Why can I not get the length in itertool.product? Python 3.6

Question

I read the other post to get the length of itertool.product() however the OP's question was never answer. I don't understand why this len() error is occuring. Here is the code:

from PIL import Image
import itertools

def function():
    rgb1 = [r,g,b]
    return itertools.product(rgb1, repeat=3)    #I added this

img = Image.open("anyImage.jpg")
r, g, b = img.split()

cartesianList = itertools.product([r,g,b], repeat=3)

for set in cartesianList:
    new_img = Image.merge("RGB", cartesianList)
    new_img.show()

Output is:

Traceback (most recent call last):
  File "C:/Users/Trevor/PycharmProjects/Pillow Test/test3.py", line 14, in <module>
    new_img = Image.merge("RGB", cartesianList)
  File "C:\Users\Trevor\AppData\Roaming\Python\Python36\site-packages\PIL\Image.py", line 2608, in merge
    if getmodebands(mode) != len(bands) or "*" in mode:
TypeError: object of type 'itertools.product' has no len()

Answer 1

for set in cartesianList:
    new_img = Image.merge("RGB", cartesianList)

Pretty sure you meant Image.merge("RGB", set) here

---

longer answer for the sake of future visitors:

Q: Why can I not get the length of an itertools.product?
A: most itertools functions return generators, which don't have a length because they are lazily evaluated (and potentially infinite).

Q: Why do I get an itertools.product error when I pass cartesianProduct into the Image.merge?
A: PIL checks that you gave it 3 channels when you specify Image.merge("RGB", [foo]), so it calls len(foo). While this works for each set in cartesianList obviously it doesn't work for cartesianList