feat: support casting string to integer or float (#59)

Author: TrevorBergeron

bigframes/dtypes.py

@@ -313,7 +313,7 @@ def cast_ibis_value(
             ibis_dtypes.string,
         ),
         ibis_dtypes.float64: (ibis_dtypes.string, ibis_dtypes.int64),
-        ibis_dtypes.string: (),
+        ibis_dtypes.string: (ibis_dtypes.int64, ibis_dtypes.float64),
         ibis_dtypes.date: (),
         ibis_dtypes.time: (),
         ibis_dtypes.timestamp: (ibis_dtypes.Timestamp(timezone="UTC"),),

tests/system/small/test_series.py

@@ -2389,6 +2389,29 @@ def test_astype(scalars_df_index, scalars_pandas_df_index, column, to_type):
     pd.testing.assert_series_equal(bf_result, pd_result)
 
 
+def test_string_astype_int():
+    pd_series = pd.Series(["4", "-7", "0", "    -03"])
+    bf_series = series.Series(pd_series)
+
+    pd_result = pd_series.astype("Int64")
+    bf_result = bf_series.astype("Int64").to_pandas()
+
+    pd.testing.assert_series_equal(bf_result, pd_result, check_index_type=False)
+
+
+def test_string_astype_float():
+    pd_series = pd.Series(
+        ["1", "-1", "-0", "000", "    -03.235", "naN", "-inf", "INf", ".33", "7.235e-8"]
+    )
+
+    bf_series = series.Series(pd_series)
+
+    pd_result = pd_series.astype("Float64")
+    bf_result = bf_series.astype("Float64").to_pandas()
+
+    pd.testing.assert_series_equal(bf_result, pd_result, check_index_type=False)
+
+
 @pytest.mark.parametrize(
     "index",
     [0, 5, -2],