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.
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