Why does a nested loop print more pairs than I expected?

Because the inner loop starts over for every item in the outer loop. The outer loop does not advance until the inner loop has finished its full pass. That means the code makes every combination of one item from the first list and one item from the second—not matches by position. [S1]

For example, assuming these lists:

colors = ["red", "blue"]
shapes = ["circle", "square", "triangle"]

for color in colors:
    for shape in shapes:
        print(color, shape)

The output is:

red circle
red square
red triangle
blue circle
blue square
blue triangle

There are 2 choices for color and 3 choices for shape, so the total is 2 × 3 = 6 pairs. The first color is used with all three shapes before the loop moves to the second color. [S1]

For this simple pattern—one inner loop that completes for every outer-loop item—the number of times the inner body runs is the outer iterable’s item count multiplied by the inner iterable’s item count. For lists of lengths 2 and 3, that is 6. [S1]

You can count the passes: red gets 3 turns through the inner loop; then blue gets 3 more. That gives 3 + 3 = 6. With 4 outer items and 5 inner items, the same pattern would run 20 times. These counts assume the loop bodies do not change what is iterated over and that there are no conditions or early exits that skip or stop iterations. If your code includes those features, inspect them too; simple multiplication may not describe its printed lines.

Use zip to iterate over corresponding items together, rather than nesting one loop inside another:

colors = ["red", "blue"]
shapes = ["circle", "square", "triangle"]

for color, shape in zip(colors, shapes):
    print(color, shape)

This pairs items in order and stops as soon as the shorter input is exhausted. [S1] With these lists, it prints red circle and blue square; triangle has no matching color, so it is not included. If you need every item from both lists to be paired, check that the input lengths are what you expect before using zip.

Ask what one output line represents. If each item from list A should be combined with every item from list B, use the nested loop. If one line should combine item 1 with item 1, item 2 with item 2, and so on, use zip.

A small planning note can make the choice clear:

Desired output: every color with every shape
Pattern: nested loops
Expected pairs: 2 × 3 = 6

Desired output: corresponding positions only
Pattern: zip(colors, shapes)
Expected pairs: 2 (the shorter list's length)

Before running the code, write down the two input lengths and a few expected output lines. Then compare that prediction with the actual output. This helps distinguish an intentional all-combinations loop from one that should pair items by position.