.extract() and .lookup() no longer extract null values, closes #186

- table.extract() and the sqlite-utils extract command now skip rows
  where every extracted column is null: the new foreign key column is
  left null and no all-null record is added to the lookup table. Rows
  with at least one non-null extracted column are extracted as before.
- The extracts= option to insert() and friends keeps None values as
  null instead of creating a lookup record for them - previously each
  insert batch added a duplicate null row to the lookup table.
- table.lookup() compares values using IS rather than = so lookup
  values containing None match existing rows correctly, instead of
  inserting a duplicate row on every call.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/test_extract.py

@@ -189,9 +189,85 @@ def test_extract_works_with_null_values(fresh_db):
     )
     assert list(fresh_db["listens"].rows) == [
         {"id": 1, "track_title": "foo", "album_id": 1},
-        {"id": 2, "track_title": "baz", "album_id": 2},
+        {"id": 2, "track_title": "baz", "album_id": None},
     ]
     assert list(fresh_db["albums"].rows) == [
         {"id": 1, "album_title": "bar"},
-        {"id": 2, "album_title": None},
+    ]
+
+
+def test_extract_null_values_single_column(fresh_db):
+    # https://github.com/simonw/sqlite-utils/issues/186
+    fresh_db["species"].insert({"id": 1, "species": "Wolf"}, pk="id")
+    fresh_db["individuals"].insert_all(
+        [
+            {"id": 10, "name": "Terriana", "species": "Fox"},
+            {"id": 11, "name": "Spenidorm", "species": None},
+            {"id": 12, "name": "Grantheim", "species": "Wolf"},
+            {"id": 13, "name": "Turnutopia", "species": None},
+            {"id": 14, "name": "Wargal", "species": "Wolf"},
+        ],
+        pk="id",
+    )
+    fresh_db["individuals"].extract("species")
+    # No null row should have been added to species
+    assert list(fresh_db["species"].rows) == [
+        {"id": 1, "species": "Wolf"},
+        {"id": 2, "species": "Fox"},
+    ]
+    assert list(fresh_db["individuals"].rows) == [
+        {"id": 10, "name": "Terriana", "species_id": 2},
+        {"id": 11, "name": "Spenidorm", "species_id": None},
+        {"id": 12, "name": "Grantheim", "species_id": 1},
+        {"id": 13, "name": "Turnutopia", "species_id": None},
+        {"id": 14, "name": "Wargal", "species_id": 1},
+    ]
+
+
+def test_extract_null_values_multiple_columns(fresh_db):
+    # A row should be extracted if at least one column is not null -
+    # only rows where ALL extracted columns are null are left alone
+    fresh_db["circulation"].insert_all(
+        [
+            {"id": 1, "title": "title one", "creator": "creator one", "year": 2018},
+            {"id": 2, "title": "title two", "creator": None, "year": 2019},
+            {"id": 3, "title": None, "creator": None, "year": 2020},
+            {"id": 4, "title": None, "creator": None, "year": 2021},
+        ],
+        pk="id",
+    )
+    fresh_db["circulation"].extract(
+        ["title", "creator"], table="books", fk_column="book_id"
+    )
+    assert list(fresh_db["books"].rows) == [
+        {"id": 1, "title": "title one", "creator": "creator one"},
+        {"id": 2, "title": "title two", "creator": None},
+    ]
+    assert list(fresh_db["circulation"].rows) == [
+        {"id": 1, "book_id": 1, "year": 2018},
+        {"id": 2, "book_id": 2, "year": 2019},
+        {"id": 3, "book_id": None, "year": 2020},
+        {"id": 4, "book_id": None, "year": 2021},
+    ]
+
+
+def test_extract_null_values_existing_lookup_table_with_null_row(fresh_db):
+    # Even if the lookup table already contains an all-null row, rows where
+    # every extracted column is null should keep a null foreign key
+    fresh_db["species"].insert({"id": 1, "species": None}, pk="id")
+    fresh_db["individuals"].insert_all(
+        [
+            {"id": 10, "name": "Terriana", "species": "Fox"},
+            {"id": 11, "name": "Spenidorm", "species": None},
+        ],
+        pk="id",
+    )
+    fresh_db["individuals"].extract("species")
+    assert list(fresh_db["species"].rows) == [
+        {"id": 1, "species": None},
+        {"id": 2, "species": "Fox"},
+    ]
+    assert list(fresh_db["individuals"].rows) == [
+        {"id": 10, "name": "Terriana", "species_id": 2},
+        {"id": 11, "name": "Spenidorm", "species_id": None},
     ]