Python - Create a Dictionary using List with None Values

The task of creating a dictionary from a list of keys in Python involves transforming a list of elements into a dictionary where each element becomes a key. Each key is typically assigned a default value, such as None, which can be updated later.

For example, if we have a list like ["A", "B", "C"], the goal is to convert it into a dictionary like {'A': None, 'B': None, 'C': None}.

Using dictionary comprehension

Dictionary comprehension is an efficient way to create dictionaries in a single line. It is highly efficient and easy to read, making it the go-to approach for initializing dictionaries with a predefined structure. This method is ideal when we need a fast and clear solution without creating intermediate objects.

li = [1, 2, 3, 4, 5]
res = {key: None for key in li}
print(res)

{1: None, 2: None, 3: None, 4: None, 5: None}

Explanation: {key: None for key in li} iterates over each element in the list li, adding each element as a key to the dictionary res with the value set to None. This results in a dictionary where each element from the list is a key, and its corresponding value is None.

Using dict.from keys()

dict.fromkeys() is a built-in function explicitly designed for initializing dictionaries with a common value. It takes an iterable of keys and a default value, which is perfect for this task. It’s straightforward, clean and optimized for initializing dictionaries, making it a great choice when clarity is a priority.

li = [1, 2, 3, 4, 5]
res = dict.fromkeys(li, None)
print(res)

{1: None, 2: None, 3: None, 4: None, 5: None}

Explanation: res = dict.fromkeys(li, None) uses the fromkeys() of the dict class to create a new dictionary. It takes an iterable li , where each element becomes a key in the dictionary and the second argument None sets the value for each key.

Using zip()

This approach combines the power of zip() and dict() to pair elements from a list with a corresponding sequence of values None in this case . It is particularly flexible, as it allows us to initialize keys with any set of corresponding values. While slightly less efficient, it’s useful when handling more dynamic scenarios.

li = [1, 2, 3, 4, 5]
res = dict(zip(li, [None] * len(li)))
print(res)

{1: None, 2: None, 3: None, 4: None, 5: None}

Explanation: dict(zip(li, [None] * len(li))) pairs elements from li with None using zip(), creating tuples like (1, None). The dict() function then converts these pairs into a dictionary where the elements from li are keys, and the value for each key is None.

Using for loop

For loop is a traditional approach that involves iterating over a list and adding key-value pairs one at a time. While not as concise or efficient as other methods, it is a straightforward way to understand dictionary construction

li = [1, 2, 3, 4, 5]
res = {} # initialize empty dictionary
for key in li:
    res[key] = None
print(res)

{1: None, 2: None, 3: None, 4: None, 5: None}

Explanation: This code iterates over each element in the list li and adds it as a key to the dictionary res, assigning None as the value for each key.