"No visitor method was defined" message unhelpful for unnamed expressions

[lucasribeiroufrj]

I have downloaded the framework parsimonious yesterday (version 0.7.0) and created the following code.
hello.py.txt

The problem is that when I run the code I get the error msg:
"parsimonious.exceptions.VisitationError: NotImplementedError: No visitor method was defined for ."

Is it a bug? There is no rule called "."!

The full error: error.txt

[jwhitlock]

You should use https://gist.github.com for pasting code - it makes it easier to view, comment, and fix. Here's your code as a gist: hello.py

I find it useful to run my code under the Python debugger, so that I can investigate what is going on: python -m pdb hello.py. In this case, it isn't a rule called . - it's a rule with no name, which is a little more apparent when you look at the generic_visit code:

        raise NotImplementedError("No visitor method was defined for %s." %
                                  node.expr_name)

That %s should be in quotes (like '%s'), so it is more apparent that a rule with an empty name is being complained about. I have a similar memory of hitting this bug.

An implicit, unnamed node is being created by the zero-or-more rule around spacing:

spacing			 = ( comment / EndOfLine / Space )*

One way to work around this is to name the implicit node:

spacing       = spacing_token*
spacing_token = (comment / EndOfLine / Space)

And now declare a visit_spacing_token member.

Another way is to define a generic_visit function that does what you want with nodes you don't care about.

[lucaswiman]

This is definitely something I've found really confusing as well.

[lucasribeiroufrj]

Humm https://gist.github.com, good to know about it!

Thank you for the debug information, I did not know about it.

Right on the spot! I have fixed using your explanation, thank you very much!!

[erikrose]

There, that should help for the moment. When somebody (or I) have a few more minutes, we could make the message more helpful in the case of an unnamed rule by having it suggest you define a generic_visitor or name the subexpression (which we can also call out into the error message by using its repr).

[cjw296]

@erikrose - I think there's a bug lurking here. Take the following example:

from parsimonious import Grammar, NodeVisitor


class Query(NodeVisitor):

    grammar = Grammar("""
expression = field clause*
clause     = op field
field      = ~"[a-z_]+"
op         = "+"
""")

    def visit_op(self, node, children):
        return ' plus '

    def visit_field(self, node, children):
        return node.match.group(0)

    def visit_clause(self, node, children):
        return ' '.join(children)

    def visit_expression(self, node, children):
        return ' '.join(children)

    def generic_visit(sef, node, children):
        raise Exception(repr((node, children)))


print Query().parse("x+y")

This blows up as follows:

    raise Exception(repr((node, children)))
parsimonious.exceptions.VisitationError: Exception: (s = 'x+y'
Node('', s, 1, 3, children=[Node('clause', s, 1, 3, children=[Node('op', s, 1, 2), RegexNode('field', s, 2, 3)])]), [' plus  y'])

Parse tree:
<Node matching "+y">  <-- *** We were here. ***
    <Node called "clause" matching "+y">
        <Node called "op" matching "+">
        <RegexNode called "field" matching "y">

How can I give a name to the subexpression here?

[erikrose]

It looks like the nameless top-level expression in the traceback is clause*: a ZeroOrMore containing the atom clause. We should do a nicer job in Node.prettily(), grabbing the expression's own pretty-printing if it has no label.

[erikrose]

In fact, that's what #59 is about: a great first patch for somebody!

[cjw296]

Okay, but still, how can I give a name to that ZeroOrMore here, so that I can define a visitor method for it?

[erikrose]

Oh! I didn't understand your question. All you should have to do is…

clauses = clause*

And then say…

expression = field clauses

[cjw296]

Have you tried your suggestion? My experience is that generic_visit will still get called even with a clauses rule defined.

[erikrose]

I just tried it, and generic_visit does not appear to be called for me in the presence of a clauses rule:

from parsimonious import Grammar, NodeVisitor


class Query(NodeVisitor):

    grammar = Grammar("""
expression = field clauses
clauses = clause*
clause     = op field
field      = ~"[a-z_]+"
op         = "+"
""")

    def visit_op(self, node, children):
        return ' plus '

    def visit_field(self, node, children):
        return node.match.group(0)

    def visit_clause(self, node, children):
        return ' '.join(children)

    def visit_expression(self, node, children):
        return ' '.join(children)

    def visit_clauses(self, node, children):
        return "ha"

print Query().parse("x+y")

x ha

[cjw296]

Hmm, well, without being able to reproduce myself, I'll retract my assertion that there's a bug lurking there until such time as I can reproduce it again ;-)

[ghost]

I'm getting the same empty node name error with this peg.

grammar = Grammar(r"""
    sexp = lp ( atom / sexp )* rp
    atom = num
    num = ~"[0-9]+" delim
    delim = &lp / &rp / ws
    lp = "("
    rp = ")"
    ws = ~"[\r\a\t\n ]+"
    """)

tree = grammar.parse('(1 2 3 (4 5))')

print LispFormatter().visit(tree)

parsimonious.exceptions.VisitationError: NotImplementedError: No visitor method was defined for .

Parse tree:
<Node matching "1 ">  <-- *** We were here. ***
    <RegexNode called "num" matching "1">
    <Node called "delim" matching " ">
        <RegexNode called "ws" matching " ">

[lonnen]

issue #59 is long closed, and I think that means all the desired remediation for this is done. Please file a new issue or reopen this one if I'm mistaken