appending data to python dictionary

You could use

dict["key"] = value_list 

so in your case:

mydict["key"] = z

as described here: Python docs

mydict = {}
print(mydict) # {}

Appending one key:

mydict['key1'] = 1
print(mydict) # {'key1': 1}

Appending multiple keys:

mydict.update({'key2': 2, 'key3': 3})
print(mydict) # {'key1': 1, 'key2': 2, 'key3': 3}

Append works for arrays, but not dictionaries.

To add to a dictionary use dict_name['item'] = 3

Another good solution (especially if you want to insert multiple items at once) would be: dict_name.update({'item': 3})

The NoneType error comes up when an instance of a class or an object you are working with has a value of None. This can mean a value was never assigned.

Also, I believe you are missing a bracket here: mydict.setdefault(items,]).append(counts[j]) It should be: mydict.setdefault(items,[]).append(counts[j])