"Generators" section misleads about the ability to iterate generatiors multiple times

[SyberiaK]

Generators are a powerful tool to save memory and improve performance. In general, they yield one value at a time and can be iterated over multiple times.

That's not true. After the first go-through the generator becomes empty.

L = [n for n in range(42_000)]
print(getsizeof(L))  # 351064 bytes
print(sum(L))  # 881979000

G = (n for n in range(42_000))
print(getsizeof(G))  # 200 bytes
print(sum(G))  # 881979000
print(sum(G))  # 0  # oops!

Also... making this kind of generator from range is kinda a wacky idea. You can just use the range itself, which also acts like a generator and saves some additional memory.
And it can actually be iterated multiple times.

from sys import getsizeof


L = [n for n in range(42_000)]
print(getsizeof(L))  # 351064 bytes
print(sum(L))  # 881979000

G = range(42_000)
print(getsizeof(G))  # 48 bytes
print(sum(G))  # 881979000
print(sum(G))  # 881979000  # yay

[Steven-Willers]

Also... making this kind of generator from range is kinda a wacky idea. You can just use the range itself, which also acts like a generator and saves some additional memory. And it can actually be iterated multiple times.

from sys import getsizeof


L = [n for n in range(42_000)]
print(getsizeof(L))  # 351064 bytes
print(sum(L))  # 881979000

G = range(42_000)
print(getsizeof(G))  # 48 bytes
print(sum(G))  # 881979000
print(sum(G))  # 881979000  # yay

@SyberiaK in Python, range() is often compared to generators due to its low memory usage. While it behaves like a generator in some ways, it is not a true generator.

For instance, in the generator example (given below), when the for loop breaks at i == 3, the generator "remembers" that iteration and continues from 4 when you convert it to a list.

gen = (i for i in range(10))

for i in gen:
    print(i)
    if i == 3:
        break

print(list(gen))  

Output:

0
1
2
3
[4, 5, 6, 7, 8, 9]

However, rangedoes not remember where you left off, leading to a reset-like behavior when evaluated again. This reduces efficiency in tasks like file reading, data collection and many others where on need to keep track.

gen= range(10)

for i in gen:
    print(i)
    if i == 3:
        break

print(list(gen))

Output:

0
1
2
3
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

While range() is memory-efficient, it lacks the advantages of a true generator. It behaves more like a lightweight list than a generator because it can be re-evaluated without remembering its state. One of the best explanation of range is given in this answer of SO.