You started with a string value, and you cannot call .append() on a string. Start with a list value instead:

if clientKey not in data:
    data[clientKey] = [ref]

Now data[clientKey] references a list object with one string in it. List objects do have an append() method.

You can do classic dict comprehension here:

signames = {k: '_'+ v for k,v in signames.items()}

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']

You need to initialize a list rather than just adding the object. Change this:

parents[parent]=kid

to this:

parents[parent] = [kid]

This will give you a list to which you can append() new objects, rather than just a string.

I feel like consolidating info about Python dictionaries:

Creating an empty dictionary

data = {}
# OR
data = dict()

Creating a dictionary with initial values

data = {'a': 1, 'b': 2, 'c': 3}
# OR
data = dict(a=1, b=2, c=3)
# OR
data = {k: v for k, v in (('a', 1), ('b',2), ('c',3))}

Inserting/Updating a single value

data['a'] = 1  # Updates if 'a' exists, else adds 'a'
# OR
data.update({'a': 1})
# OR
data.update(dict(a=1))
# OR
data.update(a=1)

Inserting/Updating multiple values

data.update({'c':3,'d':4})  # Updates 'c' and adds 'd'

Python 3.9+:

The update operator |= now works for dictionaries:

data |= {'c':3,'d':4}

Creating a merged dictionary without modifying originals

data3 = {}
data3.update(data)  # Modifies data3, not data
data3.update(data2)  # Modifies data3, not data2

Python 3.5+:

This uses a new feature called dictionary unpacking.

data = {**data1, **data2, **data3}

Python 3.9+:

The merge operator | now works for dictionaries:

data = data1 | {'c':3,'d':4}

Deleting items in dictionary

del data[key]  # Removes specific element in a dictionary
data.pop(key)  # Removes the key & returns the value
data.clear()  # Clears entire dictionary

Check if a key is already in dictionary

key in data

Iterate through pairs in a dictionary

for key in data: # Iterates just through the keys, ignoring the values
for key, value in d.items(): # Iterates through the pairs
for key in d.keys(): # Iterates just through key, ignoring the values
for value in d.values(): # Iterates just through value, ignoring the keys

Create a dictionary from two lists

data = dict(zip(list_with_keys, list_with_values))

As has been mentioned, if you create a key's value as a string, you can't use append on it, but you can if you make the key's value a list. Here is one way to do so:

newcategory=input("Enter new category: ")
oscars[newcategory]=[]
addnewnominees = 'yes'
while addnewnominees.lower() != "no":
    nominees=input("Enter new nominee: ")
    oscars[newcategory].append(nominees)
    addnewnominees = str(input("Do you want to enter more nominees: (yes/no):"))