ARROW-1706: [Python] Coerce array inputs to StructArray.from_arrays. Flip order of arguments

I flipped the argument order to be more consistent with the same methods in RecordBatch, Table. The StructArray method doesn't seem to be widely used so I'm not sure there's the need to go through a deprecation cycle

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #1512 from wesm/ARROW-1706 and squashes the following commits:

786c37d [Wes McKinney] Raise error when names is None in StructArray.from_arrays
990dda5 [Wes McKinney] Fix API change
2053d94 [Wes McKinney] Add test case
9c22949 [Wes McKinney] Flip order of arguments to StructArray.from_arrays, try to coerce non-pyarrow data to Array

Author: wesm

python/pyarrow/array.pxi

@@ -828,8 +828,23 @@ cdef class DictionaryArray(Array):
 
 
 cdef class StructArray(Array):
+
     @staticmethod
-    def from_arrays(field_names, arrays):
+    def from_arrays(arrays, names=None):
+        """
+        Construct StructArray from collection of arrays representing each field
+        in the struct
+
+        Parameters
+        ----------
+        arrays : sequence of Array
+        names : List[str]
+            Field names
+
+        Returns
+        -------
+        result : StructArray
+        """
         cdef:
             Array array
             shared_ptr[CArray] c_array
@@ -839,6 +854,11 @@ cdef class StructArray(Array):
             ssize_t length
             ssize_t i
 
+        if names is None:
+            raise ValueError('Names are currently required')
+
+        arrays = [asarray(x) for x in arrays]
+
         num_arrays = len(arrays)
         if num_arrays == 0:
             raise ValueError("arrays list is empty")
@@ -855,7 +875,7 @@ cdef class StructArray(Array):
         cdef DataType struct_type = struct([
             field(name, array.type)
             for name, array in
-            zip(field_names, arrays)
+            zip(names, arrays)
         ])
 
         c_result.reset(new CStructArray(struct_type.sp_type, length, c_arrays))

python/pyarrow/tests/test_convert_builtin.py

@@ -493,8 +493,8 @@ def test_structarray():
     strs = pa.array([u'a', None, u'c'], type=pa.string())
     bools = pa.array([True, False, None], type=pa.bool_())
     arr = pa.StructArray.from_arrays(
-        ['ints', 'strs', 'bools'],
-        [ints, strs, bools])
+        [ints, strs, bools],
+        ['ints', 'strs', 'bools'])
 
     expected = [
         {'ints': None, 'strs': u'a', 'bools': True},
@@ -529,3 +529,27 @@ def test_struct_from_dicts():
                 {'a': None, 'b': None, 'c': None},
                 {'a': None, 'b': 'bar', 'c': None}]
     assert arr.to_pylist() == expected
+
+
+def test_structarray_from_arrays_coerce():
+    # ARROW-1706
+    ints = [None, 2, 3]
+    strs = [u'a', None, u'c']
+    bools = [True, False, None]
+    ints_nonnull = [1, 2, 3]
+
+    arrays = [ints, strs, bools, ints_nonnull]
+    result = pa.StructArray.from_arrays(arrays,
+                                        ['ints', 'strs', 'bools',
+                                         'int_nonnull'])
+    expected = pa.StructArray.from_arrays(
+        [pa.array(ints, type='int64'),
+         pa.array(strs, type='utf8'),
+         pa.array(bools),
+         pa.array(ints_nonnull, type='int64')],
+        ['ints', 'strs', 'bools', 'int_nonnull'])
+
+    with pytest.raises(ValueError):
+        pa.StructArray.from_arrays(arrays)
+
+    assert result.equals(expected)

python/pyarrow/tests/test_convert_pandas.py

@@ -1154,8 +1154,8 @@ def test_structarray(self):
         strs = pa.array([u'a', None, u'c'], type=pa.string())
         bools = pa.array([True, False, None], type=pa.bool_())
         arr = pa.StructArray.from_arrays(
-            ['ints', 'strs', 'bools'],
-            [ints, strs, bools])
+            [ints, strs, bools],
+            ['ints', 'strs', 'bools'])
 
         expected = pd.Series([
             {'ints': None, 'strs': u'a', 'bools': True},