add an `edges` method; the inverse of `node_pairs`

kwant/system.py

@@ -74,7 +74,7 @@ class System2(metaclass=abc.ABCMeta):
         Raises
         ------
         IndexError
-            If any element of entries > num_entries.
+            If any element of entries >= num_entries.
         """
 
     @abc.abstractmethod
@@ -98,6 +98,29 @@ class System2(metaclass=abc.ABCMeta):
             If any element of entries >= num_entries.
         """
 
+
+    @abc.abstractmethod
+    def edges(self, heads, tails):
+        """Get the edges corresponding to some hoppings.
+
+        Parameters
+        ----------
+        heads, tails: sequence of int
+            The heads and tails of the hoppings to get.
+
+        Returns
+        -------
+        Sequence of int
+            The edges corresponding to the provided hoppings
+
+        Raises
+        ------
+        IndexError
+            If any element of heads, tails >= num_nodes
+        KeyError
+            If a given hopping does not exist as an edge in the system
+        """
+
     @abc.abstractmethod
     def neighbors(self, nodes):
         """Get the neighbors of nodes and the corresponding edges.
@@ -118,7 +141,7 @@ class System2(metaclass=abc.ABCMeta):
         Raises
         ------
         IndexError
-            If any element of nodes > num_nodes.
+            If any element of nodes >= num_nodes.
         """
 
     @abc.abstractmethod
@@ -136,7 +159,7 @@ class System2(metaclass=abc.ABCMeta):
         Raises
         ------
         IndexError
-            If any element of nodes > num_nodes.
+            If any element of nodes >= num_nodes.
         """
 
     @abc.abstractmethod
@@ -209,7 +232,7 @@ class System2(metaclass=abc.ABCMeta):
         Raises
         ------
         IndexError
-            If any element of nodes > num_nodes.
+            If any element of nodes >= num_nodes.
         KeyError
             If g is not a group element.
         """