Add first part of metapath search

References #101 and #102

src/pybel_tools/selection/metapaths.py

@@ -0,0 +1,26 @@
+# -*- coding: utf-8 -*-
+
+"""
+
+A metapath can be defined with two levels of granularity:
+
+- Low: A list of BEL functions representing the types of entities in a given path
+- High: An alternating list of BEL functions and BEL relations representing the types of entities in a given path and
+  their relations
+
+"""
+
+from pybel.constants import FUNCTION
+
+
+def convert_path_to_metapath(graph, nodes):
+    """Converts a list of nodes to their corresponding functions
+
+    :param list[tuple] nodes: A list of BEL node tuples
+    :rtype: list[str]
+    """
+    return [
+        graph.node[node][FUNCTION]
+        for node in nodes
+    ]
+