How to flatten nested python dictionaries?

You can use a simple recursive function as follows.

def flatten(d):    
    res = []  # Result list
    if isinstance(d, dict):
        for key, val in d.items():
            res.extend(flatten(val))
    elif isinstance(d, list):
        res = d        
    else:
        raise TypeError("Undefined type for flatten: %s"%type(d))

    return res


dict1 = {
    'Bob': {
        'shepherd': [4, 6, 3],
        'collie': [23, 3, 45],
        'poodle': [2, 0, 6],
    },
    'Sarah': {
        'shepherd': [1, 2, 3],
        'collie': [3, 31, 4],
        'poodle': [21, 5, 6],
    },
    'Ann': {
        'shepherd': [4, 6, 3],
        'collie': [23, 3, 45],
        'poodle': [2, 10, 8],
    }
}

print( flatten(dict1) )

You're trying to use variables that don't exist.

Use this

dict_flatted = [ i for names in dict1.values() for dog in names.values() for i in dog]