Using the result of a SPARQL query with EXISTS and AS raises an exception

[ghost]

import rdflib

QUERY = """
    SELECT * {
        BIND(
            EXISTS {
                <http://example.com/a>
                <http://example.com/b>
                <http://example.com/c>
            }
            AS ?exists
        )
    }
"""
graph = rdflib.Graph()
result = graph.query(QUERY)
for row in result:
    print(row)

ought to print

(rdflib.term.Literal('false', datatype=rdflib.term.URIRef('http://www.w3.org/2001/XMLSchema#boolean')),)

This result would agree with that of SPARQL playground. Instead, the following exception is raised.

Exception: What do I do with this CompValue? GroupGraphPatternSub_GroupGraphPatternSub_{'part': [TriplesBlock_{'triples': [[rdflib.term.URIRef('http://example.com/a'), rdflib.term.URIRef('http://example.com/b'), rdflib.term.URIRef('http://example.com/c')]], '_vars': set()}], '_vars': set()}

I observed this bug under the following conditions.

• Ubuntu 20.04
• Python 3.9.5
• rdflib 6.0.2

[ghost]

Technically it's “unhandled” (as indicated by the output) rather than a bug per se but the difference is moot.

FWIW, the RDFLib SPARQL test suite shows a basic use of EXISTS in the context of a FILTER (as opposed to a BIND):
SELECT ?l WHERE { GRAPH ex:g1 { ?a rdfs:label ?l } FILTER EXISTS { ?a a ?type }}
and the venerable SPARQL cheatsheet would seem to accord with this use pattern. Sorry can't be of more help.

[ghost]

@gjhiggins, FILTER EXISTS has a different meaning. Using EXISTS and BIND as I did conforms to W3C's grammar for SPARQL.

row	symbol	::=	expression
[60]	Bind	::=	'BIND' '(' Expression 'AS' Var ')'
[110]	Expression	::=	ConditionalOrExpression
[111]	ConditionalOrExpression	::=	ConditionalAndExpression ( '||' ConditionalAndExpression )*
[112]	ConditionalAndExpression	::=	ValueLogical ( '&&' ValueLogical )*
[113]	ValueLogical	::=	RelationalExpression
[114]	RelationalExpression	::=	NumericExpression ( '=' NumericExpression | '!=' NumericExpression | '<' NumericExpression | '>' NumericExpression | '<=' NumericExpression | '>=' NumericExpression | 'IN' ExpressionList | 'NOT' 'IN' ExpressionList )?
[115]	NumericExpression	::=	AdditiveExpression
[116]	AdditiveExpression	::=	MultiplicativeExpression ( '+' MultiplicativeExpression | '-' MultiplicativeExpression | ( NumericLiteralPositive | NumericLiteralNegative ) ( ( '*' UnaryExpression ) | ( '/' UnaryExpression ) )* )*
[117]	MultiplicativeExpression	::=	UnaryExpression ( '*' UnaryExpression | '/' UnaryExpression )*
[118]	UnaryExpression	::=	'!' PrimaryExpression | '+' PrimaryExpression | '-' PrimaryExpression | PrimaryExpression
[119]	PrimaryExpression	::=	BrackettedExpression | BuiltInCall | iriOrFunction | RDFLiteral | NumericLiteral | BooleanLiteral | Var
[121]	BuiltInCall	::=	Aggregate | 'STR' '(' Expression ')' | 'LANG' '(' Expression ')' | 'LANGMATCHES' '(' Expression ',' Expression ')' | 'DATATYPE' '(' Expression ')' | 'BOUND' '(' Var ')' | 'IRI' '(' Expression ')' | 'URI' '(' Expression ')' | 'BNODE' ( '(' Expression ')' | NIL ) | 'RAND' NIL | 'ABS' '(' Expression ')' | 'CEIL' '(' Expression ')' | 'FLOOR' '(' Expression ')' | 'ROUND' '(' Expression ')' | 'CONCAT' ExpressionList | SubstringExpression | 'STRLEN' '(' Expression ')' | StrReplaceExpression | 'UCASE' '(' Expression ')' | 'LCASE' '(' Expression ')' | 'ENCODE_FOR_URI' '(' Expression ')' | 'CONTAINS' '(' Expression ',' Expression ')' | 'STRSTARTS' '(' Expression ',' Expression ')' | 'STRENDS' '(' Expression ',' Expression ')' | 'STRBEFORE' '(' Expression ',' Expression ')' | 'STRAFTER' '(' Expression ',' Expression ')' | 'YEAR' '(' Expression ')' | 'MONTH' '(' Expression ')' | 'DAY' '(' Expression ')' | 'HOURS' '(' Expression ')' | 'MINUTES' '(' Expression ')' | 'SECONDS' '(' Expression ')' | 'TIMEZONE' '(' Expression ')' | 'TZ' '(' Expression ')' | 'NOW' NIL | 'UUID' NIL | 'STRUUID' NIL | 'MD5' '(' Expression ')' | 'SHA1' '(' Expression ')' | 'SHA256' '(' Expression ')' | 'SHA384' '(' Expression ')' | 'SHA512' '(' Expression ')' | 'COALESCE' ExpressionList | 'IF' '(' Expression ',' Expression ',' Expression ')' | 'STRLANG' '(' Expression ',' Expression ')' | 'STRDT' '(' Expression ',' Expression ')' | 'sameTerm' '(' Expression ',' Expression ')' | 'isIRI' '(' Expression ')' | 'isURI' '(' Expression ')' | 'isBLANK' '(' Expression ')' | 'isLITERAL' '(' Expression ')' | 'isNUMERIC' '(' Expression ')' | RegexExpression | ExistsFunc | NotExistsFunc
[125]	ExistsFunc	::=	'EXISTS' GroupGraphPattern

[ghost]

I didn't mean to imply that you were wrong and I'm sorry if I gave that impression. The TL;DR on this is basically “Thanks for drawing our attention to this, it will get added to the maintenance TODO list but I couldn't even venture a guess as to when it might get fixed, hope you can find a workaround in the interim.” Thanks for posting the spec, that's useful.

[ghost]

@gjhiggins, thank you.

[ghost]

Trying to work around the bug, I have found another manifestation thereof.

import rdflib

QUERY = """
    SELECT (
        EXISTS {
            <http://example.com/a>
            <http://example.com/b>
            <http://example.com/c>
        }
        AS ?exists
    )
    {}
"""
graph = rdflib.Graph()
result = graph.query(QUERY)
for row in result:
    print(row)

ought to print

(rdflib.term.Literal('false', datatype=rdflib.term.URIRef('http://www.w3.org/2001/XMLSchema#boolean')),)

Instead the following exception is raised.

Exception: What do I do with this CompValue? GroupGraphPatternSub_GroupGraphPatternSub_{'part': [TriplesBlock_{'triples': [[rdflib.term.URIRef('http://example.com/a'), PathAlternative_{'part': [PathSequence_{'part': [PathElt_{'part': rdflib.term.URIRef('http://example.com/b'), '_vars': set()}], '_vars': set()}], '_vars': set()}, rdflib.term.URIRef('http://example.com/c')]], '_vars': set()}], '_vars': set()}

[ghost]

Thanks for helping to clarify the scope of the issue. The Exception makes sense given that RDFLib only recognises EXISTS in a FILTER context.

[amediantsev]

@gjhiggins Hi. I encountered the same problem in my work. I want to ask, maybe could you recommend some temporary workaround. Also, could you share if you have some progress on that?
Thanks a lot.

[mgberg]

I'm also running into this issue. Hoping there will be a fix in the somewhat near future.

[aucampia]

@mgberg @JDawson-Camlin @amediantsev PR to fix this is here: #1794 - please review if you feel comfortable doing so.

[ghost]

@mgberg @JDawson-Camlin @amediantsev PR to fix this is here: #1794 - please review if you feel comfortable doing so.

@aucampia, the PR fixes the first manifestation of the bug, but not the second.

import rdflib

QUERY = """
    SELECT (
        EXISTS {
            <http://example.com/a>
            <http://example.com/b>
            <http://example.com/c>
        }
        AS ?exists
    )
    {}
"""
graph = rdflib.Graph()
result = graph.query(QUERY)
for row in result:
    print(row)

ought to print

(rdflib.term.Literal('false', datatype=rdflib.term.URIRef('http://www.w3.org/2001/XMLSchema#boolean')),)

Instead the following exception is raised.

Exception: What do I do with this CompValue? GroupGraphPatternSub_GroupGraphPatternSub_{'part': [TriplesBlock_{'triples': [[rdflib.term.URIRef('http://example.com/a'), PathAlternative_{'part': [PathSequence_{'part': [PathElt_{'part': rdflib.term.URIRef('http://example.com/b'), '_vars': set()}], '_vars': set()}], '_vars': set()}, rdflib.term.URIRef('http://example.com/c')]], '_vars': set()}], '_vars': set()}

This issue ought to be reopened.