len throws with 'dict_keyiterator' has no len() when calculating outgoing and incoming edges in networkx

Question

I am implementing a graph manipulation script and I was puzzled with the following error:

Traceback (most recent call last):
  File ".....py", line 12, in <module>
    print(len(graph.predecessors(i)), len(graph.successors(i)))
>>TypeError: object of type 'dict_keyiterator' has no len()<<

This is the code:

import networkx as nx

graph = nx.DiGraph()

for i in range(10):
  graph.add_node(i)

for i in range(9):
  graph.add_edge(i, i+1)

for i in range(10):
  print(len(graph.predecessors(i)), len(graph.successors(i)))

What is this dict_keyiterator and how to fix my code? Thanks!

Answer 1

The issue was eventually fixable by converting the iterator to a list:

print(len(list(graph.predecessors(i))), len(list(graph.successors(i))))

Yakym Pirozhenko suggested an alternative approach deemed faster so I checked:

def f1():
  for i in range(10):
    len(list(graph.predecessors(i)))

def f2():
  for i in range(10):
    sum(1 for _ in graph.predecessors(i))

print(timeit.timeit(f1, number=100000))
print(timeit.timeit(f2, number=100000))

And got:

0.529827729
0.652576311

Clearly, the len(list(...)) approach is faster here. I was using: Python 3.7 on Windows 10.

---

After some searching I have found a question with a similar problem and a straightforward explanation here:

In 2.x iter(some_dict) returns a dictionary-keyiterator (weird hyphen). In 3.x it's a dict_keyiterator (normal underscore).

So it seems that direct use of iter(d) where d is a dict results in the object of type dict_keyiterator in Python 3. This is one of 3 iterators replacing Python 2: d.viewkeys(), d.viewitems(), and d.viewvalues():

The corresponding iterators returned by iter() in 3.x are dict_keyiterator, dict_itemiterator, and dict_valueiterator.