"Graph" 中的 "identifier" 有什么作用？

Question

我尝试这样查询数据库：

from rdflib import Graph, Literal, URIRef
from rdflib.namespace import RDF, SKOS
from rdflib.plugins.stores import sparqlstore


# define endpoint according to https://www.stardog.com/docs/
endpoint = 'http://path/to/query'  # http://<server>:<port>/{db}/query

# create store
store = sparqlstore.SPARQLUpdateStore()

# I only want to query
store.open(endpoint)
store.setCredentials('me', 'my_pw')

# What does this actually do? That runs through
default_graph = URIRef('some:stuff')
ng = Graph(store, identifier=default_graph)
# # If identifier is not defined, it crashes
# ng = Graph(store)

rq = """
SELECT ?foo ?bar 
WHERE {
  ?something a <http://path/to/data/.ttl#SomeValues>.
  ?something <http://path/to/data/.ttl#foo> ?foo.
  ?something <http://path/to/data/.ttl#bar> ?bar.                       
}
"""

query_res = ng.query(rq)
for s, l in query_res:
    print(s, l)

很遗憾，我目前没有得到任何结果：

<head><variable name="foo"></variable><variable name="bar"></variable></head><results></results></sparql>

我的问题是，Graph 中的 identifier 在做什么，即这是否重要，如果重要，应该如何定义。当我不定义它时，代码崩溃：

Response: b'{"message":"No separator character found in the URI: N53e412e0f3a74d6eab7ed6da163463bf"}'

如果我输入任何其他有冒号或斜杠的东西，它就会运行（但查询仍然没有 return 任何东西）。

谁能简单解释一下，应该在那里放什么，这是否可能是查询不成功的原因（查询命令本身是正确的；当我从另一个工具调用它时，它工作正常）？

Answer 1

Graph constructor allows to identify an RDFLib graph. If the value is None, then blank node 的 identifier 参数用作标识符。

但是，如果 store 值是 SPARQLUpdateStore，则 identifier 值也用作 SPARQL 协议的 default-graph-uri，因此不能一个空白节点。

因此，问题是：远程三元组中默认的"unnamed"图的名称是什么？

来自 Stardog's documentation:

Naming

Stardog includes aliases for several commonly used sets of named graphs. These non-standard extensions are provided for convenience and can be used wherever named graph IRIs are expected. This includes SPARQL queries & updates, property graph operations and configuration values. Following is a list of special named graph IRIs.
          Named Graph IRI                             Refers to                
--------------------------------  ---------------------------------------------
tag:stardog:api:context:default   the default (no) context graph              
tag:stardog:api:context:all       all contexts, including the default graph    
tag:stardog:api:context:named     all named graphs, excluding the default graph

我找不到任何 public 的私有 Stardog 端点（似乎 ABS 的端点已关闭）。 DBpedia 上的示例：

from rdflib import Graph, URIRef
from rdflib.plugins.stores import sparqlstore

store = sparqlstore.SPARQLUpdateStore()
store.open('http://dbpedia.org/sparql')

default_graph = URIRef('http://people.aifb.kit.edu/ath/#DBpedia_PageRank') 
ng = Graph(store, identifier=default_graph)

rq = """
    SELECT ?foo ?foobar {
      ?foo ?foobar ?bar                       
    } LIMIT 100
"""

query_res = ng.query(rq)
for s, l in query_res:
    print(s, l)

结果与 should be. Even in your code, the name of unnamed graph is the only problem, the results obtained are correct SPARQL XML results 相似。

P.S。也许您可以尝试 sparqlwrapper instead of rdflib 来达到您的目的。

"Graph" 中的 "identifier" 有什么作用？

What does the "identifier" in "Graph" do?

python

sparql

rdflib

stardog

named-graphs