x = 'foo'
y = x
print x 
# foo
y += 'bar'
print x 
# foo
print y 
# foobar

x = [1, 2, 3]
y = x
print x 
# [1, 2, 3]
y += [3, 2, 1]
print x 
# [1, 2, 3, 3, 2, 1]
print y 
# [1, 2, 3, 3, 2, 1]

def fun(val):
    val = 'bar'
x = 'foo'
print x 
# foo
fun(x)
print x 
# foo

def fun(val):
    val[1]=-1

x = [1, 2, 3]
print x 
# [1, 2, 3]

fun(x)
print x 
# [1, -1, 3]

def fun(val):
    val = [1,2,3]
    val[1] = -2

x = [1, 2, 3]
print x 
# [1, 2, 3]
fun(x)
print x 
# [1,2, 3]