How to match addition expressions with brackets

[none-None1]

I tried matching addition expressions with brackets (e.g.: 1+1, 1+(2+3)+4) using the following code:

ParserElement.enable_left_recursion()
b=Forward()
a=b+'+'+b|'('+b+')'|Word(nums)
b<<a

However, it didn't work correctly: (1+1) can be matched, but not 1+1.

I tried many ways to fix like changing | into ^ and swapping the operands of |, but none of them worked.

What have I done wrong? Is it a bug or is it an unsupported feature?

[ptmcg]

Thanks for trying out pyparsing!

I ran your code as follows (with pyparsing 3.2.5):

from pyparsing import ParserElement, Forward, Word, nums

ParserElement.enable_left_recursion()
b=Forward()
a=b+'+'+b|'('+b+')'|Word(nums)
b<<a

print("# provided test cases")
print(b.parse_string('1 + 1'))
print(b.parse_string('(1 + 1)'))

# use run_tests to try lots of tests and capture failures
b.run_tests("""\
    # binary addition
    1+1

    # binary addition grouped with parentheses
    (1+1)

    # binary addition with multiple operands
    1+2+3

    # binary addition with multiple operands and parentheses
    (2+7+11)+31

    # this line should fail, to show what the error message looks like
    1+a
    """)

And got these results:

# provided test cases
['1', '+', '1']
['(', '1', '+', '1', ')']

# binary addition
1+1
['1', '+', '1']

# binary addition grouped with parentheses
(1+1)
['(', '1', '+', '1', ')']

# binary addition with multiple operands
1+2+3
['1', '+', '2', '+', '3']

# binary addition with multiple operands and parentheses
(2+7+11)+31
['(', '2', '+', '7', '+', '11', ')', '+', '31']

# this line should fail, to show what the error message looks like
1+a
1+a
 ^
ParseException: Expected end of text, found '+'  (at char 1), (line:1, col:2)
FAIL: Expected end of text, found '+'  (at char 1), (line:1, col:2)

So I don't see your error, here a couple of other suggestions:

• Python code is a little cleaner-looking if you follow at least some of the PEP8 recommendations, like whitespace around operators:

   a=b+'+'+b|'('+b+')'|Word(nums)
   b<<a
  

  vs

   a = b + '+' + b | '(' + b + ')' | Word(nums)
   b << a
  

• Try adding pyparsing Group around the parenthesized part of 'a' to get some structure in your results.

   a = b + '+' + b | Group('(' + b + ')') | Word(nums)
  

• Try changing your literal '+' to a pyparsing expression that accepts both "+" and "-" operators, since they are at the same level of operator precedence:

   add_sub = one_of("+ -")
   a = b + add_sub + b | '(' + b + ')' | Word(nums)
  

• Use run_tests to test your parsers, much easier to work with than writing your own calls to parse_string. (The comments and blank lines are optional, you can just insert tests if you like.)

[none-None1]

Thanks for replying!

I ran your code and it did output the expected results, but I used a.parseString instead of b.parseString. They should be equivalent as b is a Forward of a. However, with a.parseString, I didn't get the expected results.

In [4]: a.parseString('1+1')
Out[4]: ParseResults(['1'], {})

In [5]: b.parseString('1+1')
Out[5]: ParseResults(['1', '+', '1'], {})

Is there anything wrong with my understanding of Forward?

[ptmcg]

Ok, that is a puzzler, and I can see that same behavior as well. I'll try to dig into this in the coming weeks (busy time of year).