recursion print error nested dict() object in python

Question

Let's make it a bit easier.
Code:

level3 = {'a':'aa'}                                                             
level2 = {'b':level3, 'd':level3}                                               
level1 = {'j':level2, 'k':level2}                                               

def print_rec(node = None):                                                 
    if node is None:                                                            
        node = level1                                                           
    if node == 'aa':                                                            
        return                                                                  
    for key, successor in node.items():                                         
        print(key,":",node.get(key))                                            
        print_rec(successor)                                                

print_rec()

Outputs:

k : {'d': {'a': 'aa'}, 'b': {'a': 'aa'}}
d : {'a': 'aa'}
a : aa
b : None
Traceback (most recent call last):
  File "test.py", line 13, in <module>
    print_rec(level1)
  File "test.py", line 11, in print_rec
    print_rec(node)
  File "test.py", line 11, in print_rec
    print_rec(node)
  File "test.py", line 8, in print_rec
    for key in node:
TypeError: 'NoneType' object is not iterable

I think node = node.get(key) will be executed only if the key is in node. So why the new node will get a NoneType? Anyone can help?

flornquake · Accepted Answer · 2013-08-27 15:19:50Z

4

In the for loop, it seems like you're using the name node for two different things:

for key in node:                                                            
    print(key,":",node.get(key))                                            
    node = node.get(key)                                                    
    print_rec(node)

When in the first iteration, you change the value of node. In the second iteration, when you do node.get(key), you're using the new node, but you want to be using the original node.

This should help:

for key in node:
    print(key,":",node.get(key))
    successor = node.get(key)
    print_rec(successor)

It can be written even more concisely like this:

for key, successor in node.items():
    print(key,":",successor)
    print_rec(successor)

edited Aug 27, 2013 at 15:19

answered Aug 27, 2013 at 15:04

flornquake

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5 Comments

Gabriel Over a year ago

Thank you, it works. Could you show me why "node = node.get(key)" will lead to an unexpected result? If the copy is shallow, can I regard the "node" as a pointer? If so, "node = node.get(key)" will be fine, I think.

flornquake Over a year ago

I've added an explanation.

Gabriel Over a year ago

Thanks. I modified it with a default argument so that when I encapsulate this method in a class, user can call it without accessing private field. I wonder if there more concise way to achieve this?

Gabriel Over a year ago

And under some situation, if recursion returns with a None, my way may lead to an infinite loop, I think.

flornquake Over a year ago

I don't understand what you're asking. If you want help on this, simply create a new question :)

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