GH-44651: [Python] Allow from_buffers to work with StringView on Pyth…

…on (#44701)

### Rationale for this change

Currently `from_buffers` is not working with StringView on Python because we validate against num_buffers. This only take into account the mandatory buffers but does not take into account the variadic_spec that can be present for both string_view and binary_view

### What changes are included in this PR?

Take into account whether the type contains a variadic_spec for the non-mandatory buffers and only check lower_bound number of buffers.

### Are these changes tested?

Yes, I've added a couple of tests.

### Are there any user-facing changes?

We are exposing a new method on the Python DataType. `has_variadic_buffers` which tells us whether the number of buffers expected is only lower-bounded by num_buffers.
* GitHub Issue: #44651

Authored-by: Raúl Cumplido <[email protected]>
Signed-off-by: Raúl Cumplido <[email protected]>

python/pyarrow/array.pxi

@@ -1174,7 +1174,12 @@ cdef class Array(_PandasConvertible):
                              "({0}) did not match the passed number "
                              "({1}).".format(type.num_fields, len(children)))
 
-        if type.num_buffers != len(buffers):
+        if type.has_variadic_buffers:
+            if type.num_buffers > len(buffers):
+                raise ValueError("Type's expected number of buffers is at least "
+                                 "{0}, but the passed number is "
+                                 "{1}.".format(type.num_buffers, len(buffers)))
+        elif type.num_buffers != len(buffers):
             raise ValueError("Type's expected number of buffers "
                              "({0}) did not match the passed number "
                              "({1}).".format(type.num_buffers, len(buffers)))

python/pyarrow/includes/libarrow.pxd

@@ -158,6 +158,7 @@ cdef extern from "arrow/api.h" namespace "arrow" nogil:
 
     cdef cppclass CDataTypeLayout" arrow::DataTypeLayout":
         vector[CBufferSpec] buffers
+        optional[CBufferSpec] variadic_spec
         c_bool has_dictionary
 
     cdef cppclass CDataType" arrow::DataType":

python/pyarrow/tests/test_array.py

@@ -651,6 +651,32 @@ def test_string_binary_from_buffers():
     assert copied.null_count == 0
 
 
+def test_string_view_from_buffers():
+    array = pa.array(
+        [
+            "String longer than 12 characters",
+            None,
+            "short",
+            "Length is 12"
+        ], type=pa.string_view())
+
+    buffers = array.buffers()
+    copied = pa.StringViewArray.from_buffers(
+        pa.string_view(), len(array), buffers)
+    copied.validate(full=True)
+    assert copied.to_pylist() == [
+        "String longer than 12 characters",
+        None,
+        "short",
+        "Length is 12"
+    ]
+
+    match = r"number of buffers is at least 2"
+    with pytest.raises(ValueError, match=match):
+        pa.StringViewArray.from_buffers(
+            pa.string_view(), len(array), buffers[0:1])
+
+
 @pytest.mark.parametrize('list_type_factory', [
     pa.list_, pa.large_list, pa.list_view, pa.large_list_view])
 def test_list_from_buffers(list_type_factory):

python/pyarrow/tests/test_types.py

@@ -887,6 +887,14 @@ def test_types_weakref():
     assert wr() is None  # not a singleton
 
 
+def test_types_has_variadic_buffers():
+    for ty in get_many_types():
+        if ty in (pa.string_view(), pa.binary_view()):
+            assert ty.has_variadic_buffers
+        else:
+            assert not ty.has_variadic_buffers
+
+
 def test_fields_hashable():
     in_dict = {}
     fields = [pa.field('a', pa.int32()),

python/pyarrow/types.pxi

@@ -326,6 +326,22 @@ cdef class DataType(_Weakrefable):
         """
         return self.type.layout().buffers.size()
 
+    @property
+    def has_variadic_buffers(self):
+        """
+        If True, the number of expected buffers is only
+        lower-bounded by num_buffers.
+
+        Examples
+        --------
+        >>> import pyarrow as pa
+        >>> pa.int64().has_variadic_buffers
+        False
+        >>> pa.string_view().has_variadic_buffers
+        True
+        """
+        return self.type.layout().variadic_spec.has_value()
+
     def __str__(self):
         return frombytes(self.type.ToString(), safe=True)