python dictionary key error when appending key in loop

Question

why am i getting a key error when trying to iterate through a list of key value pairs and count how many times the key occurs? the error i'm getting is something like KeyError: 3, which means the key doesn't exist. can i not add it like this? self.node_degree[source] += 1

class PageRank:
    def __init__(self, edge_file):

        self.node_degree = {}
        self.max_node_id = 0
        self.edge_file = edge_file

    def read_edge_file(self, edge_file):
        with open(edge_file) as f:
            for line in f:
                if line[0] != '%':
                    val = line.split()
                    yield int(val[0]), int(val[1])

    def get_max_node_id(self):
        return self.max_node_id

    def calculate_node_degree(self):

        for source,target in self.read_edge_file(self.edge_file):   
            self.node_degree[source] += 1

score 3 · Accepted Answer · answered Nov 06 '19 at 03:07

3

try this

for source,target in self.read_edge_file(self.edge_file):   
    try:
        self.node_degree[source] += 1
    except: 
        self.node_degree[source] = 1

You tried to add 1 to None coz you didn't create the key in dict if there is no such key in dict so in except creating new key of dict will do the work.

answered Nov 06 '19 at 03:07

Tserenjamts

579
6
15

I like this answer too. I use `except KeyError` to be more specific. – butterflyeffect Jun 22 '22 at 22:52

score 0 · Answer 2 · answered Nov 06 '19 at 03:11

0

Try

self.node_degree[source] = self.node_degree.get(source, 0) + 1

answered Nov 06 '19 at 03:11

Anatoliy R

1,749
2
14
20

Not sure why this got downvoted, I'd say using the `get` method is the canonical way to approach this. – wwii Nov 06 '19 at 17:22

python dictionary key error when appending key in loop

2 Answers2