database_ellcurves: access files through features, for simpler configuration

Author: tornaria

src/sage/features/databases.py

@@ -65,6 +65,34 @@ def __init__(self, name="cremona"):
                             description="Cremona's database of elliptic curves")
 
 
+class DatabaseEllcurves(StaticFile):
+    r"""
+    A :class:`~sage.features.Feature` which describes the presence of
+    William Stein's database of interesting curves.
+
+    EXAMPLES::
+
+        sage: from sage.features.databases import DatabaseEllcurves
+        sage: bool(DatabaseEllcurves().is_present())  # optional - database_ellcurves
+        True
+    """
+    def __init__(self):
+        r"""
+        TESTS::
+
+            sage: from sage.features.databases import DatabaseEllcurves
+            sage: isinstance(DatabaseEllcurves(), DatabaseEllcurves)
+            True
+        """
+        from sage.env import ELLCURVE_DATA_DIR
+        StaticFile.__init__(self, "database_ellcurves",
+                            filename='rank0',
+                            search_path=ELLCURVE_DATA_DIR,
+                            spkg="elliptic_curves",
+                            type="standard",
+                            description="William Stein's database of interesting curve")
+
+
 class DatabaseGraphs(StaticFile):
     r"""
     A :class:`~sage.features.Feature` which describes the presence of
@@ -211,6 +239,7 @@ def all_features():
     return [PythonModule('conway_polynomials'),
             DatabaseCremona(),
             DatabaseCremona('cremona_mini'),
+            DatabaseEllcurves(),
             DatabaseGraphs(),
             DatabaseJones(),
             DatabaseKnotInfo(),

src/sage/schemes/elliptic_curves/ec_database.py

@@ -132,8 +132,10 @@ def rank(self, rank, tors=0, n=10, labels=False):
             sage: elliptic_curves.rank(6, n=3, labels=True)
             []
         """
-        from sage.env import ELLCURVE_DATA_DIR
-        data = os.path.join(ELLCURVE_DATA_DIR, 'rank%s' % rank)
+        from sage.features.databases import DatabaseEllcurves
+        db = DatabaseEllcurves()
+        data = os.path.join(os.path.dirname(db.absolute_filename()),
+                            f'rank{rank}')
         try:
             f = open(data)
         except OSError: