How to append lists in a dictionary in Python?

Question

Hey everyone this code is working fine just one thing to deal with. It overwrites the multiple entries against a key. I need to avoid the overwriting and to save all those entries. Can you help me in this please?

#!/usr/bin/python

import sys
import fileinput

#trys to create dictionary from african country list
dic = {}

for line in sys.stdin:
    lst = line.split('|')
    links = lst[4].split()
    new=links[0] + ' ' + links[len(links)-1]
    dic[new]=lst[4] # WARNING: overwrites multiple entriess at the moment! :)

for newline in fileinput.input('somefile.txt'):
    asn = newline.split()
    new = asn[0] + ' ' + asn[1]
    if new in dic:
            print "found from " + asn[0] + " to " + asn[1]
            print dic[new]

Note: Sys.stdin takes a string in the following format; 1.0.0.0/24|US|158.93.182.221|GB|7018 1221 3359 3412 2543 1873

You shouldn't use list as a variable name since it shadows the builtin function list(). Also you're miss ing a colon in the if statement, which BTW should probably look like this: if new in dic:. At least, that's the pythonic way. — pillmuncher
– pillmuncher, Commented Aug 29, 2011 at 21:24
What do you expect dic[dic[new]] to do, and why? (Hint: what does dic[new] do?) — Karl Knechtel
– Karl Knechtel, Commented Aug 30, 2011 at 4:43

agf · Accepted Answer · 2011-08-29 21:41:39Z

5

You've got a number of problems with your code. The simplest way to do what you describe is to use a defaultdict, which gets rid of the explicit if and has_key (which you should replace by new in dic anyway):

#trys to create dictionary from african country list
from collections import defaultdict

dic = defaultdict(list)   # automatically creates lists to append to in the dict

for line in sys.stdin:
    mylist = line.split('|')    # call it mylist instead of list
    links = mylist[4].split()
    new = links[0] + ' ' + links[-1]   # can just use -1 to reference last item
    dic[new].append(mylist[4])         # append the item to the list in the dict
                                # automatically creates an empty list if needed

See eryksun's comment on Gerrat's answer if you're on an old version of Python without defaultdict.

edited Aug 29, 2011 at 21:41

answered Aug 29, 2011 at 21:29

agf

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Comments

Gerrat · Accepted Answer · 2011-08-29 21:28:48Z

1

There is no method called appendlist. use append:

dic[dic[new]].append(list[4])

Also, it's inadvisable to use list as a variable name.
It is a builtin in python.

Also, this entire section of code:

    if ( dic.has_key(new))
        dic[dic[new]].appendlist(list[4])
    else:
       dic[dic[new]] = [list[4]]

should instead probably be:

    if new in dic:  # this is the preferrable way to test this
        dic[new].append(list[4])
    else:
       dic[new] = [list[4]]

edited Aug 29, 2011 at 21:28

answered Aug 29, 2011 at 21:23

Gerrat

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13 Comments

Sohaib Over a year ago

Hi, I have just corrected the mistakes I had made. But still code: dic[dic[new]] = [list[4]] is not working and gives error "'type' subject is not subscriptable".

agf Over a year ago

@Sohaib - as both @Gerrat and I said in our answers, dic[dic[new]] is wrong, however, that shouldn't give a trying to subscript a type object (not "subject"!) error. You must have accidentally typed dict instead of dic, as dict is a type object and dic is not.

Sohaib Over a year ago

@Gerrat: I have tried your preferred way to test it and encountered an error "'str' object has no attribute 'append'". I am actually trying to create a dictionary which will contain multiple values against a key, each value is a string. I would then read content of a new file and on comparing keys in dictionary I'll print those values in of key.

Gerrat Over a year ago

@Sohaib: it sounds like you've used: dic[new] = lst[4], not dic[new] = [lst[4]] as you've posted...is the code above exactly what you're running?

Sohaib Over a year ago

@Gerrat: yes, as it is in code, I have used it in the same way.

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