ARROW-4434: [Python] Allow creating trivial StructArray

python/pyarrow/array.pxi

@@ -1317,29 +1317,31 @@ cdef class StructArray(Array):
             ssize_t num_arrays
             ssize_t length
             ssize_t i
+            DataType struct_type
 
         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")
-
-        length = len(arrays[0])
-
-        c_arrays.resize(num_arrays)
-        for i in range(num_arrays):
-            array = arrays[i]
-            if len(array) != length:
-                raise ValueError("All arrays must have the same length")
-            c_arrays[i] = array.sp_array
-
-        cdef DataType struct_type = struct([
-            field(name, array.type)
-            for name, array in
-            zip(names, arrays)
+        if num_arrays != len(names):
+            raise ValueError("The number of names should be equal to the "
+                             "number of arrays")
+
+        if num_arrays > 0:
+            length = len(arrays[0])
+            c_arrays.resize(num_arrays)
+            for i in range(num_arrays):
+                array = arrays[i]
+                if len(array) != length:
+                    raise ValueError("All arrays must have the same length")
+                c_arrays[i] = array.sp_array
+        else:
+            length = 0
+
+        struct_type = struct([
+            field(name, array.type) for name, array in zip(names, arrays)
         ])
 
         c_result.reset(new CStructArray(struct_type.sp_type, length, c_arrays))

python/pyarrow/filesystem.py

@@ -385,11 +385,11 @@ def _ensure_filesystem(fs):
     # interface and return it
     if not issubclass(fs_type, FileSystem):
         for mro in inspect.getmro(fs_type):
-            if mro.__name__ is 'S3FileSystem':
+            if mro.__name__ == 'S3FileSystem':
                 return S3FSWrapper(fs)
             # In case its a simple LocalFileSystem (e.g. dask) use native arrow
             # FS
-            elif mro.__name__ is 'LocalFileSystem':
+            elif mro.__name__ == 'LocalFileSystem':
                 return LocalFileSystem.get_instance()
 
         raise IOError('Unrecognized filesystem: {0}'.format(fs_type))

python/pyarrow/tests/test_convert_builtin.py

@@ -867,6 +867,11 @@ def test_empty_range():
 
 
 def test_structarray():
+    arr = pa.StructArray.from_arrays([], names=[])
+    assert arr.type == pa.struct([])
+    assert len(arr) == 0
+    assert arr.to_pylist() == []
+
     ints = pa.array([None, 2, 3], type=pa.int64())
     strs = pa.array([u'a', None, u'c'], type=pa.string())
     bools = pa.array([True, False, None], type=pa.bool_())
@@ -883,6 +888,10 @@ def test_structarray():
     pylist = arr.to_pylist()
     assert pylist == expected, (pylist, expected)
 
+    # len(names) != len(arrays)
+    with pytest.raises(ValueError):
+        pa.StructArray.from_arrays([ints], ['ints', 'strs'])
+
 
 def test_struct_from_dicts():
     ty = pa.struct([pa.field('a', pa.int32()),