DescIndex(column) for descending index columns, refs #260

docs/python-api.rst

@@ -1866,6 +1866,17 @@ By default the index will be named ``idx_{table-name}_{columns}`` - if you want
         index_name="good_dogs_by_age"
     )
 
+To create an index in descending order for a column, wrap the column name in ``db.DescIndex()`` like this:
+
+.. code-block:: python
+
+    from sqlite_utils.db import DescIndex
+
+    db["dogs"].create_index(
+        ["is_good_dog", DescIndex("age")],
+        index_name="good_dogs_by_age"
+    )
+
 You can create a unique index by passing ``unique=True``:
 
 .. code-block:: python

sqlite_utils/db.py

@@ -144,6 +144,10 @@ class InvalidColumns(Exception):
     pass
 
 
+class DescIndex(str):
+    pass
+
+
 _COUNTS_TABLE_CREATE_SQL = """
 CREATE TABLE IF NOT EXISTS [{}](
    [table] TEXT PRIMARY KEY,
@@ -1156,6 +1160,13 @@ def create_index(self, columns, index_name=None, unique=False, if_not_exists=Fal
             index_name = "idx_{}_{}".format(
                 self.name.replace(" ", "_"), "_".join(columns)
             )
+        columns_sql = []
+        for column in columns:
+            if isinstance(column, DescIndex):
+                fmt = "[{}] desc"
+            else:
+                fmt = "[{}]"
+            columns_sql.append(fmt.format(column))
         sql = (
             textwrap.dedent(
                 """
@@ -1167,7 +1178,7 @@ def create_index(self, columns, index_name=None, unique=False, if_not_exists=Fal
             .format(
                 index_name=index_name,
                 table_name=self.name,
-                columns=", ".join("[{}]".format(c) for c in columns),
+                columns=", ".join(columns_sql),
                 unique="UNIQUE " if unique else "",
                 if_not_exists="IF NOT EXISTS " if if_not_exists else "",
             )

tests/test_create.py

@@ -1,6 +1,7 @@
 from sqlite_utils.db import (
     Index,
     Database,
+    DescIndex,
     ForeignKey,
     AlterError,
     NoObviousTable,
@@ -739,6 +740,19 @@ def test_create_index_if_not_exists(fresh_db):
     dogs.create_index(["name"], if_not_exists=True)
 
 
+def test_create_index_desc(fresh_db):
+    dogs = fresh_db["dogs"]
+    dogs.insert({"name": "Cleo", "twitter": "cleopaws", "age": 3, "is good dog": True})
+    assert [] == dogs.indexes
+    dogs.create_index([DescIndex("age"), "name"])
+    sql = fresh_db.execute(
+        "select sql from sqlite_master where name='idx_dogs_age_name'"
+    ).fetchone()[0]
+    assert sql == (
+        "CREATE INDEX [idx_dogs_age_name]\n" "    ON [dogs] ([age] desc, [name])"
+    )
+
+
 @pytest.mark.parametrize(
     "data_structure",
     (