python-How to solve KeyError: 2?

python dictionary keyerror
jinglei picture jinglei · Feb 14, 2016 · Viewed 29.4k times · Source

I have a dict which is formatted as {int:[]}

When I was trying to set value to a key-value pair where the value list is NULL, I got KeyError: 2

tags = {}
tags.setdefault(int,[])
for tag_id in (db.session.query(PostTagRel).filter(PostTagRel.id == post_id).first().tag_id.split(',')):
            tag = db.session.query(Tag).filter(Tag.tag_id == tag_id).first().tag_name
            tags[post_id].append(tag)

What should I do?

Answer

schwobaseggl picture schwobaseggl · Feb 14, 2016

In order to set a general default value for all keys, you can use defaultdict:

from collections import defaultdict

d = defaultdict(list)    
d[0].append(1)

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