class Test1(object):
    incr_amount = 1
    def increment(self, num):
        return num + self.incr_amount

    def increment_2(self, num):
        Test1.incr_amount = 2
        return num + self.incr_amount

a = Test1()
b = Test1()

assert a.increment(10) == 11
assert a.increment_2(10) == 12
try:
    assert b.increment(10) == 11
except AssertionError:
    pass
    print('b.increment fails because the incr_amount was set to something else in *a*')


class Test2(object):
    def __init__(self):
        self.incr_amount = 1

    def increment(self, num):
        return num + self.incr_amount

    def increment_2(self, num):
        self.incr_amount = 2
        return num + self.incr_amount

a = Test2()
b = Test2()

assert a.increment(10) == 11
assert a.increment_2(10) == 12
assert b.increment(10) == 11  # This now works because the separate instances have different values

try:
    assert a.increment(10) == 11
except AssertionError:
    pass
    print('a.increment fails, though, because a.increment_2 set the incr_amount to something else')

# These highlight the problem with one kind of side effect in terms of bugs in the implementation
# (the increment function needs to set incr_amount before it does any work).  However:
#
# (1) In complex code, these sorts of bugs are harder to see
#
# (2) With threading, the same thing occurs even if the function sets the values because one thread
#     can be running the increment() code while the other thread can be running the increment_2() code
#     which creates a race condition on what self.incr_amount is set to.