate(diff, 1)
         for diff_line_no, line in diff_lines:
@@ -552,6 +555,10 @@ def _parse(self, diff: Iterable, encoding: Optional[str],
                 if current_file is not None and not (current_file.target_file == target_file):
                     raise UnidiffParseError('Target without source: %s' % line)
                 if current_file is None:
+                    if source_file is None:
+                        # a target header requires a preceding source header
+                        raise UnidiffParseError(
+                            'Target without source: %s' % line)
                     # add current file to PatchSet
                     current_file = PatchedFile(
                         patch_info, source_file, target_file,
@@ -559,6 +566,9 @@ def _parse(self, diff: Iterable, encoding: Optional[str],
                         diff_line_no=diff_line_no)
                     self.append(current_file)
                     patch_info = None
+                    # the source header has been consumed by this file
+                    source_file = None
+                    source_timestamp = None
                 else:
                     current_file.target_timestamp = target_timestamp
                 continue