How to Merge Two Lists in Python

Merge two Python lists into a single combined list by appending each element with a simple loop, achieving the same result as the + operator.

Easy Python 3.9+ Aug 9, 2026 Lists & loops 14 views 0 copies

lists merge loops beginner

Python code

14 lines
Python 3.9+
Run it 
def merge_lists(list_a, list_b):
    merged = []
    for item in list_a:
        merged.append(item)
    for item in list_b:
        merged.append(item)
    return merged


if __name__ == "__main__":
    fruits = ["apple", "banana"]
    vegetables = ["carrot", "spinach"]
    result = merge_lists(fruits, vegetables)
    print(result)

Output

stdout 
['apple', 'banana', 'carrot', 'spinach']

How it works

This helper function creates a new empty list and then iterates through each input list separately, appending every item to the merged result. The for loops guarantee order is preserved, with all elements from list_a coming before list_b. While the built-in + operator or extend() method is more concise, writing the loop explicitly shows exactly how list concatenation works under the hood and makes the logic readable for beginners.

Common mistakes

  • Forgetting that the function creates a new list rather than modifying the originals
  • Using `append` with the entire list instead of iterating to add individual elements
  • Assuming the input lists remain unchanged after merging (they do, since we return a new list)

Variations

  1. Use the `+` operator: `result = fruits + vegetables`
  2. Use `list_a.extend(list_b)` to modify the first list in place, or a list comprehension with nested loops

Real-world use cases

  • Combining two API response pages into one list before displaying results to the user.
  • Merging user-provided tags from multiple configuration sources into a single list for validation.
  • Joining separate error logs from different services into one list for aggregated reporting.

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