Add functionality to remove sites and connections from a topology

gmso/core/topology.py

@@ -647,6 +647,58 @@ def get_scaling_factors(self, *, molecule_id=None):
             ]
         )
 
+    def remove_site(self, site):
+        """Remove a site from the topology.
+
+        Parameters
+        ----------
+        site : gmso.core.Site
+            The site to be removed.
+
+        Notes
+        -----
+        When a site is removed, any connections that site belonged
+        to are also removed.
+
+        See Also
+        --------
+        gmso.core.topology.Topology.iter_connections_by_site
+            The method that shows all connections belonging to a specific site
+        """
+        if site not in self._sites:
+            raise ValueError(
+                f"Site {site} is not currently part of this topology."
+            )
+        for connection in self.iter_connections_by_site(site):
+            self.remove_connection(connection)
+        self._sites.remove(site)
+
+    def remove_connection(self, connection):
+        """Remove a connection from the topology.
+
+        Parameters
+        ----------
+        connection : gmso.abc.abstract_conneciton.Connection
+            The connection to be removed from the topology
+
+        Notes
+        -----
+        The sites that belong to this connection are
+        not removed from the topology.
+        """
+        if connection not in self.connections:
+            raise ValueError(
+                f"Connection {connection} is not currently part of this topology."
+            )
+        if isinstance(connection, gmso.core.bond.Bond):
+            self._bonds.remove(connection)
+        elif isinstance(connection, gmso.core.angle.Angle):
+            self._angles.remove(connection)
+        elif isinstance(connection, gmso.core.dihedral.Dihedral):
+            self._dihedrals.remove(connection)
+        elif isinstance(connection, gmso.core.improper.Improper):
+            self._impropers.remove(connection)
+
     def set_scaling_factors(self, lj, electrostatics, *, molecule_id=None):
         """Set both lj and electrostatics scaling factors."""
         self.set_lj_scale(
@@ -831,7 +883,6 @@ def add_connection(self, connection, update_types=False):
             Improper: self._impropers,
         }
         connections_sets[type(connection)].add(connection)
-
         if update_types:
             self.update_topology()
 
@@ -1367,6 +1418,40 @@ def iter_sites_by_molecule(self, molecule_tag):
         else:
             return self.iter_sites("molecule", molecule_tag)
 
+    def iter_connections_by_site(self, site, connections=None):
+        """Iterate through this topology's connections which contain
+        this specific site.
+
+        Parameters
+        ----------
+        site : gmso.core.Site
+            Site to limit connections search to.
+        connections : set or list or tuple, optional, default=None
+            The connection types to include in the search.
+            If None, iterates through all of a site's connections.
+            Options include "bonds", "angles", "dihedrals", "impropers"
+
+        Yields
+        ------
+        gmso.abc.abstract_conneciton.Connection
+            Connection where site is in Connection.connection_members
+
+        """
+        if connections is None:
+            connections = ["bonds", "angles", "dihedrals", "impropers"]
+        else:
+            connections = set([option.lower() for option in connections])
+            for option in connections:
+                if option not in ["bonds", "angles", "dihedrals", "impropers"]:
+                    raise ValueError(
+                        "Valid connection types are limited to: "
+                        '"bonds", "angles", "dihedrals", "impropers"'
+                    )
+        for conn_str in connections:
+            for conn in getattr(self, conn_str):
+                if site in conn.connection_members:
+                    yield conn
+
     def create_subtop(self, label_type, label):
         """Create a new Topology object from a molecule or graup of the current Topology.
 

gmso/tests/test_topology.py

@@ -52,6 +52,19 @@
         top.add_site(site)
         assert top.n_sites == 1
 
+    def test_remove_site(self, ethane):
+        ethane.identify_connections()
+        for site in ethane.sites[2:]:
+            ethane.remove_site(site)
+        assert ethane.n_sites == 2
+        assert ethane.n_connections == 1
+
+    def test_remove_site_not_in_top(self, ethane):
+        top = Topology()
+        site = Atom(name="site")
+        with pytest.raises(ValueError):
+            top.remove_site(site)
+
     def test_add_connection(self):
         top = Topology()
         atom1 = Atom(name="atom1")
@@ -64,6 +77,26 @@
 
         assert len(top.connections) == 1
 
+    def test_remove_connection(self):
+        top = Topology()
+        atom1 = Atom(name="atom1")
+        atom2 = Atom(name="atom2")
+        connect = Bond(connection_members=[atom1, atom2])
+
+        top.add_connection(connect)
+        top.add_site(atom1)
+        top.add_site(atom2)
+        top.remove_connection(connect)
+        assert top.n_connections == 0
+
+    def test_remove_connection(self):
+        top = Topology()
+        atom1 = Atom(name="atom1")
+        atom2 = Atom(name="atom2")
+        connect = Bond(connection_members=[atom1, atom2])
+        with pytest.raises(ValueError):
+            top.remove_connection(connect)
+
     def test_add_box(self):
         top = Topology()
         box = Box(2 * u.nm * np.ones(3))
@@ -905,6 +938,42 @@
             for site in labeled_top.iter_sites_by_molecule(molecule_name):
                 assert site.molecule.name == molecule_name
 
+    @pytest.mark.parametrize(
+        "connections",
+        ["bonds", "angles", "dihedrals", "impropers"],
+    )
+    def test_iter_connections_by_site(self, ethane, connections):
+        type_dict = {
+            "bonds": Bond,
+            "angles": Angle,
+            "dihedrals": Dihedral,
+            "impropers": Improper,
+        }
+        ethane.identify_connections()
+        site = ethane.sites[0]
+        for conn in ethane.iter_connections_by_site(
+            site=site, connections=[connections]
+        ):
+            assert site in conn.connection_members
+            assert isinstance(conn, type_dict[connections])
+
+    def test_iter_connections_by_site_none(self, ethane):
+        ethane.identify_connections()
+        site = ethane.sites[0]
+        for conn in ethane.iter_connections_by_site(
+            site=site, connections=None
+        ):
+            assert site in conn.connection_members
+
+    def test_iter_connections_by_site_bad_param(self, ethane):
+        ethane.identify_connections()
+        site = ethane.sites[0]
+        with pytest.raises(ValueError):
+            for conn in ethane.iter_connections_by_site(
+                site=site, connections=["bond"]
+            ):
+                pass
+
     def test_write_forcefield(self, typed_water_system):
         forcefield = typed_water_system.get_forcefield()
         assert "opls_111" in forcefield.atom_types