Returning the product of a list

Without using lambda:

from operator import mul
# from functools import reduce # python3 compatibility
reduce(mul, list, 1)

it is better and faster. With python 2.7.5

from operator import mul
import numpy as np
import numexpr as ne
# from functools import reduce # python3 compatibility

a = range(1, 101)
%timeit reduce(lambda x, y: x * y, a)   # (1)
%timeit reduce(mul, a)                  # (2)
%timeit np.prod(a)                      # (3)
%timeit ne.evaluate("prod(a)")          # (4)

In the following configuration:

a = range(1, 101)  # A
a = np.array(a)    # B
a = np.arange(1, 1e4, dtype=int) #C
a = np.arange(1, 1e5, dtype=float) #D

Results with python 2.7.5

       |     1     |     2     |     3     |     4     |
-------+-----------+-----------+-----------+-----------+
 A       20.8 µs     13.3 µs     22.6 µs     39.6 µs     
 B        106 µs     95.3 µs     5.92 µs     26.1 µs
 C       4.34 ms     3.51 ms     16.7 µs     38.9 µs
 D       46.6 ms     38.5 ms      180 µs      216 µs

Result: np.prod is the fastest one, if you use np.array as data structure (18x for small array, 250x for large array)

with python 3.3.2:

       |     1     |     2     |     3     |     4     |
-------+-----------+-----------+-----------+-----------+
 A       23.6 µs     12.3 µs     68.6 µs     84.9 µs     
 B        133 µs      107 µs     7.42 µs     27.5 µs
 C       4.79 ms     3.74 ms     18.6 µs     40.9 µs
 D       48.4 ms     36.8 ms      187 µs      214 µs

Is python 3 slower?

from functools import reduce

a = [1, 2, 3]
reduce(lambda x, y: x * y, a, 1)

Starting Python 3.8, a prod function has been included to the math module in the standard library:

math.prod(iterable, *, start=1)

which returns the product of a start value (default: 1) times an iterable of numbers:

import math

math.prod([2, 3, 4]) # 24

Note that if the iterable is empty, this will produce 1 (or the start value if provided).

import operator
reduce(operator.mul, list, 1)

I remember some long discussions on comp.lang.python (sorry, too lazy to produce pointers now) which concluded that your original product() definition is the most Pythonic.

Note that the proposal is not to write a for loop every time you want to do it, but to write a function once (per type of reduction) and call it as needed! Calling reduction functions is very Pythonic - it works sweetly with generator expressions, and since the sucessful introduction of sum(), Python keeps growing more and more builtin reduction functions - any() and all() are the latest additions...

This conclusion is kinda official - reduce() was removed from builtins in Python 3.0, saying:

"Use functools.reduce() if you really need it; however, 99 percent of the time an explicit for loop is more readable."

See also The fate of reduce() in Python 3000 for a supporting quote from Guido (and some less supporting comments by Lispers that read that blog).

P.S. if by chance you need product() for combinatorics, see math.factorial() (new 2.6).

I've tested various solutions with perfplot (a small project of mine) and found that

numpy.prod(lst)

is by far the fastest solution (if the list isn't very short).

Code to reproduce the plot:

import perfplot
import numpy

import math
from operator import mul
from functools import reduce

from itertools import accumulate


def reduce_lambda(lst):
    return reduce(lambda x, y: x * y, lst)


def reduce_mul(lst):
    return reduce(mul, lst)


def forloop(lst):
    r = 1
    for x in lst:
        r *= x
    return r


def numpy_prod(lst):
    return numpy.prod(lst)


def math_prod(lst):
    return math.prod(lst)


def itertools_accumulate(lst):
    for value in accumulate(lst, mul):
        pass
    return value


b = perfplot.bench(
    setup=numpy.random.rand,
    kernels=[
        reduce_lambda,
        reduce_mul,
        forloop,
        numpy_prod,
        itertools_accumulate,
        math_prod,
    ],
    n_range=[2 ** k for k in range(20)],
    xlabel="len(a)",
)
b.save("out.png")
b.show()

One option is to use numba and the @jit or @njit decorator. I also made one or two little tweaks to your code (at least in Python 3, "list" is a keyword that shouldn't be used for a variable name):

@njit
def njit_product(lst):
    p = lst[0]  # first element
    for i in lst[1:]:  # loop over remaining elements
        p *= i
    return p

For timing purposes, you need to run once to compile the function first using numba. In general, the function will be compiled the first time it is called, and then called from memory after that (faster).

njit_product([1, 2])  # execute once to compile

Now when you execute your code, it will run with the compiled version of the function. I timed them using a Jupyter notebook and the %timeit magic function:

product(b)  # yours
# 32.7 µs ± 510 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each)

njit_product(b)
# 92.9 µs ± 392 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each)

Note that on my machine, running Python 3.5, the native Python for loop was actually the fastest. There may be a trick here when it comes to measuring numba-decorated performance with Jupyter notebooks and the %timeit magic function. I am not sure that the timings above are correct, so I recommend trying it out on your system and seeing if numba gives you a performance boost.

The fastest way I found was, using while:

mysetup = '''
import numpy as np
from find_intervals import return_intersections 
'''

# code snippet whose execution time is to be measured
mycode = '''

x = [4,5,6,7,8,9,10]
prod = 1
i = 0
while True:
    prod = prod * x[i]
    i = i + 1
    if i == len(x):
        break
'''

# timeit statement for while:
print("using while : ",
timeit.timeit(setup=mysetup,
              stmt=mycode))

# timeit statement for mul:
print("using mul : ",
    timeit.timeit('from functools import reduce;
    from operator import mul;
    c = reduce(mul, [4,5,6,7,8,9,10])'))

# timeit statement for mul:
print("using lambda : ",      
    timeit.timeit('from functools import reduce;
    from operator import mul;
    c = reduce(lambda x, y: x * y, [4,5,6,7,8,9,10])'))

and the timings are:

>>> using while : 0.8887967770060641

>>> using mul : 2.0838719510065857

>>> using lambda : 2.4227715369997895

Python 3 result for the OP's tests: (best of 3 for each)

with lambda: 18.978000981995137
without lambda: 8.110567473006085
for loop: 10.795806062000338
with lambda (no 0): 26.612515013999655
without lambda (no 0): 14.704098362999503
for loop (no 0): 14.93075215499266

I'm not sure about the fastest way, but here is the short code to get product of any collection without importing any library or module.

eval('*'.join(map(str,l)))

Here is the code:

product = 1 # Set product to 1 because when you multiply it you don't want you answer to always be 0
my_list = list(input("Type in a list: ").split(", ")) # When input, the data is a string, so you need to convert it into a list and split it to make it a list.
for i in range(0, len(my_list)):
  product *= int(my_list[i])
print("The product of all elements in your list is: ", product)