How to Write a Nested For Loop in One Line Python?

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:

Exercise: Instead of printing the outputs, store them in three lists of tuples. Print the lists after creating them so that your output has three lines!

Method 1: Nested List Comprehension

Write a Nested For Loop in One Line Python

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 for and if statements.

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 exec(...) function.

# 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.

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