Question

I want to keep astructure saved over iterations of a function, in order to keep data to use it for later purpose.
There is a simple way to do it, in using a global variable.

The thing is, anyone could access the variable.
So, is there a way to create a variable accessing only by the function, but that would not be erased when running this function?

Ex :

mylist = []

def test():
    global mylist

    if mylist:
        my_list.append(1)

    other_stuff, using my_list

Is exactly what I want to do, but my_list could be accessed by anyone else.

(ok I know, the example is completely dumb)

Sorry for the formulation, but I could not come with something simpler

EDIT : Ok, so with so different (and all interesting) solutions. I’ll quickly explain the idea of what I want to do 🙂

Let’s imagine I want to calculate is a number is a prime. This is usually computationnaly coslty.

So I got a is_prime(value) function, returning False or True.

value can be any number, but there are chances (in my case) that value takes several times the same value (litteraly ^^).

In this case, I could use a (not to long ) list of primes I have already found to quickly check and save computations.
But my prime_list is no use in a function returning true/false.

So here is my question :).

Hope to clarify some minds (including me! ).

Answer 1

Here is a case where a list as the default value of a parameter can come in handy:

def test(myList=[]):

    myList.append(1)
    other_stuff, using myList

I leave the if myList off, since it will exist (it was created when test was defined). (Plus, if myList tests if the list is non-empty, not defined).

myList, as a function parameter, is local to test. When test is defined, it is initialized to a list. The value of the list persists for the life of your program.

Compare to the standard advice of

def test(myList = None):
    if myList is None:
        myList = []

which is necessary when you want a fresh empty list inside test at each call if no list is provided by the call.