Unexpected parse action behavior

[eamathw]

Can someone explain why this behaves the way it does?
It prints:

comparison
compchain
comparison
compchain
comparison
comparison

I'm expecting:

comparison
compchain
comparison
compchain
comparison

It seems to be calling the parse actions on failed matches.

import pyparsing as pp
 
def _comparison_action( results: pp.ParseResults):
    print('comparison')
def _compchain_action( results: pp.ParseResults):
    print('compchain')

exp_flip = pp.CaselessLiteral('FLIP')

exp_chain = pp.CaselessLiteral('CHAIN')

exp_comparison =  pp.Word(pp.alphanums)+ pp.Word(pp.alphanums)
exp_comparison.set_parse_action(_comparison_action)

exp_compchain = exp_comparison('comparison') +exp_chain('chain')
exp_compchain.set_parse_action(_compchain_action)

exp_all = pp.ZeroOrMore( exp_compchain('compchain')) +exp_comparison('comparison') +pp.Opt( exp_flip('flip') )


commands = """
    plugin1  plugin2
    chain 
    plugin3  plugin4 
    chain 
    plugin5  plugin6 
    flip"""   

r = exp_all.parseString( commands, parse_all=True) 

[eamathw]

I made a mistake in my description.
I'm expecting :

comparison
compchain
comparison
compchain
comparison

In short, I get an extra match on "comparison".

Updated above.

[ptmcg]

The last comparison is a successful match of comparison, it just comes as part of a failed match on chain.

I made some changes to your code that make this process clearer (and might be easier to do than adding parse actions to trace the parsing behavior):

exp_comparison = pp.Word(pp.alphanums) + pp.Word(pp.alphanums)
exp_comparison.set_parse_action(_comparison_action)
exp_comparison.set_name("exp_comparison").set_debug()  # <-- added

exp_compchain = exp_comparison('comparison') + exp_chain('chain')
exp_compchain.set_parse_action(_compchain_action)
exp_compchain.set_name("exp_compchain").set_debug()  # <-- added

which generates this output:

C:\Users\ptmcg\dev\pyparsing\gh\pyparsing\venv\Scripts\python.exe C:\Users\ptmcg\dev\pyparsing\gh\pyparsing\scratch\issue_512.py 
Match exp_compchain at loc 5(2,5)
      plugin1  plugin2
      ^
Match exp_comparison at loc 5(2,5)
      plugin1  plugin2
      ^
comparison
Matched exp_comparison -> ['plugin1', 'plugin2']
compchain
Matched exp_compchain -> ['plugin1', 'plugin2', 'CHAIN']
Match exp_compchain at loc 37(4,5)
      plugin3  plugin4 
      ^
Match exp_comparison at loc 37(4,5)
      plugin3  plugin4 
      ^
comparison
Matched exp_comparison -> ['plugin3', 'plugin4']
compchain
Matched exp_compchain -> ['plugin3', 'plugin4', 'CHAIN']
Match exp_compchain at loc 70(6,5)
      plugin5  plugin6 
      ^
Match exp_comparison at loc 70(6,5)
      plugin5  plugin6 
      ^
comparison
Matched exp_comparison -> ['plugin5', 'plugin6']
Match exp_compchain failed, ParseException raised: Expected 'CHAIN', found 'flip'  (at char 92), (line:7, col:5)
Match exp_comparison at loc 70(6,5)
      plugin5  plugin6 
      ^
comparison
Matched exp_comparison -> ['plugin5', 'plugin6']

Process finished with exit code 0

Right now, the only way to conditionalize the comparison parse action based on whether the chain will match will be to do a lookahead using FollowedBy:

exp_compchain = pp.FollowedBy(
    exp_comparison().set_parse_action(None) + exp_chain().set_parse_action(None)
    ) + exp_comparison('comparison') + exp_chain('chain')

I had to create copies of exp_comparison and exp_chain with no parse actions to put in the lookahead FollowedBy.

[eamathw]

That makes sense now. Thank you!