How to get the length of an itertools.product?

问题:

I am using itertools to run a numerical simulation iterating over all possible combinations of my input parameters. In the example below, I have two parameters and six possible combinations:

import itertools

x = [0, 1]
y = [100, 200, 300]

myprod = itertools.product(x, y)

for p in myprod:
    print p[0], p[1]
    # run myfunction using p[0] as the value of x and p[1] as the value of y

How can I get the size of myprod (six, in the example)? I'd need to print this before the for loop starts.

I understand myprod is not a list. I can calculate len(list(myprod)), but this consumes the iterator so the for loop no longer works.

I tried:

myprod2=copy.deepcopy(myprod)
mylength = len(list(myprod2))

but this doesn't work, either. I could do:

myprod2=itertools.product(x,y)
mylength = len(list(myprod2))

but it's hardly elegant and pythonic!

回答1:

To implement Kevin's answer for an arbitrary number of source iterables, combining reduce and mul:

>>> import functools, itertools, operator
>>> iters = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
>>> functools.reduce(operator.mul, map(len, iters), 1)
27
>>> len(list(itertools.product(*iters)))
27

Note that this will not work if your source iterables are themselves iterators, rather than sequences, for the same reason your initial attempts to get the length of the itertools.product failed. Python generally and itertools specifically can work in a memory-efficient way with iterators of any length (including infinite!) so finding out lengths up-front isn't really a case it was designed to deal with.

回答2:

How about:

mylength = len(x) * len(y)

回答3:

Alternative solution I used:

import itertools

param = (('a', 'b'), (1, 2)) # a list of lists

# Calculate all combinations
combinations = itertools.product(*param)

# Calculate number of combinations
total_combinations = 1
for i in param:
    total_combinations = total_combinations * len(i)

回答4:

While this doesn't answer the question directly, very often we want to find the length of generators to estimate the progress/runtime.

For this, do consider using tqdm's (version >= 4.42.0) wrappers around generator functions that don't forget the lengths of iterators (tqdm is a progressbar library). E.g.,

from tqdm.contrib.itertools import product
from time import sleep
for i, j in product(range(3), range(4)):
    sleep(1)

will show a progress bar. The length of the product is shown as the total of the tqdm object (e.g.., the 6 in 3/6 [00:03<00:03] shown).