Question

我正在尝试使用parcon来解析布尔值语句和表达式的简单语法，如下所示：

foo = True
bar = foo or False

这是我正在尝试的语法定义：

from parcon import (
    Word,
    Literal,
    SignificantLiteral,
    Forward,
    Return,
    alpha_chars,
)

m_boolean = Literal("True")[lambda x: True] | Literal("False")[lambda x: False]

m_and = SignificantLiteral('and')(name='AND')
m_or = SignificantLiteral('or')(name='OR')

m_logical_operator = m_and | m_or

m_identifier = (Word(alpha_chars) - m_logical_operator)

m_expression = Forward()

m_subexpression = '(' + m_expression + ')'

m_term = m_boolean | m_identifier | m_subexpression

m_expression_tail= Forward()
m_infix_expression = m_term + m_expression_tail
m_expression_tail << (
    (m_logical_operator + m_expression + m_expression_tail)
    |
    (Return(None))
)

m_expression << (m_term | m_infix_expression)

def test(toker, tests):
    print '\n'

    l = [x.strip() for x in tests.split('\n') if x.strip() != '']
    for test in l:
        print test, ':',
        print toker.parse_string(test)

test(m_subexpression, '''
(True)
(foo)
''')
test(m_term, '''
True
foo
(True)
''')
test(m_infix_expression, '''
a and b
True and False
''')
test(m_expression, '''
True
( True )
True and False
a and b
not True
not a
True or (b)
a and (False)
foo
(foo)
(a or b) and (c or d)
''')

它适用于我的一些测试，但在解析m_expression时我遇到了这个错误：

True and False :
Traceback (most recent call last):
  File "boolean_parcon.py", line 78, in <module>
    ''')
  File "boolean_parcon.py", line 18, in test
    print toker.parse_string(test)
  File "/usr/local/lib/python2.7/dist-packages/parcon/__init__.py", line 620, in parse_string
    self, getattr(self,'name',''), format_failure(result.expected)))
parcon.ParseException: Parse (Forward() ) failure: At position 4: expected one of EOF

我不明白为什么。我想我已经消除了任何无限左递归的可能性。该错误似乎表明True and False规则正在解析m_term并放弃。在m_term失败之后，我会想到，接下来会尝试m_infix_expression。有什么指针吗？

或者，是否有人可以推荐一种语法定义工具，它可能比parcon更适合我？

Python：用parcon解析一个简单的语法

0 个答案: