Function calls with mutable objects: Python specifics to keep in mind.

fun_call_mutable_obj.py

## lists are mutable in python;
##  list objects are passed by reference to functions and MAY be changed!
def changer(a,b):
  b[0] = 'qq'
  a = 2
X = 1
L = [1,2]
changer(X,L)
X,L # tuple: now is (1, ['qq',2])
## mutable object from enclosing scope was modified by a function.

## to avoid modifying mutables by a function:
##  COPY object instead of passing by reference!
changer(X, L[:])
## L[:] copies list L to a new object rather than passing the original object by reference.