Change NestedDtype.from_string to raise TypeError

Previously it was ValueError which is inconsistent with the behavior
expected by pandas.

Fixes #22

src/nested_pandas/series/dtype.py

@@ -70,17 +70,17 @@ def construct_from_string(cls, string: str) -> Self:  # type: ignore[name-define
 
         Raises
         ------
-        ValueError
+        TypeError
             If the string is not a valid nested type string or if the element types
             are parametric pyarrow types.
         """
         if not string.startswith("nested<") or not string.endswith(">"):
-            raise ValueError("Not a valid nested type string, expected 'nested<...>'")
+            raise TypeError("Not a valid nested type string, expected 'nested<...>'")
         fields_str = string.removeprefix("nested<").removesuffix(">")
 
         field_strings = fields_str.split(", ")
         if len(field_strings) == 0:
-            raise ValueError(
+            raise TypeError(
                 "Not a valid nested type string, expected at least a single field inside "
                 "'nested<x: [type], ...>'"
             )
@@ -90,12 +90,12 @@ def construct_from_string(cls, string: str) -> Self:  # type: ignore[name-define
             try:
                 field_name, field_type = field_string.split(": ", maxsplit=1)
             except ValueError as e:
-                raise ValueError(
+                raise TypeError(
                     "Not a valid nested type string, expected 'nested<x: [type], ...>', got invalid field "
                     f"string '{field_string}'"
                 ) from e
             if not field_type.startswith("[") or not field_type.endswith("]"):
-                raise ValueError(
+                raise TypeError(
                     "Not a valid nested type string, expected 'nested<x: [type], ...>', got invalid field "
                     f"type string '{field_type}'"
                 )
@@ -105,7 +105,7 @@ def construct_from_string(cls, string: str) -> Self:  # type: ignore[name-define
             try:
                 pa_value_type = pa.type_for_alias(value_type)
             except ValueError as e:
-                raise ValueError(
+                raise TypeError(
                     f"Parsing pyarrow specific parameters in the string is not supported yet: {value_type}. "
                     "Please use NestedDtype() or NestedDtype.from_fields() instead."
                 ) from e

tests/nested_pandas/series/test_accessor.py

@@ -4,6 +4,7 @@
 import pytest
 from nested_pandas import NestedDtype
 from nested_pandas.series.ext_array import NestedExtensionArray
+from nested_pandas.series.packer import pack_flat
 from numpy.testing import assert_array_equal
 from pandas.testing import assert_frame_equal, assert_series_equal
 
@@ -503,3 +504,28 @@ def test___len__():
     series = pd.Series(struct_array, dtype=NestedDtype(struct_array.type), index=[0, 1])
 
     assert len(series.nest) == 2
+
+
+def test_to_flat_dropna():
+    """Test that to_flat() gives a valid dataframe, based on GH22
+
+    https://github.com/lincc-frameworks/nested-pandas/issues/22
+    """
+
+    flat = pd.DataFrame(
+        data={"c": [0.0, 2, 4, 1, np.NaN, 3, 1, 4, 1], "d": [5, 4, 7, 5, 3, 1, 9, 3, 4]},
+        index=[0, 0, 0, 1, 1, 1, 2, 2, 2],
+    )
+    nested = pack_flat(flat, name="nested")
+
+    new_flat = nested.nest.to_flat()
+    # .dropna() was failing in the issue report
+    filtered = new_flat.dropna(subset="c")
+
+    assert_frame_equal(
+        filtered,
+        pd.DataFrame(
+            data={"c": [0.0, 2, 4, 1, 3, 1, 4, 1], "d": [5, 4, 7, 5, 1, 9, 3, 4]},
+            index=[0, 0, 0, 1, 1, 2, 2, 2],
+        ),
+    )

tests/nested_pandas/series/test_dtype.py

@@ -99,6 +99,29 @@ def test_name_vs_construct_from_string(fields):
     assert dtype == NestedDtype.construct_from_string(dtype.name)
 
 
+@pytest.mark.parametrize(
+    "s",
+    [
+        "float",  # not a nested type
+        "nested(f: [int64])",  # must be <> instead
+        "ts<in64>",  # 'ts' was a previous name, now we use 'nested'
+        "nested",  # no type specified
+        "nested<a: [int64]",  # missed closing bracket
+        "nested<>",  # no field specified
+        "nested<int64>",  # no field name specified
+        "nested<[int64]>",  # no field name specified
+        "nested<a:[int64]>",  # separator must be ": " with space
+        "nested<a: int64>",  # missed [] - nested list
+        "nested<a: [complex64]>",  # not an arrow type
+        "nested<a: [list<item: double>]>",  # complex arrow types are not supported
+    ],
+)
+def test_construct_from_string_raises(s):
+    """Test that we raise an error when constructing NestedDtype from invalid string."""
+    with pytest.raises(TypeError):
+        NestedDtype.construct_from_string(s)
+
+
 def test_construct_array_type():
     """Test that NestedDtype.construct_array_type() returns NestedExtensionArray."""
     assert NestedDtype.construct_array_type() is NestedExtensionArray