Avoid Flattening When Merging Python Lists: Keep a List of Lists to Print Grouped Output with Nested Loops

Learn why merging Python lists with + flattens your data and breaks nested loops. See the correct list-of-lists way to preserve grouping, print aisle labels.

When you merge multiple lists in Python the wrong way, you can accidentally flatten your data and lose the structure your loops depend on. A common symptom is that a nested loop prints a group label only once, then proceeds to output all items as a single block. The fix is not about loops at all—it’s about the shape of the data you’re iterating.

Problem

You want to combine several aisles, then iterate with two nested for loops to print the aisle number before each group of items. The following snippet demonstrates the issue: only one aisle header is printed, followed by all items in one long sequence.

# Each aisle contains a variety of items
lane_a = ["Tomatoes", "Carrots", "Lettuce"]
lane_b = ["Eggs", "Butter", "Cream"]
lane_c = ["Pasta", "Rice", "Cereal"]
# combine all lists
lanes = [lane_a + lane_b + lane_c]

section_no = 1
# iterate through the lists in lanes
for section in lanes:
    print("Aisle: ", section_no)  # print aisle number before each sublist

    # print each item of each sublist
    for element in section:
        print(f"Contains {element}")
    section_no += 1

What’s going wrong

The core of the problem is how the lists are combined. Using the plus operator inside square brackets creates a single, concatenated list wrapped in another list. In other words, you end up with one outer element that contains every item. The outer loop runs exactly once, so you only see one aisle label. The inner loop then walks through every product without boundaries between aisles.

Solution

Instead of concatenating, keep a true nested structure by placing the individual lists inside a container as separate elements. This preserves grouping and allows the nested loops to print a header for each sublist.

# Each aisle contains a variety of items
lane_a = ["Tomatoes", "Carrots", "Lettuce"]
lane_b = ["Eggs", "Butter", "Cream"]
lane_c = ["Pasta", "Rice", "Cereal"]
# build a list of lists (preserve grouping)
lanes = [lane_a, lane_b, lane_c]

section_no = 1
# iterate through each sublist and print a header, then its items
for section in lanes:
    print("Aisle: ", section_no)
    for element in section:
        print(f"Contains {element}")
    section_no += 1

Why this matters

Iteration strategy follows data shape. If you need grouped output with nested loops, your data must remain nested. Concatenation flattens lists and removes boundaries, which directly affects control flow and the resulting output. Preserving a list of lists keeps grouping explicit, makes the outer loop meaningful, and avoids subtle bugs where labels appear only once.

Takeaways

Before writing or debugging loops, confirm the structure you’re iterating over. If you expect multiple groups, ensure you have a list of lists rather than a single flattened list. When grouping is correct, the nested loops naturally yield the desired output. If you prefer avoiding manual counters, enumerate can provide indices cleanly, but the essential part is keeping the data nested.