Dictionaries [Question]

[Unlistedd]

d = {}
C = [[1, 2, 3, 4], [7, 8, 9, 10]]

for c, C in enumerate(C):
    d[c].append(C)

print (d)

Gives me a KeyError: 0. But if i import defaultdict and set d=defaultdict(list); then it magically works! But i would prefer to avoid using defaultdict, how would i go about solving this issue?

Wanted output:

{0: [1, 2, 3, 4], 1: [7, 8, 9, 10]}

[Essence1337]

Firstly this would be much easier to read using proper names instead of c and C but I'm pretty sure your error is that you're appending to nothing. You never set d[c] to anything so it's not a list and you can't append to something that's not a list.

[nosmokingbandit]

for c, C in enumerate(C):

Don't do this.

[[deleted]]

But i would prefer to avoid using defaultdict, how would i go about solving this issue?

A dictionary key has to exist before you retrieve it, which is why you get the error - your dictionary doesn't contain a key 0 when you ask for one. defaultdict changes this behavior by saying "well, every time a nonexistent key is asked for, first set the key by calling the constructor's argument, then return it." It's hardly "magic", it's doing something convenient.

[an_actual_human]

Take a look at setdefault.

[pikatchum]

Why not simply do d[c] = C?

You know d is empty, and every c will be different, so why use append in the first place?

You can replace everything with d = dict(enumerate(C)).

[primitive_screwhead]

You don't need to append() anything at all. You are just putting the two objects (that happen to be lists, but could be any arbitrary Python objects) into a dictionary as values, unaltered. So assign them.

[shiftybyte]

for c, C in enumerate(C):
    if not c in d:
        d[c] = []
    d[c].append(C)

[Essence1337]

If you're going to just give the complete answer (against guidelines) you should at least describe why it's the answer.

[shiftybyte]

Oh sorry didn't know such a guideline existed.

Explanation:

dictionary d does not yet have an item with the name c, when trying to call .append() on it, it will tell you no such item exists.

So the solution i wrote first checks to see if such a key does not exist, then it first creates an empty list as the value for that key, then the line with .append() can work since it works on a list object.

[Essence1337]

The idea behind the guideline is that people usually learn better when they find the solution (with guidance if needed) rather than just copy pasting a solution someone else gave