Adding keys and values to a dictionary

[deanhystad]

This is the dictionary you are trying to make:

Output:
{3:'I', 3:'love', '2':'self.py!'}

Lets see what happens if we try to create the dictionary from a literal.

inverse_dict = {3:'I', 3:'love', 2:'self.py!'}
print(inverse_dict)

Output:
{3: 'love', '2': 'self.py!'}

Hmmm, same problem.

Lets try building the dictionary step by step:

inverse_dict = {}
inverse_dict[3] = 'I'
inverse_dict[3] = 'love'
inverse_dict[2] = 'self.py!'
print(inverse_dict)

Output:
{3: 'love', '2': 'self.py!'}

Hopefully this last example makes it clear why 'love' replaces 'I' when you SET inverse_dict[3] = 'love'. The keys in a dictionary are unique. When you set the key:value in a dictionary it sets the value associated with the key. If the key does not exist the key is added.

This is a potential problem with inverting a dictionary. A dictionary does not care if the values are unique, only the keys. When you swap keys and values the dictionary will happily overwrite any duplicate values. You could make this a key error.

course_dict = {'I': 3, 'love': 3, 'self.py!': 2}

def inverse_dict(my_dict):
    new_dict = {}
    for key, value in my_dict.items():
        if value in new_dict:
            raise KeyError(f'Key {value} already exists')
        new_dict[value] = key
    return new_dict
 
print(inverse_dict(course_dict))

Output:
Traceback (most recent call last):
  File "...", line 14, in <module>
    print(inverse_dict(course_dict))
  File "...", line 8, in inverse_dict
    raise KeyError(f'Key {value} already exists')
KeyError: 'Key 3 already exists'