Allow multiple sub-section of the same kind in metainfo.

nomad/metainfo/metainfo.py

@@ -886,16 +886,37 @@ class MSection(metaclass=MObjectMeta):
         """ Returns the number of sub sections for the given sub section definition. """
         return self.m_data.m_sub_section_count(self, sub_section_def)
 
-    def m_create(self, section_cls: Type[MSectionBound], **kwargs) -> MSectionBound:
+    def m_create(
+            self, section_cls: Type[MSectionBound], sub_section_def: 'SubSection' = None,
+            **kwargs) -> MSectionBound:
         """ Creates a section instance and adds it to this section provided there is a
         corresponding sub section.
+
+        Args:
+            section_cls: The section class for the sub-secton to create
+            sub_section_def: If there are multiple sub-sections for the given class,
+                this must be used to explicitely state the sub-section definition.
         """
 
         section_def = section_cls.m_def
-        sub_section_def = self.m_def.all_sub_sections_by_section.get(section_def, None)
-        if sub_section_def is None:
+        sub_section_defs = self.m_def.all_sub_sections_by_section.get(section_def, [])
+        n_sub_section_defs = len(sub_section_defs)
+        if n_sub_section_defs == 0:
             raise TypeError('There is no sub section to hold a %s in %s.' % (section_def, self.m_def))
 
+        if n_sub_section_defs > 1 and sub_section_def is None:
+            raise MetainfoError(
+                'There are multiple sub section to hold a %s in %s, '
+                'but no sub-section was explicitely given.' % (section_def, self.m_def))
+
+        if sub_section_def is not None and sub_section_def not in sub_section_defs:
+            raise MetainfoError(
+                'The given sub-section class %s does not match the given sub-section '
+                'definition %s.' % (section_cls, sub_section_def))
+
+        if sub_section_def is None:
+            sub_section_def = sub_section_defs[0]
+
         sub_section = section_cls(**kwargs)
         self.m_add_sub_section(sub_section_def, sub_section)
 
@@ -1620,7 +1641,7 @@ class Section(Definition):
         all_sub_sections_by_section:
             A helper property that gives all sub-section definition including inherited ones
             as a dictionary that maps section classes (i.e. Python class objects) to
-            :class:`SubSection`.
+            lists of :class:`SubSection`.
     """
 
     section_cls: Type[MSection] = None
@@ -1640,7 +1661,7 @@ class Section(Definition):
         self.all_properties: Dict[str, Union['SubSection', Quantity]] = dict()
         self.all_quantities: Dict[str, Quantity] = dict()
         self.all_sub_sections: Dict[str, SubSection] = dict()
-        self.all_sub_sections_by_section: Dict['Section', 'SubSection'] = dict()
+        self.all_sub_sections_by_section: Dict['Section', List['SubSection']] = dict()
 
     def on_add_sub_section(self, sub_section_def, sub_section):
         if sub_section_def == Section.quantities:
@@ -1650,7 +1671,8 @@ class Section(Definition):
         if sub_section_def == Section.sub_sections:
             self.all_properties[sub_section.name] = sub_section
             self.all_sub_sections[sub_section.name] = sub_section
-            self.all_sub_sections_by_section[sub_section.sub_section] = sub_section
+            self.all_sub_sections_by_section.setdefault(
+                sub_section.sub_section, []).append(sub_section)
 
     def on_set(self, quantity_def, value):
         if quantity_def == Section.base_sections:
@@ -1677,13 +1699,6 @@ class Section(Definition):
                 assert definition.name not in names, 'All names in a section must be unique.'
                 names.add(definition.name)
 
-    def c_unique_sub_sections(self):
-        sub_sections = set()
-        for sub_section in self.all_sub_sections.values():
-            assert sub_section.sub_section not in sub_sections, \
-                'The same section definition can only be used in one sub-section'
-            sub_sections.add(sub_section.sub_section)
-
 
 class Package(Definition):
     """ Packages organize metainfo defintions alongside Python modules

tests/test_metainfo.py

@@ -118,7 +118,8 @@ class TestM2:
         assert len(Run.m_def.sub_sections) == 3
         assert Run.m_def.all_sub_sections['systems'] in Run.m_def.sub_sections
         assert Run.m_def.all_sub_sections['systems'].sub_section == System.m_def
-        assert Run.m_def.all_sub_sections_by_section[System.m_def].sub_section == System.m_def
+        assert len(Run.m_def.all_sub_sections_by_section[System.m_def]) == 1
+        assert Run.m_def.all_sub_sections_by_section[System.m_def][0].sub_section == System.m_def
 
     def test_properties(self):
         assert len(Run.m_def.all_properties) == 6
@@ -184,11 +185,12 @@ class TestM2:
                 m_def = Section(extends_base_section=True)
                 name = Quantity(type=int)
 
-    def test_unique_sub_sections(self):
-        with assert_exception(MetainfoError):
-            class TestSection(MSection):  # pylint: disable=unused-variable
-                one = SubSection(sub_section=System)
-                two = SubSection(sub_section=System)
+    def test_multiple_sub_sections(self):
+        class TestSection(MSection):  # pylint: disable=unused-variable
+            one = SubSection(sub_section=System)
+            two = SubSection(sub_section=System)
+
+        assert len(TestSection.m_def.all_sub_sections_by_section[System.m_def]) == 2
 
     def test_dimension_exists(self):
         with assert_exception(MetainfoError):
@@ -246,13 +248,9 @@ class TestM1:
     def test_defaults(self):
         assert len(System().periodic_dimensions) == 3
         assert System().atom_labels is None
-        try:
+
+        with assert_exception(AttributeError):
             System().does_not_exist
-            assert False, 'Supposed unreachable'
-        except AttributeError:
-            pass
-        else:
-            assert False, 'Expected AttributeError'
 
     def test_m_section(self):
         assert Run().m_def == Run.m_def
@@ -279,31 +277,16 @@ class TestM1:
         assert parsing.m_parent_index == -1
 
     def test_wrong_type(self):
-        try:
+        with assert_exception(TypeError):
             Run().code_name = 1
-            assert False, 'Supposed unreachable'
-        except TypeError:
-            pass
-        else:
-            assert False, 'Expected TypeError'
 
     def test_wrong_shape_1(self):
-        try:
+        with assert_exception(TypeError):
             Run().code_name = ['name']
-            assert False, 'Supposed unreachable'
-        except TypeError:
-            pass
-        else:
-            assert False, 'Expected TypeError'
 
     def test_wrong_shape_2(self):
-        try:
+        with assert_exception(TypeError):
             System().atom_labels = 'label'
-            assert False, 'Supposed unreachable'
-        except TypeError:
-            pass
-        else:
-            assert False, 'Expected TypeError'
 
     def test_np(self):
         system = System()
@@ -353,13 +336,8 @@ class TestM1:
     def test_derived(self):
         system = System()
 
-        try:
+        with assert_exception(DeriveError):
             assert system.n_atoms == 3
-            assert False, 'supposed unreachable'
-        except DeriveError:
-            pass
-        else:
-            assert False, 'supposed unreachable'
 
         system.atom_labels = ['H', 'H', 'O']
         assert system.n_atoms == 3
@@ -389,3 +367,16 @@ class TestM1:
         system.atom_labels = ['H']
         system.atom_positions = []
         assert len(system.m_validate()) > 0
+
+    def test_multiple_sub_sections(self):
+        class TestSection(MSection):  # pylint: disable=unused-variable
+            one = SubSection(sub_section=System)
+            two = SubSection(sub_section=System)
+
+        test_section = TestSection()
+        with assert_exception():
+            test_section.m_create(System)
+
+        test_section.m_create(System, TestSection.one)
+
+        assert test_section.one is not None