Unique API for single/multi-patch Domain objects (#173)

In order to improve the compatibility between single and multi-patch
domains, we add:

- `subdomains` and `mappings` attributes to the `Domain` class, which
always return a tuple (fixes #171);
- the possibility of calling the `join` function (which usually creates
multipatch domains) with a single patch.

We also increase the library version to 0.19.1.

---------

Co-authored-by: Yaman Güçlü <yaman.guclu@gmail.com>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name            = "sympde"
-version         = "0.19.0"
+version         = "0.19.1"
 description     = "Symbolic calculus for partial differential equations (and variational forms)"
 readme          = "README.rst"
 requires-python = ">= 3.8, < 3.13"

sympde/topology/domain.py

@@ -6,7 +6,7 @@
 import yaml
 import os
 
-from collections import abc
+from collections import abc, OrderedDict
 from typing import Union as TypeUnion, Optional, List, Dict, Iterable, TYPE_CHECKING
 # Union clashes with core.basic.Union
 
@@ -180,6 +180,20 @@ def mapping(self) -> Optional[Mapping]:
         """The mapping that maps the logical domain to the physical domain"""
         return self.args[3]
 
+    @property
+    def subdomains(self) -> tuple:
+        """returns subdomains as tuple of Domains"""
+        if isinstance( self.interior, iterable_types):
+            subs = self.interior
+        else:
+            subs = [self.interior]
+        return tuple(subs)
+
+    @property
+    def mappings(self) -> OrderedDict:
+        return OrderedDict([(P.logical_domain, P.mapping)
+                           for P in self.subdomains])
+
     @property
     def logical_domain(self) -> Domain:
         """The domain is the image of the logical_domain under the mapping"""
@@ -481,6 +495,11 @@ def join(cls, patches, connectivity, name):
         assert isinstance(connectivity, (tuple, list))
         assert isinstance(name, str)
 
+        if len(patches) == 1:
+            # single patch domain: return the patch
+            assert len(connectivity) == 0
+            return patches[0]
+
         assert all(p.dim==patches[0].dim for p in patches)
         dim = int(patches[0].dim)