I've been looking through a tutorial and book but I can find no mention of a built in product function i.e. of the same type as sum(), but I could not find anything such as prod().
Is the only way I could find the product of items in a list by importing the mul() operator?
4 Answers
Pronouncement
Yes, that's right. Guido rejected the idea for a built-in prod() function because he thought it was rarely needed.
Python 3.8 Update
In Python 3.8, prod() was added to the math module:
>>> from math import prod >>> prod(range(1, 11)) 3628800 Alternative with reduce()
As you suggested, it is not hard to make your own using reduce() and operator.mul():
def prod(iterable): return reduce(operator.mul, iterable, 1) >>> prod(range(1, 5)) 24 In Python 3, the reduce() function was moved to the functools module, so you would need to add:
from functools import reduce Specific case: Factorials
As a side note, the primary motivating use case for prod() is to compute factorials. We already have support for that in the math module:
>>> import math >>> math.factorial(10) 3628800 Alternative with logarithms
If your data consists of floats, you can compute a product using sum() with exponents and logarithms:
>>> from math import log, exp >>> data = [1.2, 1.5, 2.5, 0.9, 14.2, 3.8] >>> exp(sum(map(log, data))) 218.53799999999993 >>> 1.2 * 1.5 * 2.5 * 0.9 * 14.2 * 3.8 218.53799999999998 There is no product in Python, but you can define it as
def product(iterable): return reduce(operator.mul, iterable, 1) Or, if you have NumPy, use numpy.product.
Since the reduce() function has been moved to the module functools python 3.0, you have to take a different approach.
You can use functools.reduce() to access the function:
product = functools.reduce(operator.mul, iterable, 1) Or, if you want to follow the spirit of the python-team (which removed reduce() because they think for would be more readable), do it with a loop:
product = 1 for x in iterable: product *= x from numpy import multiply, product list1 = [2,2,2] list2 = [2,2,2] mult = 3 prod_of_lists = multiply(list1,list2) >>>[4,4,4] prod_of_list_by_mult = multiply(list1,mult) >>>[6,6,6] prod_of_single_array = product(list1) >>>8 numpy has many really cool functions for lists!
2
