ArrayData: Make name optional in get_array

The `ArrayData` was designed to be able to store multiple numpy arrays.
While useful, it forced users to be more verbose than necessary when
only storing a single array as an explicit array name is always required:

    node = ArrayData()
    node.set_array('some_key', numpy.array([]))
    node.get_array('some_key')

The `get_array` method is updated to allow `None` for the `name`
argument as long as the node only stores a single array so that it can
return the correct array unambiguously. This simplifies typical user
code significantly:

    node = ArrayData(numpy.array([]))
    node.get_array()

aiida/orm/nodes/data/array/array.py

@@ -130,14 +130,28 @@ def get_iterarrays(self) -> Iterator[tuple[str, 'ndarray']]:
         for name in self.get_arraynames():
             yield (name, self.get_array(name))
 
-    def get_array(self, name: str) -> 'ndarray':
+    def get_array(self, name: str | None = None) -> 'ndarray':
         """
         Return an array stored in the node
 
-        :param name: The name of the array to return.
+        :param name: The name of the array to return. The name can be omitted in case the node contains only a single
+            array, which will be returned in that case. If ``name`` is ``None`` and the node contains multiple arrays or
+            no arrays at all a ``ValueError`` is raised.
+        :raises ValueError: If ``name`` is ``None`` and the node contains more than one arrays or no arrays at all.
         """
         import numpy
 
+        if name is None:
+            names = self.get_arraynames()
+            narrays = len(names)
+
+            if narrays == 0:
+                raise ValueError('`name` not specified but the node contains no arrays.')
+            if narrays > 1:
+                raise ValueError('`name` not specified but the node contains multiple arrays.')
+
+            name = names[0]
+
         def get_array_from_file(self, name: str) -> 'ndarray':
             """Return the array stored in a .npy file"""
             filename = f'{name}.npy'

docs/source/nitpick-exceptions

@@ -130,6 +130,7 @@ py:class disk_objectstore.utils.LazyOpener
 py:class frozenset
 
 py:class numpy.bool_
+py:class ndarray
 
 py:class paramiko.proxy.ProxyCommand
 

tests/orm/nodes/data/test_array.py

@@ -9,6 +9,7 @@
 ###########################################################################
 """Tests for the :mod:`aiida.orm.nodes.data.array.array` module."""
 import numpy
+import pytest
 
 from aiida.orm import ArrayData, load_node
 
@@ -43,3 +44,20 @@ def test_constructor():
     assert sorted(node.get_arraynames()) == ['a', 'b']
     assert (node.get_array('a') == arrays['a']).all()
     assert (node.get_array('b') == arrays['b']).all()
+
+
+def test_get_array():
+    """Test :meth:`aiida.orm.nodes.data.array.array.ArrayData:get_array`."""
+    node = ArrayData()
+    with pytest.raises(ValueError, match='`name` not specified but the node contains no arrays.'):
+        node.get_array()
+
+    node = ArrayData({'a': numpy.array([]), 'b': numpy.array([])})
+    with pytest.raises(ValueError, match='`name` not specified but the node contains multiple arrays.'):
+        node.get_array()
+
+    node = ArrayData({'a': numpy.array([1, 2])})
+    assert (node.get_array() == numpy.array([1, 2])).all()
+
+    node = ArrayData(numpy.array([1, 2]))
+    assert (node.get_array() == numpy.array([1, 2])).all()