Handling syntax errors in the input

This section explains how to handle parsing errors.

If your grammar is correct but you get input string with syntax error parser will raise NoMatch exception with the information where in the input stream error has occurred and what the parser expect to see at that location.

By default, if NoMatch is not caught you will get detailed explanation of the error at the console. The exact location will be reported, the context (part of the input where the error occurred) and all the rules that were tried at that location.


parser = ParserPython(calc)
# 'r' in the following expression can't be recognized by
# calc grammar
input_expr = "23+4/r-89"
parse_tree = parser.parse(input_expr)

As there is an error in the input_expr string (r is not expected) the following traceback will be printed:

Traceback (most recent call last):
arpeggio.NoMatch: Expected '+' or '-' or 'number' or 
  '(' at position (1, 6) => '23+4/*r-89'.

The place in the input stream is marked by * and the position in (line, column) is given ((1, 6)).

If you wish to handle syntax errors gracefully you can catch NoMatch in your code and inspect its attributes.

  parser = ParserPython(calc)
  input_expr = "23+4/r-89"
  parse_tree = parser.parse(input_expr)
except NoMatch as e:
  # Do something with e
  # Call e.eval_attrs() if you want to create a custom message
  # call str(e) for the default message

NoMatch class has the following attributes:

  • rules - A list of ParsingExpression rules that are the sources of the exception.
  • position - A position in the input stream where exception occurred.
  • parser - A Parser instance used for parsing.

After eval_attrs() method is called on the exception (happens automatically if __str__ is called) these additional attributes are available:

  • message - An error message.
  • context - A context (part of the input) in which error occurred.
  • line, col - A line and column in the input stream where exception occurred.

Arpeggio is a backtracking parser, which means that it will go back and try another alternatives when the match does not succeeds. Nevertheless, it will report the furthest place in the input where it failed. Arpeggio will report all Match rules that failed at that position.