Python generator object getting modified after being saved

Question

I'm iterating through and want to save generator objects as I go along.

In the example below, I want the variable "gen" (the generator returned) to return "Tom" and "Tommy", but I find that when I mutate the original list the generator was called on, it also changes my generator object.

Desired behavior would be to save these generator objects as I go along, without having them being modified.

I understand that a list is mutable, but if this was run with a function, it would save the output to the variable, and that output could NOT be changed.

(for more context, I'm building a dictionary of generators where point-in-time matters, I chose a generator because the computation is pretty heavy and want to save it until the end)

# simple class
class Name:
    def __init__(self, name):
        self.name = name

# simple generator
def generator_function(lst):
    for obj in lst:
        yield obj.name

lst = [Name("Tom"), Name("Tommy")]

# create gen object
gen= generator_function(lst)


# mutate list
lst[0] = Name("Andrea")


# >>> Prints out :    "Andrea" , "Tommy"  
# >>> Desired output: "Tom", "Tommy"
for ii in gen:
    print(ii)

Answer 1

As you said yourself, python lists are variable. Therefore, you will have to make a new copy for every generator you create.

This can be achieved easily by modifying your call to generator_function :

gen = generator_function(list(lst))

Answer 2

I'm not personally crazy about hoping the user remembers to copy their list when passing to the generator. In this case, you can return a generator expression:

from typing import List, Iterable

def generator_function(lst: List[Name]) -> Iterable[str]:
    return (obj.name for obj in lst.copy())

I've also added annotations since I think it's important, especially here where your generator is accessing a .name property.