How to keep variables fixed within a while loop

Question

I have two variables (variable_1 and variable_2). They are outputs of an algorithm and they are always different, since the algorithm contains some random part.

And I have a very long and complex function afterwards, that takes these variables as inputs. Its basic structure is:

def function(variable_1, variable_2):
    switch = True
    while switch:
      variable_1
      variable_2
      inner_function(variable_1, variable_2):
         ~changes variable_1 and variable_2 randomly~
      ~changed variable_1 and variable_2 are then transformed with data structure comprehensions.~
      ~in the end, there is a condition. If variable_1 and variable_2 meet this condition, switch is turned to False and the function ends. If not, the while loop shall start again, but with the original values of variable_1 and variable_2.~

The intention of the function is to output the changed variables. The problem is that it does not work. If the while loop runs through one iteration, and the switch is still True at the end of this iteration, variable_1 and variable_2 are not set back to their original values. How can I do this? (Please keep in mind that I do not want to fix variable_1 and variable_2 for my entire code that comes before or afterwards).

Sorry for not giving a minimally reproduceable example. I could not come up with one, considering the length and complexity of the function.

Edit: If I hardcode the variables (meaning that I write variable_1 = "its value" and variable_2 = "its value" above the inner function, it works. But I do not want to do this.

Are you saying that `inner_function` is defined inside `function()` and so has access to `variable_1` and `variable_2` to be able to change them? — quamrana, Aug 17 '21 at 20:47
seriously? store them in other variables and reset at beginning of loop — Garr Godfrey, Aug 17 '21 at 20:47
Then that other function cannot change `variable_1` and `variable_2`. — quamrana, Aug 17 '21 at 20:50
@Garr Godfrey I tried this. It did not work. Unless there is a reset command that I do not know. — lonyen11, Aug 17 '21 at 20:50
@quamrana What would be a solution or workaround for this problem then? — lonyen11, Aug 17 '21 at 20:52
@quamrana: the function cannot assign other stuff to these names, but if the objects pointed to by these names are mutable (eg, lists), the `inner_fucntion` can of course modify them — H. Doebler, Aug 17 '21 at 20:53
This is not the actual code. You should provide a minimal example of real code that exhibits the problem you're facing. — jarmod, Aug 17 '21 at 20:54
So, these `variable_1` cannot be `int`s or `str`s. What are they? — quamrana, Aug 17 '21 at 20:54
@quamrana Sorry for not mentioning this. They are both lists of lists, storing strings inside. — lonyen11, Aug 17 '21 at 20:55
Make a deep copies of the lists when passing them to the function. — Barmar, Aug 17 '21 at 20:56
@Prophet Yes. It is similar to what quamrana has posted. His approach is a little bit more straightforward though. And it works for my case. — lonyen11, Aug 17 '21 at 21:17

score 1 · Accepted Answer · answered Aug 17 '21 at 20:57

So you just need to make a deepcopy:

import copy

def function(variable_1o, variable_2o):
    switch = True
    while switch:
      variable_1 = copy.deepcopy(variable_1o)
      variable_2 = copy.deepcopy(variable_2o)
      inner_function(variable_1, variable_2)

Prophet · Answer 2 · 2021-08-17T21:01:19.373

Here you actually asking: "How to pass variables by value, not by reference".
One of approaches to do this is to put the original variables to the list, then to create another list from that list, and then to pass the second list as a parameter to the function. This will imitate passing variables by value, not by reference.
Like following:

def function(variable_1, variable_2):
original_list = [variable_1, variable_2]
    switch = True
    while switch:
      copy_list = original_list[:]      
      inner_function(copy_list):
      #here you can use, change etc the data in copy_list safely
         ~changes variable_1 and variable_2 randomly~
      ~changed variable_1 and variable_2 are then transformed with data structure comprehensions.~
      ~in the end, there is a condition. If variable_1 and variable_2 meet this condition, switch is turned to False and the function ends. If not, the while loop shall start again, but with the original values of variable_1 and variable_2.~

How to keep variables fixed within a while loop

2 Answers2