Python: return a specific value while keys not defined

Question

I have a dict h, the keys are int numbers:

from collections import defaultdict

h = defaultdict(dict)
h[1]=['chr1','12','20','1']
h[2]=['chr1','15','20','2']
print(h)

defaultdict(<class 'dict'>, {1: ['chr1', '12', '20', '1'], 2: ['chr1', '15', '20', '2']})

it works:

for i in range(1, 3):
    print(h[i][3])

1
2

but it's possible there are keys which were not defined and have no values, in which case, I want h[i][3] returns value 0. e.g. if I run:

for i in range(1, 5):
    print(h[i][3])

although I have used from collections import defaultdict, the result shows:

1
2
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-45-212d01eedb83> in <module>
      1 for i in range(1, 5):
----> 2     print(h[i][3])

KeyError: 3

My question is, how could I make h[i][3] returns 0 when this i is not defined and then the result should be:

Possible duplicate? [Return None if Dictionary key is not available](https://stackoverflow.com/questions/6130768/return-none-if-dictionary-key-is-not-available) — Eric, Apr 21 '20 at 16:28

score 1 · Answer 1 · answered Apr 21 '20 at 16:28

1

Use a try except.

try:
    for i in range(1, 5):
    print(h[i][3])
except KeyError as e:
    print('Key error - %s' % str(e))
    print(0)

answered Apr 21 '20 at 16:28

Sri

2,281
2
17
24

Swazy · Answer 2 · 2020-04-21T17:01:42.400

1

You could use h.get(i,[0]*4)[3]. Should do the trick. Although this assumes you won't index higher than 3. If you index higher you can return a longer array of zeros within the get method.

edited Apr 21 '20 at 17:01

answered Apr 21 '20 at 16:29

Swazy

378
3
10

score 1 · Accepted Answer · answered Apr 21 '20 at 16:37

The problem is that your values in the dictionary are lists.

So for h[i] defaultdict might help you if i doesn't exist but for the inner access, it can't help since there is no defaultlist which will return something if there is an empty list.

The best you can do is:

h = defaultdict(list)
h[1]=['chr1','12','20','1']
h[2]=['chr1','15','20','2']
for i in range(1, 5):
    print(h[i][3] if h.get(i) and len(h.get(i))>=4 else 0) # checks also that there is at least 4 elements in the list

Python: return a specific value while keys not defined

3 Answers3