fix(text_editor.py): update error messages for clarity regarding file handling

feat(text_editor.py): add support for append mode when line_start exceeds total lines
test(text_editor.py): add unit test for appending content in append mode

Author: tumf

src/mcp_text_editor/text_editor.py

@@ -253,20 +253,18 @@ async def edit_file_contents(
                     current_content = ""
                     current_hash = ""
                     lines = []
-                elif current_hash != expected_hash:
-                    lines = []
                 elif current_content and expected_hash == "":
                     return {
                         "result": "error",
-                        "reason": "Unexpected error",
+                        "reason": "Unexpected error - Cannot treat existing file as new",
                         "file_hash": None,
                         "content": None,
                     }
                 elif current_hash != expected_hash:
                     return {
                         "result": "error",
                         "reason": "Hash mismatch - file has been modified",
-                        "hash": None,
+                        "file_hash": None,
                         "content": current_content,
                     }
                 else:
@@ -358,9 +356,12 @@ async def edit_file_contents(
                 if not os.path.exists(file_path) or not current_content:
                     # New file or empty file - treat as insertion
                     is_insertion = True
+                elif line_start_zero >= len(lines):
+                    # Append mode - line_start exceeds total lines
+                    is_insertion = True
                 else:
                     # For existing files:
-                    # range_hash is required for all modifications
+                    # range_hash is required for modifications
                     if not expected_range_hash:
                         return {
                             "result": "error",

tests/test_text_editor.py

@@ -737,3 +737,34 @@ async def test_invalid_line_range(editor, tmp_path):
 
     assert result["result"] == "error"
     assert "End line must be greater than or equal to start line" in result["reason"]
+
+
+@pytest.mark.asyncio
+async def test_append_mode(editor, tmp_path):
+    """Test appending content when line_start exceeds total lines."""
+    # Create a test file
+    test_file = tmp_path / "test_append.txt"
+    original_content = "Line 1\nLine 2\nLine 3\n"
+    test_file.write_text(original_content)
+
+    # Read the content and get hash
+    content, start, end, file_hash, total_lines, size = await editor.read_file_contents(
+        str(test_file)
+    )
+
+    # Attempt to append content with line_start > total_lines
+    append_content = "Appended Line\n"
+    result = await editor.edit_file_contents(
+        str(test_file),
+        file_hash,
+        [
+            {
+                "line_start": total_lines + 1,  # Start beyond current line count
+                "contents": append_content,
+                # No line_end or range_hash needed for append mode
+            }
+        ],
+    )
+
+    assert result["result"] == "ok"
+    assert test_file.read_text() == original_content + append_content