Appending values to lists in a python dictionary

You should use append to add to the list. But also here are few code tips:

I would use dict.setdefault or defaultdict to avoid having to specify the empty list in the dictionary definition.

If you use prev to to filter out duplicated values you can simplfy the code using groupby from itertools Your code with the amendments looks as follows:

import itertools
def make_drug_dictionary(data):
    drug_dictionary = {}
    for key, row in itertools.groupby(data, lambda x: x[11]):
        drug_dictionary.setdefault(key,[]).append(row[?])
    return drug_dictionary

If you don't know how groupby works just check this example:

>>> list(key for key, val in itertools.groupby('aaabbccddeefaa'))
['a', 'b', 'c', 'd', 'e', 'f', 'a']

If you want to append to the lists of each key inside a dictionary, you can append new values to them using + operator (tested in Python 3.7):

mydict = {'a':[], 'b':[]}
print(mydict)
mydict['a'] += [1,3]
mydict['b'] += [4,6]
print(mydict)
mydict['a'] += [2,8]
print(mydict)

and the output:

{'a': [], 'b': []}
{'a': [1, 3], 'b': [4, 6]}
{'a': [1, 3, 2, 8], 'b': [4, 6]}

mydict['a'].extend([1,3]) will do the job same as + without creating a new list (efficient way).

You can use the update() method as well

d = {"a": 2}
d.update({"b": 4})
print(d) # {"a": 2, "b": 4}

It sounds as if you are trying to setup a list of lists as each value in the dictionary. Your initial value for each drug in the dict is []. So assuming that you have list1 that you want to append to the list for 'MORPHINE' you should do:

drug_dictionary['MORPHINE'].append(list1)

You can then access the various lists in the way that you want as drug_dictionary['MORPHINE'][0] etc.

To traverse the lists stored against key you would do:

for listx in drug_dictionary['MORPHINE'] :
  do stuff on listx

To append entries to the table:

for row in data:
    name = ???     # figure out the name of the drug
    number = ???   # figure out the number you want to append
    drug_dictionary[name].append(number)

To loop through the data:

for name, numbers in drug_dictionary.items():
    print name, numbers

how do i append a number into the drug_dictionary?

Do you wish to add "a number" or a set of values?

I use dictionaries to build associative arrays and lookup tables quite a bit.

Since python is so good at handling strings, I often use a string and add the values into a dict as a comma separated string

drug_dictionary = {} 

drug_dictionary={'MORPHINE':'',
         'OXYCODONE':'',
         'OXYMORPHONE':'',
         'METHADONE':'',
         'BUPRENORPHINE':'',
         'HYDROMORPHONE':'',
         'CODEINE':'',
         'HYDROCODONE':''}


drug_to_update = 'MORPHINE'

try: 
   oldvalue = drug_dictionary[drug_to_update] 
except: 
   oldvalue = ''

# to increment a value

   try: 
      newval = int(oldval) 
      newval += 1
   except: 
      newval = 1 


   drug_dictionary[drug_to_update] = "%s" % newval

# to append a value  

   try: 
      newval = int(oldval) 
      newval += 1
   except: 
      newval = 1 


   drug_dictionary[drug_to_update] = "%s,%s" % (oldval,newval) 

The Append method allows for storing a list of values but leaves you will a trailing comma

which you can remove with

drug_dictionary[drug_to_update][:-1]

the result of the appending the values as a string means that you can append lists of values as you need too and

print "'%s':'%s'" % ( drug_to_update, drug_dictionary[drug_to_update]) 

can return

'MORPHINE':'10,5,7,42,12,'

vowels = ("a","e","i","o","u") #create a list of vowels  
my_str = ("this is my dog and a cat") # sample string to get the vowel count
count = {}.fromkeys(vowels,0) #create dict initializing the count to each vowel to 0                                                                    
for char in my_str :  
    if char in count:  
       count[char] += 1  

print(count)