ENH: advanced articles don't have volumes, remove required status from those fields

src/citeable/_entries.py

@@ -155,7 +155,7 @@ class Article(CitationBase):
     """An ``@article`` BibTeX entry."""
 
     journal: str
-    volume: int
+    volume: int | None
     pages: str | None
     article_number: str | None
     number: int | None
@@ -166,8 +166,8 @@ def __init__(
         title: str,
         year: int,
         journal: str,
-        volume: int,
         *,
+        volume: int | None = None,
         pages: str | None = None,
         article_number: str | None = None,
         number: int | None = None,
@@ -178,10 +178,6 @@ def __init__(
         app: str | None = None,
     ) -> None:
         require_field(journal, "journal", "Article")
-        require_field(volume, "volume", "Article")
-        if pages is None and article_number is None:
-            msg = "Article requires 'pages' or 'article_number'; both are None"
-            raise ValueError(msg)
 
         self._init_base(
             author, title, year, doi=doi, url=url, note=note, key=key, app=app
@@ -199,8 +195,9 @@ def __str__(self) -> str:
             _format_bibtex_field("title", self.title),
             _format_bibtex_field("journal", self.journal),
             _format_bibtex_field("year", str(self.year)),
-            _format_bibtex_field("volume", str(self.volume)),
         ]
+        if self.volume is not None:
+            lines.append(_format_bibtex_field("volume", str(self.volume)))
         if self.number is not None:
             lines.append(_format_bibtex_field("number", str(self.number)))
         if self.pages is not None:
@@ -217,8 +214,9 @@ def _repr_fields(self) -> list[tuple[str, object]]:
             ("title", self.title),
             ("year", self.year),
             ("journal", self.journal),
-            ("volume", self.volume),
         ]
+        if self.volume is not None:
+            fields.append(("volume", self.volume))
         if self.pages is not None:
             fields.append(("pages", self.pages))
         if self.article_number is not None:

tests/test_entries.py

@@ -44,27 +44,17 @@ def test_article_with_article_number():
     assert a.pages is None
 
 
-def test_article_missing_pages_and_article_number():
-    with pytest.raises(ValueError, match=r"pages.*article_number"):
-        Article(
-            author=["Smith, Jane"],
-            title="Something",
-            year=2024,
-            journal="Nature",
-            volume=1,
-        )
-
-
-def test_article_missing_volume():
-    with pytest.raises(ValueError, match="Article requires 'volume'"):
-        Article(
-            author=["Smith, Jane"],
-            title="Something",
-            year=2024,
-            journal="Nature",
-            volume=None,  # type: ignore[arg-type]
-            pages="1-10",
-        )
+def test_article_advance_access():
+    a = Article(
+        author=["Smith, Jane"],
+        title="Something",
+        year=2024,
+        journal="Nature",
+        doi="10.1234/example",
+    )
+    assert a.volume is None
+    assert a.pages is None
+    assert a.article_number is None
 
 
 def test_article_missing_journal():