Aggregate numeric values
Compute a total from values held in a list, tuple, or other iterable.
Python · Function
sum
sum() adds the items of an iterable from left to right, starting with an optional initial value. It returns the resulting total and is intended primarily for numeric items.
How it works
sum() consumes an iterable and combines its items from left to right, beginning with start, which defaults to 0. It returns the resulting total; the items are normally numeric and start must not be a string.
Syntax
sum(iterable, /, *, start=0)Parameters
| Name | Type | Description |
|---|---|---|
iterable | iterable | An iterable whose items are added. |
start | object | Initial value added before the iterable's items; defaults to 0 and cannot be a string. |
Returns
The total of start and the iterable's items.
Key points
- The calculation begins with start and incorporates each item in iteration order.
- Use sum() primarily for iterables containing numeric values.
- A string cannot be used as the initial value.
Common mistakes
- Do not pass a string as start; the documented function rejects that type of initial value.
Example: Sum values with an initial total
numbers = [1, 2, 3]
total = sum(numbers, start=10)
print(total) # 16Adds the iterable's values from left to right, beginning with the supplied starting value.
When to use it
Add to an initial total
Begin an accumulation with an existing offset or subtotal.
Sum an iterable
Consume generated values without first materializing them into a collection.
Interview questions
QWhat does Python's sum() do?
sum() accepts an iterable and an optional keyword-only start value that defaults to 0. It adds the start value and iterable items from left to right, then returns the total.
Follow-up: What does sum([], start=10) return? It returns 10 because the empty iterable contributes no items and the initial value remains.
QWhat kinds of inputs does sum() expect?
The iterable's items are normally numbers, so numeric values can be accumulated directly. The function processes them in iteration order rather than requiring a sequence.
Follow-up: Can a generator be passed? Yes: because the parameter is an iterable, a generator can supply the items as it is consumed.
QHow does the start argument affect the result?
start is optional and defaults to 0, and it must not be a string. Supplying another suitable initial value lets the total begin from a nonzero value.
Follow-up: Why is an explicit start useful? It lets the caller include an initial offset without separately adding it after the iterable is summed.
Related
math.fsum()iter()