TST: new test for str to small float conversion dtype (#29372)

* add str to small float conversion type test

* restrucutre test

Author: ganevgv

pandas/tests/frame/test_dtypes.py

@@ -1063,6 +1063,18 @@ def test_asarray_homogenous(self):
         expected = np.array([[1, 1], [2, 2]], dtype="object")
         tm.assert_numpy_array_equal(result, expected)
 
+    def test_str_to_small_float_conversion_type(self):
+        # GH 20388
+        np.random.seed(13)
+        col_data = [str(np.random.random() * 1e-12) for _ in range(5)]
+        result = pd.DataFrame(col_data, columns=["A"])
+        expected = pd.DataFrame(col_data, columns=["A"], dtype=object)
+        tm.assert_frame_equal(result, expected)
+        # change the dtype of the elements from object to float one by one
+        result.loc[result.index, "A"] = [float(x) for x in col_data]
+        expected = pd.DataFrame(col_data, columns=["A"], dtype=float)
+        tm.assert_frame_equal(result, expected)
+
 
 class TestDataFrameDatetimeWithTZ:
     def test_interleave(self, timezone_frame):