sqlite-utils convert command and db[table].convert(...) method

Closes #251, closes #302.

tests/test_convert.py

@@ -0,0 +1,77 @@
+from sqlite_utils.db import BadMultiValues
+import pytest
+
+
+@pytest.mark.parametrize(
+    "columns,fn,expected",
+    (
+        (
+            "title",
+            lambda value: value.upper(),
+            {"title": "MIXED CASE", "abstract": "Abstract"},
+        ),
+        (
+            ["title", "abstract"],
+            lambda value: value.upper(),
+            {"title": "MIXED CASE", "abstract": "ABSTRACT"},
+        ),
+    ),
+)
+def test_convert(fresh_db, columns, fn, expected):
+    table = fresh_db["table"]
+    table.insert({"title": "Mixed Case", "abstract": "Abstract"})
+    table.convert(columns, fn)
+    assert list(table.rows) == [expected]
+
+
+@pytest.mark.parametrize(
+    "drop,expected",
+    (
+        (False, {"title": "Mixed Case", "other": "MIXED CASE"}),
+        (True, {"other": "MIXED CASE"}),
+    ),
+)
+def test_convert_output(fresh_db, drop, expected):
+    table = fresh_db["table"]
+    table.insert({"title": "Mixed Case"})
+    table.convert("title", lambda v: v.upper(), output="other", drop=drop)
+    assert list(table.rows) == [expected]
+
+
+def test_convert_output_multiple_column_error(fresh_db):
+    table = fresh_db["table"]
+    with pytest.raises(AssertionError) as excinfo:
+        table.convert(["title", "other"], lambda v: v, output="out")
+        assert "output= can only be used with a single column" in str(excinfo.value)
+
+
+@pytest.mark.parametrize(
+    "type,expected",
+    (
+        (int, {"other": 123}),
+        (float, {"other": 123.0}),
+    ),
+)
+def test_convert_output_type(fresh_db, type, expected):
+    table = fresh_db["table"]
+    table.insert({"number": "123"})
+    table.convert("number", lambda v: v, output="other", output_type=type, drop=True)
+    assert list(table.rows) == [expected]
+
+
+def test_convert_multi(fresh_db):
+    table = fresh_db["table"]
+    table.insert({"title": "Mixed Case"})
+    table.convert(
+        "title", lambda v: {"upper": v.upper(), "lower": v.lower()}, multi=True
+    )
+    assert list(table.rows) == [
+        {"title": "Mixed Case", "upper": "MIXED CASE", "lower": "mixed case"}
+    ]
+
+
+def test_convert_multi_exception(fresh_db):
+    table = fresh_db["table"]
+    table.insert({"title": "Mixed Case"})
+    with pytest.raises(BadMultiValues):
+        table.convert("title", lambda v: v.upper(), multi=True)