Clamp negative rows in translate_row_col_to_index

src/prompt_toolkit/document.py

@@ -325,16 +325,16 @@ def translate_row_col_to_index(self, row: int, col: int) -> int:
 
         Negative row/col values are turned into zero.
         """
+        # Clamp before indexing. A negative row is a valid list index and
+        # would otherwise select a line from the end of the document.
+        if row < 0:
+            row = 0
         try:
             result = self._line_start_indexes[row]
             line = self.lines[row]
         except IndexError:
-            if row < 0:
-                result = self._line_start_indexes[0]
-                line = self.lines[0]
-            else:
-                result = self._line_start_indexes[-1]
-                line = self.lines[-1]
+            result = self._line_start_indexes[-1]
+            line = self.lines[-1]
 
         result += max(0, min(col, len(line)))
 

tests/test_document.py

@@ -66,6 +66,15 @@ def test_translate_index_to_position(document):
     assert pos == (0, 0)
 
 
+def test_translate_row_col_to_index_clamps_negative_row():
+    # Negative rows are documented to become zero. List indexing would
+    # otherwise select lines from the end of the document.
+    document = Document("one\ntwo\nthree")
+    for row in (-1, -2, -3, -4):
+        assert document.translate_row_col_to_index(row, 0) == 0
+    assert document.translate_row_col_to_index(-1, 1) == 1
+
+
 def test_is_cursor_at_the_end(document):
     assert Document("hello", 5).is_cursor_at_the_end
     assert not Document("hello", 4).is_cursor_at_the_end