To create a list of n placeholder elements, multiply the list of a single placeholder element with n. For example, use [None] * 5 to create a list [None, None, None, None, None] with five elements None. You can then overwrite some elements with index assignments. In the example, lst[2] = 42 would result in the changed list [None, None, 42, None, None].
Let’s play with an interactive code shell before you’ll dive into the detailed solution!
Exercise: Initialize the list with n=20 placeholder elements -1 and run the code.
Next, you’ll learn about the more formal problem and dive into the step-by-step solution.
Problem: Given an integer n. How to initialize a list with n placeholder elements?
# n=0 --> [] # n=1 --> [None] # n=5 --> [None, None, None, None, None]
Solution: Use the list concatenation operation *.
n = 5 lst = [None] * n print(lst) # [None, None, None, None, None]
You can modify the element n as you like. In subsequent operations, you can overwrite all placeholder None list elements using simple index assignment operations:
lst[0] = 'Alice' lst[1] = 0 lst[2] = 42 lst[3] = 12 lst[4] = 'hello' print(lst) # ['Alice', 0, 42, 12, 'hello']
However, there’s a small problem if you want to create a list with mutable objects (such as a list of lists):
lst = [[]] * n print(lst) # [[], [], [], [], []] lst[2].append(42) print(lst) # [[42], [42], [42], [42], [42]]
Changing one list element changes all list elements because all list elements refer to the same list object in memory:
The solution is to use list comprehension (see my detailed blog tutorial on list comprehension for a complete guide):
lst = [[] for _ in range(n)] print(lst) # [[], [], [], [], []] lst[2].append(42) print(lst) # [[], [], [42], [], []]
In the following visualization, you can see how each element now refers to an independent list object in memory:
Exercise: Run the visualization and convince yourself that only one element is modified! Why is this the case?
Where to Go From Here?
Enough theory. Let’s get some practice!
Coders get paid six figures and more because they can solve problems more effectively using machine intelligence and automation.
To become more successful in coding, solve more real problems for real people. That’s how you polish the skills you really need in practice. After all, what’s the use of learning theory that nobody ever needs?
You build high-value coding skills by working on practical coding projects!
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🚀 If your answer is YES!, consider becoming a Python freelance developer! It’s the best way of approaching the task of improving your Python skills—even if you are a complete beginner.
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