Why does Python sometimes not create new objects for function arguments? -

i assumed whenever passed object function, automatically created separate copy you modify without changing original. however, following code terminates error every time.

>>> testvar = ['a' in range(100000)] >>> def func(arg):         while arg testvar:             arg.pop() >>> func(testvar) traceback (most recent call last):   file "<pyshell#47>", line 1, in <module>     thing(m)   file "<pyshell#43>", line 3, in thing     obj.pop() indexerror: pop empty list 

from: https://docs.python.org/3.4/library/copy.html

assignment statements in python not copy objects, create bindings between target , object. collections mutable or contain mutable items, copy needed 1 can change 1 copy without changing other. module provides generic shallow , deep copy operations (explained below).

when calling function, doing following:

func(arg=testvar)

if want have actual copy of data modify function following:

def func(arg):     arg_copy = arg[:]     # mutate arg_copy here without mutating arg