Adding keys and values to a dictionary

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Adding keys and values to a dictionary - giladal - Nov-19-2020

Hey Guys.

Trying to add value, key in this order to a dictionary.
Checking with the debugger it shows that index 0 is added to new_dict, and then in the second iteration, index 1 replaces index 0 and then index 2 is added correctly.

Any idea why?

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


def inverse_dict(my_dict):
    new_dict = {}
    for key, value in my_dict.items():
        new_dict.update({value: key})
       

    return new_dict

print(inverse_dict(course_dict))

RE: Adding keys and values to a dictionary - bowlofred - Nov-19-2020

Keys in a dictionary are unique, but the starting values you have are not unique. So simply reversing the order of values and keys gives a key collision. What do you think should be the answer in your final dictionary if you asked it for the key 3? inverse_dict[3] == ?

If you didn't want to lose data, you could make the value a container, like a set or a list and append any conflicts together.

RE: Adding keys and values to a dictionary - giladal - Nov-19-2020

(Nov-19-2020, 04:39 PM)bowlofred Wrote: Keys in a dictionary are unique, but the starting values you have are not unique. So simply reversing the order of values and keys gives a key collision. What do you think should be the answer in your final dictionary if you asked it for the key 3? inverse_dict[3] == ?

If you didn't want to lose data, you could make the value a container, like a set or a list and append any conflicts together.

Thanks for your quick reply.

You are right, I got the exercise wrong. It involves making a list. Ill give it a go now.

Thanks again!

RE: Adding keys and values to a dictionary - deanhystad - Nov-19-2020

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'