Literal equality fails for datatypes `None` and `XSD.string`

[jfschaefer]

Example:

>>> from rdflib import Literal
>>> from rdflib.namespace import XSD
>>> Literal('foo') == Literal('foo', datatype=XSD.string)
False

According to https://www.w3.org/TR/rdf11-concepts/#section-Graph-Literal, every literal has a datatype (and a simple literal without a datatype should be considered syntactic sugar for literals of datatype http://www.w3.org/2001/XMLSchema#string).

The implementation in rdflib seems to follow the original RDF (not RDF 1.1), which allowed missing datatypes.

Related issues:

• Literal should always have a datatype #1326 proposes to always set a datatype (which would work, but might break things).

PS:
A similar problem would presumably occur with RDF.langString, except that setting both lang and datatype results in an error (which might also be an issue).

[aucampia]

This is a bug, but fixing it without a major version bump may be somewhat controversial.

[aucampia]

One option to solve this is to enforce that rdflib.terms.Literal always has a datatype, but then we won't be able to support RDF 1.0 anymore. I'm somewhat okay with this, I think it would be nice to be able to support 1.0 and 1.1 - but I think 1.1. support is more important.

[aucampia]

I think the best option is to just not allow null literals.