CODEWITHSUNDEEP
parsingrecursionpyparsing
dfdashh
dfdashh

Reputation: 13

pyparsing with a recursive grammar

I'm having trouble getting myself to properly parse a recursive grammar using pyparsing. Test #5 in the code below fails, despite my thinking that it would recognize it as three matches of the "param" parser (two of which are nested under one "parent"):

import pyparsing as p

DOUBLE_QUOTE  = p.Word('"')
SINGLE_QUOTE  = p.Word("'")
COMMA         = p.Suppress(p.Word(","))
EQUALS        = p.Suppress(p.Word("="))
RIGHT_PAREN   = p.Suppress(p.Word(")"))
LEFT_PAREN    = p.Suppress(p.Word("("))
WORD          = p.Word(p.alphanums + '<' + '<' + '>' + '/' + '.' + ':' + \
        ';' + '-' + '_' + '$' + '+' + '*' + '&' + '!' + '%' + '?' + '@' + '\\')
QUOTED_STRING = p.QuotedString("'") | p.QuotedString('"')
value         = WORD | QUOTED_STRING
value_list    = value + p.ZeroOrMore(COMMA + value)
keyword       = WORD
pv1           = value
pv2           = (LEFT_PAREN + value_list + RIGHT_PAREN)
pv3           = p.Forward()
param         = keyword + EQUALS + p.Group(p.OneOrMore(pv3) | pv2 | pv1)
pv3 << (LEFT_PAREN + param + RIGHT_PAREN)

parser = p.OneOrMore(p.Group(param))

tests = []
tests.append("""l1=v1""")
tests.append("""l1=(v1,v2,v3)""")
tests.append("""l1=(v1,v2,v3) l1=(v4, v5, v6)""")
tests.append("""l1=(l2=v1)""")
tests.append("""l1=v1 l1=v2""")

# This test fails
tests.append("""l1=(l2=(l3=v1))""")

results = []
for (i, test_string) in enumerate(tests):
    try:
        results.append(parser.parseString(test_string))
    except Exception as e:
        print("Failed test #{}".format(i))
        print(e)

Where did I go wrong here?

Upvotes: 1

Views: 513

Answers (1)

B8vrede
B8vrede

Reputation: 4532

It took me some time to figure this one out as I was checking whether your recursion was correct. But it turned out that your code was fine expect for 2 lines of code at the top of your code (which I assumed was corrected)

The error was caused by the fact that you set the parenthese using p.Word instead of p.Literal. So by changing you code into this it should work:

RIGHT_PAREN   = p.Suppress(p.Literal(")"))
LEFT_PAREN    = p.Suppress(p.Literal("("))

Just a reminder from the PyParsing wiki:

Literal - construct with a string to be matched exactly

Word - one or more contiguous characters; construct with a string containing the set of allowed initial characters, and an optional second string of allowed body characters;

Upvotes: 3

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