Problem: How to append elements to a list using a single line for loop?
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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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
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