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Sum a list of numbers in Python

Sum a list of numbers in Python

David Y.

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In Python, how do I add all the numbers in a list to get a total?

The Solution

Python provides a built-in `sum` function, which returns the sum of all items in an iterable (such as a list). For example:

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total = sum(numbers)
print(total) # Will print 15 (1 + 2 + 3 + 4 + 5)
```

`sum` is an example of a fold (also called reduce), which is a type of higher-order functions that produces a single result from a sequence of items. It does this by applying a function (in this case, addition) to the first two items (1 + 2), getting a result (3) and then applying the same function to that result and the next item (3 + 4). This process continues until the list is consumed, and then the final result is returned.

`sum` takes an optional `start` parameter, which provides a first value to use in the first addition. For example:

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```numbers = [1, 2, 3, 4, 5]

total = sum(numbers, 6)
print(total) # Will print 21 (6 + 1 + 2 + 3 + 4 + 5)
```

We can write our own folds using the `reduce` function from Python’s built-in `functools` module. For example, here’s how we might find the product of all numbers in a list:

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```from functools import reduce

numbers = [1, 2, 3, 4, 5]
product = reduce(lambda x, y: x * y, numbers)

print(product)  # Will print 120 (1 * 2 * 3 * 4 * 5)
```

We supply `reduce` with two arguments: a function which takes two arguments and a list. It will then apply this function to the contents of the list in the manner described above, ultimately producing a single result. We’ve used an anonymous lambda function here, but `reduce` will also work with a standard function, as below:

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```from functools import reduce

def multiply(x, y):
return x * y

numbers = [1, 2, 3, 4, 5]
product = reduce(multiply, numbers)

print(product)  # Will print 120 (1 * 2 * 3 * 4 * 5)
```

`reduce` supports an optional `initializer` parameter, which works the same as the `start` parameter in `sum`.

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```from functools import reduce

numbers = [1, 2, 3, 4, 5]
product = reduce(lambda x, y: x * y, numbers, 6)

print(product)  # Will print 720 (6 * 1 * 2 * 3 * 4 * 5)
```
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