CODEWITHSUNDEEP
pythonsparqlrdflib
user3523375
user3523375

Reputation: 153

ASK SPARQL queries in rdflib

I'm trying to learn SPARQL and I'm using python's rdflib to train. I've done a few tries, but any ASK query always seems to give me back a True result. For instance, i tried the following:

#!/usr/bin/python
# -*- coding: utf-8 -*-
import rdflib
mygraph=rdflib.Graph();
mygraph.parse('try.ttl',format='n3');
results=mygraph.query("""
ASK {?p1 a <http://false.com>}
 """)
print bool(results)

The result is true, even if there is no subject of type false.com in 'try.ttl'. Can anyone explain me why? Thank you in advance for your help!

UPDATE: Reading the rdflib manual, I found out that results is of type list and (in my case) should contain a single boolean with the return value from the ask query. I tried the following: for x in results: print x And I got "None". I'm guessing I don't use the query method in the right way.

Upvotes: 2

Views: 1374

Answers (1)

Joshua Taylor
Joshua Taylor

Reputation: 85843

The documentation doesn't actually says that it's of type list, but that you can iterate over it, or you can convert it to a boolean:

If the type is "ASK", iterating will yield a single bool (or bool(result) will return the same bool)

This means that print bool(results), as you've done, should work. In fact, your code does work for me:

$ touch try.ttl
$ cat try.ttl # it's empty

$ cat test.py # same code
#!/usr/bin/python
# -*- coding: utf-8 -*-
import rdflib
mygraph=rdflib.Graph();
mygraph.parse('try.ttl',format='n3');
results=mygraph.query("""
ASK {?p1 a <http://false.com>}
 """)
print bool(results)

$ ./test.py # the data is empty, so there's no match
False

If we add some data to the file that would make the query return true, we get true:

$ cat > try.ttl 
<http://example.org> a <http://false.com> .

$ cat try.ttl 
<http://example.org> a <http://false.com> .

$ ./test.py 
True

Maybe you're using an older version of the library? Or a newer version and a bug was introduced? I'm using 4.0.1:

$ python
Python 2.7.3 (default, Feb 27 2014, 19:58:35) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pkg_resources
>>> pkg_resources.get_distribution("rdflib").version
'4.0.1'

Upvotes: 5

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