Summary: To write a nested for loop in a single line of Python code, use the one-liner code
[print(x, y) for x in iter1 for y in iter2] that iterates over all values
x in the first iterable and all values
y in the second iterable.
Problem: How to write a nested for loop as a Python one-liner? Roughly speaking, you want to iterate over two or more iterables that are nested into each other. Here’s an example of a multi-liner with two nested loops:
iter1 = [1, 2, 3, 4] iter2 = ['a', 'b', 'c'] for x in iter1: for y in iter2: print(x, y) ''' 1 a 1 b 1 c 2 a 2 b 2 c 3 a 3 b 3 c 4 a 4 b 4 c '''
How to accomplish this in a single line?
Let’s dive into multiple methods! Here’s a quick overview:
Method 1: Nested List Comprehension
The first method makes use of the powerful feature of list comprehension:
# Method 1: Nested List Comprehension [print(x, y) for x in iter1 for y in iter2]
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
Here’s a short video tutorial on list comprehension in case you need a quick refresher:
Method 2: exec()
You can always one-linerize any multi-liner by using Python’s built-in
# Method 2: exec() exec("for x in iter1:\n for y in iter2:\n print(x, y)")
You wrote the multi-liner as a one-liner string using the newline character
'\n'. Note that you must ensure that the three lines are properly indented.
Method 3: For Loop with List Comprehension
Again, you use list comprehension—but now only for the inner loop.
# Method 3: For Loop with List Comprehension for x in iter1: [print(x, y) for y in iter2]
Note that many coders would consider this to be “unpythonic” because you create a list consisting only of
None values—the return values from the
print() function calls.
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.
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.