Python loops and accumulators

Build a total one iteration at a time.

On this page

Before you start

Variables, lists, and assignment.

Build a total one iteration at a time.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

A for loop visits each item from an iterable. An accumulator keeps the partial result between iterations.

Initialize total once before the loop. Moving that initialization inside the loop would discard the previous partial sum. An empty list leaves the total at zero.

Code example

Python

def total_of(values):
    total = 0
    for value in values:
        total += value
    return total

print(total_of([2, 4, 6]))
assert total_of([]) == 0
assert total_of([-2, 2]) == 0

Output

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

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Guided walkthroughReady

1def total_of(values):

2 total = 0

3 for value in values:

4 total += value

5 return total

7print(total_of([2, 4, 6]))

8assert total_of([]) == 0

9assert total_of([-2, 2]) == 0

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Call total_of with three values.	`values = [2, 4, 6]`	—
Start the accumulator.	`total = 0`	—
First iteration adds 2.	`value = 2, total = 2`	—
Second iteration adds 4.	`value = 4, total = 6`	—
Third iteration adds 6.	`value = 6, total = 12`	—
Iteration ends and the function returns 12.	`return = 12`	—
Print the returned total.	`return = 12`	12
The two assertions check empty input and cancellation.	`return = 12`	12

Behind the scenes

How Python processes this code

Python runs this source through its interpreter. It creates objects for values and resolves names as each statement runs.

Cost and common mistakes

O(n) time and O(1) auxiliary space for n fixed-size numbers.

Do not reset the accumulator inside the loop.
Inputs must support addition with the chosen starting value.

Try it yourself

Adapt the function to count only positive values in [-2, 0, 3, 5].

Reveal answer and reasoning

Start count at 0 and increment only when value > 0. The answer is 2.