def connect_bd():
    global con
    ...        

def get_from_bd():
    connect_bd()

    with con:
        ...

class A:
    def __init__(self, arg_list=[]):
        self._args = arg_list

    def append(self, item):
        self._args.append(item)

    def iterate(self):
        for item in self._args:
            print(f"{item} ")

def run():
    a = A()
    b = A()
    b.append(1)
    b.append(2)
    a.append(3)
    b.iterate()

run()

W>Лично я над примерно таким довольно долго медитировал (пока не домедитировал до знания, что это типа здесь вполне нормальное поведение)

W>class A:
W>    def __init__(self, arg_list=[]):
W>        self._args = arg_list
W>

W>"Счастливой отладки!" как говорится

def run():
    a = A([])
    b = A([])

$ cat t1.c
#include <stdio.h>
void aaa();
void bbb();
int main() {
  extern int moo;
  moo = 0x12345678;
  printf("%x\n", moo);
  aaa();
  bbb();
}
$ cat t2.c
int moo = 999;
netch@lessie:/tmp>>
$ cat t3.c
void aaa() {
  extern short moo;
  moo = 123;
}
$ cat t4.c
#include <stdio.h>
void bbb() {
   extern int moo;
   printf("%x\n", moo);
}
$ gcc t1.c t2.c t3.c t4.c
$ ./a.out 
12345678
1234007b

Though a bit surprising at first, a moment’s consideration explains this. On one hand, requiring global for assigned variables provides a bar against unintended side-effects. On the other hand, if global was required for all global references, you’d be using global all the time. You’d have to declare as global every reference to a built-in function or to a component of an imported module. This clutter would defeat the usefulness of the global declaration for identifying side-effects.