Say, you want to convert a list of lists
lst=[[1, 2], [3, 4]] into a single list
[1, 2, 3, 4]. How to achieve this? There are different options:
- List comprehension
[x for l in lst for x in l]assuming you have a list of lists
[*lst, *lst]assuming you have a list of two lists
- Using the
extend()method of Python lists to extend all lists in the list of lists.
Find examples of all three methods in the following code snippet:
lst = [[1, 2], [3, 4]] # Method 1: List Comprehension flat_1 = [x for l in lst for x in l] # Method 2: Unpacking flat_2 = [*lst, *lst] # Method 3: Extend Method flat_3 =  for l in lst: flat_3.extend(l) ## Check results: print(flat_1) # [1, 2, 3, 4] print(flat_2) # [1, 2, 3, 4] print(flat_3) # [1, 2, 3, 4]
Due its simplicity and efficiency, the first list comprehension method is superior to the other two methods.
You can check out these methods yourself in our interactive code shell:
Do you need to dive even deeper into this code snippet? Try the Python teacher code execution to see the memory objects created by this code:
Where to Go From Here?
Enough theory, let’s get some practice!
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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
To help students reach higher levels of Python success, he founded the programming education website Finxter.com. He’s author of the popular programming book Python One-Liners (NoStarch 2020), coauthor of the Coffee Break Python series of self-published books, computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.
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