Added negative number support in the tokenizer.

tokenizer.py

@@ -24,6 +24,17 @@ def get_tokens_from_expression_string(expression_string):
 			parsing_number = True
 			tmp = tmp + c
 			continue
+		
+		elif c == '-' and not parsing_number:
+			if expression_string[i + 1] == '.' or expression_string[i + 1].isdigit():
+				tmp = "-"
+				parsing_number = True
+			else:
+				tokens.append(token.Token(c, token.TokenType.get_operator(c)))
+				parsing_number = False
+				tmp = ""
+				continue
+		
 		elif c == '(':
 			if parsing_number:
 				tokens.append(token.Token(tmp, token.TokenType.constant))
@@ -73,6 +84,11 @@ def get_tokens_from_expression_string(expression_string):
 	
 	return tokens
 
+def peek_list(tokens):
+    if tokens:
+        return tokens[-1]
+    else:
+        raise IndexError("The token list is empty.", tokens)
 
 def replace_variables(tokens, new_constants):
 	newTokenList = []