Support for modifying string/bytes fields.

tests/test_blendfile_modification.py

@@ -0,0 +1,34 @@
+from shutil import copyfile
+
+from blender_asset_tracer import blendfile
+from abstract_test import AbstractBlendFileTest
+
+
+class ModifyUncompressedTest(AbstractBlendFileTest):
+    def setUp(self):
+        self.orig = self.blendfiles / 'linked_cube.blend'
+        self.to_modify = self.orig.with_name('linked_cube_modified.blend')
+
+        copyfile(str(self.orig), str(self.to_modify))  # TODO: when requiring Python 3.6+, remove str()
+        self.bf = blendfile.BlendFile(self.to_modify, mode='r+b')
+
+        self.assertFalse(self.bf.is_compressed)
+
+    def tearDown(self):
+        if self.to_modify.exists():
+            self.to_modify.unlink()
+
+    def test_change_path(self):
+        library = self.bf.code_index[b'LI'][0]
+
+        # Change it from absolute to relative.
+        library[b'filepath'] = b'//basic_file.blend'
+        library[b'name'] = b'//basic_file.blend'
+
+        # Reload the blend file to inspect that it was written properly.
+        self.bf.close()
+        self.bf = blendfile.BlendFile(self.to_modify, mode='r+b')
+
+        library = self.bf.code_index[b'LI'][0]
+        self.assertEqual(b'//basic_file.blend', library[b'filepath'])
+        self.assertEqual(b'//basic_file.blend', library[b'name'])