ParseError should be hashable

[jshprentz]

ParseError defines a __eq__ method, but no __hash__ method. The default __hash__ method fails because the error attribute is a list. This breaks PyTest and Python's traceback.format_exception_only function.

Discovery

During testing of a new grammar, PyTest reported an internal error when Parsley raised a ParseError.

INTERNALERROR>   File "/home/joel/.virtualenvs/meetup2xibo/lib/python3.5/site-packages/_pytest/_code/code.py", line 481, in exconly
INTERNALERROR>     lines = format_exception_only(self.type, self.value)
INTERNALERROR>   File "/usr/lib/python3.5/traceback.py", line 136, in format_exception_only
INTERNALERROR>     return list(TracebackException(etype, value, None).format_exception_only())
INTERNALERROR>   File "/usr/lib/python3.5/traceback.py", line 439, in __init__
INTERNALERROR>     _seen.add(exc_value)
INTERNALERROR> TypeError: unhashable type: 'ParseError'

The TypeError is raised by Python standard library function traceback.format_exception_only.

A Simple Test

The following code demonstrates the use of traceback.format_exception_only and raises the TypeError without involving PyTest.

from parsley import makeGrammar, ParseError
import sys
import traceback

def format_exception():
    (last_type, last_value, last_traceback) = sys.exc_info()
    return traceback.format_exception_only(last_type, last_value)

def parse(text):
    parser = makeGrammar("foo = 'a'", {})
    try:
        return parser(text).foo()
    except ParseError:
        return format_exception()

def divide(x, y):
    try:
        return x / y
    except ZeroDivisionError:
        return format_exception()

def test():
    print(divide(6, 2))
    print(divide(6, 0))
    print(parse('a'))
    print(parse('b'))

test()

Running the test code with Python 3.5 gives the following results:

• The quotient is printed.
• The ZeroDivisionError is formatted.
• The parser recognizes 'a'.
• The parser raises a ParseError when parsing 'b', but traceback.format_exception_only fails to format the error.

$ python foo.py
3.0
['ZeroDivisionError: division by zero\n']
a
Traceback (most recent call last):
  File "foo.py", line 28, in <module>
    test()
  File "foo.py", line 26, in test
    print(parse('b'))
  File "foo.py", line 14, in parse
    return format_exception()
  File "foo.py", line 7, in format_exception
    return traceback.format_exception_only(last_type, last_value)
  File "/usr/lib/python3.5/traceback.py", line 136, in format_exception_only
    return list(TracebackException(etype, value, None).format_exception_only())
  File "/usr/lib/python3.5/traceback.py", line 439, in __init__
    _seen.add(exc_value)
TypeError: unhashable type: 'ParseError'

Workaround

The following code monkey patches PyError to add a __hash__ method.

def parse_error_hash(self):
    """Define missing ParseError.__hash__()."""
    return hash((self.position, self.formatReason()))

ParseError.__hash__ = parse_error_hash

Rerunning the test code with the monkey patch gives successful results.

o$ python foo.py
3.0
['ZeroDivisionError: division by zero\n']
a
["ometa.runtime.ParseError: \nb\n^\nParse error at line 1, column 0: expected the character 'a'. trail: [foo]\n\n"]