Initialize a Dictionary with Only Keys from a List – Python
Last Updated : 28 Jan, 2025
The task of initializing a dictionary with only keys from a list in Python involves creating a dictionary where the keys are derived from the elements of the list and each key is assigned a default value .
For example, given a list like [“A”, “B”, “C”], the goal is to transform this list into a dictionary where the keys are “A”, “B”, and “C”, all mapped to the default value None. After initialization, the dictionary will look like this: {‘A’: None, ‘B’: None, ‘C’: None}.
Using dictionary comprehension
Dictionary comprehension is one of the most efficient way to initialize a dictionary when the keys are already available in a list . It allows us to construct a dictionary in a single, concise statement, where each key in the list is mapped to a specified value. This method combines clarity and performance, making it a favorite choice for many Python developers.
Python li = ["Paras", "Jain", "Cyware"] d = {key: None for key in li} print(d) Output{'Paras': None, 'Jain': None, 'Cyware': None} Explanation: {key: None for key in li} loops over each element in li, uses each element as a dictionary key and assigns None as the corresponding value. This process results in a new dictionary with the keys from the list and None as their values .
Using dict.fromkeys()
fromkeys() method is another efficient approach in Python to initialize a dictionary from a list of keys. This method is specifically designed to handle cases like this, where we want to initialize all keys with the same value. It’s both concise and built for this exact scenario, making it a good alternative to dictionary comprehension.
Python li = ["Paras", "Jain", "Cyware"] d = dict.fromkeys(li) print(d)
Output{'Paras': None, 'Jain': None, 'Cyware': None} Explanation: dict.fromkeys(li) creates a dictionary with keys from li, and assigns None as the value for each key.
Using map()
Another method to initialize a dictionary is by using the map() in combination with the dict() constructor. While this approach is less common for this specific task, it is still an option. map() function transforms the list into key-value pairs and dict() then converts it into a dictionary.
Python li = ["Paras", "Jain", "Cyware"] d = dict(map(lambda key: (key, None), li)) print(d)
Output{'Paras': None, 'Jain': None, 'Cyware': None} Explanation: map() applies the lambda function to each element of the list li, creating tuples of each element paired with None. dict() constructor then converts these tuples into a dictionary where each element of the list becomes a key, and the value for each key is None.
Using for loop
for loop is the most basic way to initialize a dictionary from a list of keys. It’s highly efficient , making it a good option for beginners. However, it’s not efficient as the methods mentioned above.
Python li = ["Paras", "Jain", "Cyware"] d = {} # initialize empty dictionary for key in li: d.setdefault(key, None) print(d) Output{'Paras': None, 'Jain': None, 'Cyware': None} Explanation: setdefault() checks if a key exists in the dictionary. If not, it adds the key with a default value None .For each item in the list li, setdefault(key, None) is called, adding each key with None as the value.
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