Append a list to a list within a dictionary in Python

Question

So i am simply trying to append my list containing a set of numbers, to a value in a dictionary which is currently a list.

I don't know whether it is important but some of the lists in the dictionary are empty and some contain integers.

I assumed this would be simple and tried the most obvious thing to me which was appending the list to that value like this.

middlenumberdic[y].append(numberlist)

However this gave me the following error:

    middlenumberdic[y].append(numberlist)
KeyError: 786

So i decided that the problem was that lists couldn't apppend with lists so i broke it up into individual integers

for value in numberlist:
    middlenumberdic[y].append(value)

but this just gave me the same KeyError thing

    middlenumberdic[y].append(value)
KeyError: 792

From my point of view i would've thought that middlenumberdic[y] would return [123,456,789] (as that is what is assigned to it) and then that i could just append to that list but apparently i can't.

Before doing all this i tried middlenumberdic[y] = numberlistand although this didn't result in an error all it did was overwrite my existing list rather than adding to it.

After reading How to append to a list in a dictionary? and Append to a list within a dictionary i was still none the wiser which is why i have posted this.

For the first question the answer given was much the same as the solution i used which didn't work and the other question was to complicated for me to understand.

This is my first question i've asked on this site so i'm sorry if i've done something wrong.

P.S Their is more script but i thought only this would be neccesary

Apparently there are no entries in the dictionary `middlenumberdic` with the keys `786` or `792` - what exactly does `middlenumberdic` contain? — Tim Pietzcker, Jan 31 '16 at 11:59
best if you create a [mcve]. It's not clear currently what exactly your input data values are. — Robert Longson, Jan 31 '16 at 11:59
Really sorry, couldn't quite get your question. Can you provide sample input and output so that we can easily understand? — Ahsanul Haque, Jan 31 '16 at 12:00
Adding to a list in a dictionary is no problem. `d = { "a" : [] }; d["a"].append(4); d` prints `{'a': [4]}`. — oarfish, Jan 31 '16 at 12:01
When you get an error, it's a good idea to read the documentation about that error: https://wiki.python.org/moin/KeyError — pseudoDust, Jan 31 '16 at 12:02
Did you first initialise your dictionary with empty lists? It seems like there is no entry at the key `y` you are trying to append. — lab, Jan 31 '16 at 12:04

score 1 · Answer 1 · answered Jan 31 '16 at 12:02

1

Your problem is not with the appending part. It's with accessing your list in the dictionary. The KeyError means there are not elements with index 786 or 792.

In [1]: a = dict()

In [2]: a[5] = []

In [3]: a
Out[3]: {5: []}

In [4]: a[5]
Out[4]: []

In [5]: a[5].append([3,4,5])

In [6]: a
Out[6]: {5: [[3, 4, 5]]}

answered Jan 31 '16 at 12:02

Diziet Asahi

38,379
7
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Sorry i hadn't understood what the Key Error meant – Joe Clinton Jan 31 '16 at 12:23

score 0 · Accepted Answer · answered Jan 31 '16 at 12:06

0

I think you are trying to access with a key that doesn't exist. So, you are getting this KeyError. In that case,

for value in numberlist:
     middlenumberdic[y].append(value)

Replace this with below:

for value in numberlist:
    middlenumberdic.setdefault(y,[]).append(value)

I think it should work now.

answered Jan 31 '16 at 12:06

Ahsanul Haque

10,676
4
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Thankyou, this worked perfectly. it seems that i had not intialized the dictionary with empty values. – Joe Clinton Jan 31 '16 at 12:21
Yes, you are right. What `setdefault(key,[])` does is, if it couldn't find the key, it initializes the key with an empty list. Only then you can append some value to it. – Ahsanul Haque Jan 31 '16 at 12:26

Append a list to a list within a dictionary in Python

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