Accessing DeclaredNamespace keywords with dict style from rebound variable causes mypy failure

[ajnelson-nist]

I have a variable-spelling pattern for rdflib.Namespace objects, prefixing them with NS_. To keep things consistent between namespaces custom to the script I'm working on, as well as with built-in rdflib namespaces, I tend to have this in my scripts' module-level variables area:

from rdflib import RDFS
NS_RDFS = RDFS

This hits a weird-looking error from mypy if I need to use a dictionary spelling for accessing a namespace member. While not common, SHACL happens to include a few namespace members whose suffixes are Python keywords, sh:class and sh:not among them.

Here is a shell transcript of a set of Python programs, with comments indicating what fails, and the mypy report across all of them:

$ mypy --version
mypy 1.3.0 (compiled: yes)


$ head -n 20 try_*
==> try_1.py <==
import rdflib
# This works:
rdflib.DCTERMS["title"]
# And this works:
rdflib.SH["not"]

==> try_2.py <==
from rdflib import SH
# This works:
SH["not"]

==> try_3.py <==
import rdflib
# This works:
rdflib.Namespace(str(rdflib.SH))["not"]

==> try_4.py <==
import rdflib
NS_DCTERMS = rdflib.DCTERMS
NS_SH = rdflib.SH
# This works:
NS_DCTERMS.title
# But this fails:
NS_DCTERMS["title"]
# This works:
NS_SH.alternativePath
# But this fails:
NS_SH["alternativePath"]
# And this fails:
NS_SH["not"]


$ mypy try_*
try_4.py:7: error: Type application targets a non-generic function or class  [misc]
try_4.py:7: error: Name "title" is not defined  [name-defined]
try_4.py:11: error: Type application targets a non-generic function or class  [misc]
try_4.py:11: error: Name "alternativePath" is not defined  [name-defined]
try_4.py:13: error: Type application targets a non-generic function or class  [misc]
try_4.py:13: error: Invalid type comment or annotation  [valid-type]
Found 6 errors in 1 file (checked 4 source files)

It's only try_4.py that has errors. try_3.py demonstrates a workaround by stripping out the DefinedNamespace functionality. For my purposes, that prevents me typo'ing the namespace IRI, but now I lose typo-checking of the namespace members.

Is this an RDFLib issue, or a Mypy issue?

[aucampia]

I think this is an RDFLib issue, I think the problem is that this is not annotated with @classmethod:

rdflib/rdflib/namespace/__init__.py

Lines 243 to 262 in f278b86

	@lru_cache(maxsize=None)
	def __getitem__(cls, name: str, default=None) -> URIRef:
	name = str(name)
	if name in _DFNS_RESERVED_ATTRS:
	raise AttributeError(
	f"DefinedNamespace like object has no attribute {name!r}"
	)
	if str(name).startswith("__"):
	# NOTE on type ignore: This seems to be a real bug, super() does not
	# implement this method, it will fail if it is ever reached.
	return super().__getitem__(name, default) # type: ignore[misc] # undefined in superclass
	if (cls._warn or cls._fail) and name not in cls:
	if cls._fail:
	raise AttributeError(f"term '{name}' not in namespace '{cls._NS}'")
	else:
	warnings.warn(
	f"Code: {name} is not defined in namespace {cls.__name__}",
	stacklevel=3,
	)
	return cls._NS[name]

Though I could be wrong.

It is a bit odd that rdflib.DCTERMS["title"] does not raise a type error.