Pfthb

I can't find an answer.
I read a dictionary from file, it has a string value for a key.
Then I check if a new value is already in the dictionary, if it isn't then I want
to append it to the previous string and to get a list of values for a given key.

For example if I have a dictionary

dict = "vehicles": "car", "animals": "cat"

and the following condition is true

if "dog" not in dict["animals"]:

I'd like to get the output

dict = "vehicles": "car", "animals": ["cat", "dog"] 

Another way is to try append the element to the list and if it fails make the dictionary value a list:

dic = "vehicles": "car", "animals": "cat"
if "dog" not in dic["animals"]:
 try:
 dic["animals"].append("dog")
 except:
 dic["animals"] = [dic["animals"], "dog"]

Gives output:

'vehicles': 'car', 'animals': ['cat', 'dog']

In this way if you have a single element the except part will be executed, making the value a list of string. If you then try to add another element it will be appended to the list.

start with a dictionary with values as set of strings

d = "vehicles": "car", "animals": "cat"

Then adding twice a new value only adds it once (and lookup is fast since it uses hashes):

d["animals"].add('dog')
d["animals"].add('dog')

>>> d
'animals': 'dog', 'cat', 'vehicles': 'car'

If you have the dictionary as input like in your question, you can transform it easily with values as set with dictionary comprehension like this:

loaded_d = "vehicles": "car", "animals": "cat" # dict just loaded from file
d = key:value for key,value in loaded_d.items()

>>> if "dog" not in dict["animals"]:
... dict["animals"] = [dict["animals"], "dog"]

Should be the easies way I think

dict = "vehicles": "car", "animals": "cat"
if "dog" not in dict["animals"]:
 dict["animals"].add("dog")

You just need to update animals list by using append method.

dict = "vehicles": "car", "animals": "cat"

def appendItem(item):
 if item not in dict["animals"]:
 if not isinstance(dict["animals"], list):
 dict["animals"] = [dict["animals"]]
 dict["animals"].append(item)

appendItem("dog")
appendItem("dog")
appendItem("rabbit")
appendItem("cat")

Output

=> 'vehicles': 'car', 'animals': ['cat', 'dog', 'rabbit']