Report line number of error

lib/wificom/punchbag.py

@@ -3,21 +3,26 @@
 Handles the DigiROM tree.
 '''
 
+NO_CONTENT = -1
+LEADING_SPACE = -2
+
 def count_tabs(line):
 	'''
-	Count leading tabs on line. None for comment or no content.
+	Count leading tabs on line.
+	NO_CONTENT for comment, blank line, or tabs only.
+	LEADING_SPACE if a space was found before text begins.
 	'''
 	tabs = 0
 	for character in line:
 		if character == "\t":
 			tabs += 1
 		elif character in "#\r\n":
-			return None
+			return NO_CONTENT
 		elif character == " ":
-			raise ValueError("Tabs are required")
+			return LEADING_SPACE
 		else:
 			return tabs
-	return None
+	return NO_CONTENT
 
 class DigiROM_Node:  #pylint:disable=invalid-name
 	'''
@@ -52,23 +57,25 @@ def children(self):
 			f.seek(pos)
 			f.readline()
 		prev_tabs = start_tabs
-		tabs = None
+		tabs = NO_CONTENT
 		while True:
-			if tabs is not None:
+			if tabs != NO_CONTENT:
 				prev_tabs = tabs
 			pos = f.tell()
 			line = f.readline()
 			if line == "":
 				# EOF
 				break
 			tabs = count_tabs(line)
-			if tabs is None:
+			if tabs == LEADING_SPACE:
+				self._error("Tabs required", pos)
+			if tabs == NO_CONTENT:
 				continue
 			if tabs <= start_tabs:
 				break
 			tab_step = tabs - prev_tabs
 			if tab_step == 0 or tab_step > 1:
-				raise ValueError("Layout error")
+				self._error("Layout error", pos)
 			if tabs == start_tabs + 1:
 				result.append(DigiROM_Node(line.strip(), pos))
 			elif tab_step == 1:
@@ -93,9 +100,17 @@ def digirom(self, node):
 	def pick(self, node):
 		'''Move to the menu option at the chosen node.'''
 		if node.leaf_pos is not None:
-			raise ValueError("No menu here")
+			self._error("No menu here", node.pos)
 		self._menu_path.append(node.pos)
 	def back(self):
 		'''Step back one menu level. Ignored if at the root.'''
 		if len(self._menu_path) > 0:
 			self._menu_path.pop()
+	def _error(self, message, pos):
+		'''Raise ValueError with start-of-line seek position converted to line number.'''
+		f = self._file_obj
+		f.seek(0)
+		line_number = 0
+		while f.tell() <= pos and f.readline() != "":
+			line_number += 1
+		raise ValueError(f"L{line_number}: {message}")