Example: You hope to accomplish something like this where you create an initial list (this one is empty) and you append multiple elements to it:
# WRONG CODE: >>> .append(i) for i in range(5) [0, 1, 2, 3, 4]
However, this statement doesn’t work! Is there a one-line for loop to append elements to a given list?
Let’s dive into several methods to accomplish this! Here’s a quick overview:
Exercise: Can you modify the code to append elements in a tuple to the given list in a single line of code?
Let’s dive into the three methods in more detail!
Method 1: Use List Comprehension
If you don’t need to add elements to a given list but you’re fine to create a new list, list comprehension is your best shot!
# Method 1 lst = [i for i in range(5)] print(lst) # [0, 1, 2, 3, 4]
This one-liner accomplishes the desired result—but it does create a new list. Let’s quickly recap how list comprehension works in this video:
List comprehension is a compact way of creating lists. The simple formula is
[expression + context].
- Expression: What to do with each list element?
- Context: What elements to select? The context consists of an arbitrary number of
[x for x in range(3)] creates the list
[0, 1, 2].
But is there another way if you have a list and you just want to append elements to this list?
Method 2: Single-Line For Loop with append()
Sure! You can write blocks in a single line—if the block body itself is not nested!
# Method 2 friends = ['Ann', 'Alice'] new_friends = ['Bob', 'Liz'] # One-Liner: for f in new_friends: friends.append(f) # Results print(friends) # ['Ann', 'Alice', 'Bob', 'Liz']
You use the
list.append() method repeatedly for each element in the iterable
new_friends that contains the elements to be appended to the original list
friends. The fact that the for loop is compressed in a single line doesn’t make this one-liner ambiguous so Python is okay with it.
Method 3: extend()
However, a much better option to append all elements in a given iterable to a given list is to use the
# Method 3 friends = ['Ann', 'Alice'] new_friends = ['Bob', 'Liz'] # One-Liner: friends.extend(new_friends) # Results print(friends) # ['Ann', 'Alice', 'Bob', 'Liz']
The one-liner is much shorter and even faster. You can find a detailed speed comparison here.
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Python programmers will improve their computer science skills with these useful one-liners.
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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.
His passions are writing, reading, and coding. But his greatest passion is to serve aspiring coders through Finxter and help them to boost their skills. You can join his free email academy here.