pyArango - create edge with specified _key

I have a graph in ArangoDB. How to create edge with specified _key value? My code:

edge_attributes = {"_key": "ab", "count": 0}
graph.createEdge(collection_edges_name, node_from_id, node_to_id, edge_attributes)

I can see proper values of count as well as _from and _to, but _key is some random number.

How I can create an edge with a specific _key? I want to specify key to fast querying edges by key as well as preventing multiple edges from node A to node B.

I prepared a workaround for this problem. I create an instance of Edge class with a specified name of the collection with edges and then I call:

edge_attributes = {"_key": edge_key,
                   "_from": parent_id,
                   "_to": node_to_id,
                   "count": 0}
edge = my_edges_collection.createDocument(edge_attributes)
edge.save()

This solution creates a document with the proper key and id.