Add legacyMode to options and document its use.

Author: mielvds

README.md

@@ -41,6 +41,7 @@ memory footprint in order to operate.
 ## Requirements
 
 * Python (3.10 or later)
+* rdflib (7.3 or later)
 * [Requests](http://docs.python-requests.org/) (optional)
 * [aiohttp](https://aiohttp.readthedocs.io/) (optional)
 
@@ -128,6 +129,15 @@ normalized = jsonld.normalize(
 # that can be used for hashing, comparison, etc.
 ```
 
+`jsonld.to_rdf()` returns an `rdflib.Dataset` when no output `format` is
+requested. To keep the RDF.js-like dataset `dict` used by PyLD versions lower
+than 4, pass
+`{'legacyMode': True}`:
+
+```python
+dataset = jsonld.to_rdf(doc, {'legacyMode': True})
+```
+
 ## Features & conformance
 
 This library aims to conform with the following W3C Recommendations:
@@ -421,4 +431,4 @@ See [`LICENSE`](./LICENSE) file for details.
 The PyLD library is maintained by [Miel Vander
 Sande](https://github.com/mielvds), [Anatoly
 Scherbakov](https://github.com/anatoly-scherbakov) and [Digital
-Bazaar](https://github.com/digitalbazaar) (Original authors of `PyLD`).
+Bazaar](https://github.com/digitalbazaar) (Original authors of `PyLD`).

docs/reference/to_rdf.md

@@ -12,6 +12,10 @@
       show_bases: false
       heading_level: 3
 
+When `format` is not set, `jsonld.to_rdf()` returns an
+[`rdflib.Dataset`](https://rdflib.readthedocs.io/). Set `legacyMode` to `True`
+to return the RDF.js-like dataset `dict` used by PyLD versions lower than 4.
+
 ## Example
 
 {{ example('to_rdf.py') }}

lib/pyld/options.py

@@ -178,6 +178,9 @@ class ToRdfOptions(ProcessingOptions, total=False):
     rdfDirection: Literal['i18n-datatype']
     """Only `i18n-datatype` supported."""
 
+    legacyMode: bool
+    """`True` to return the RDF.js-like dataset `dict` used by PyLD versions lower than 4 instead of an `rdflib.Dataset` when `format` is not set (default: `False`)."""
+
 
 class FromRdfOptions(TypedDict, total=False):
     format: Literal['application/n-quads']