python: unintentionally modifying parameters passed into a function

Question

A few times I accidentally modified the input to a function. Since Python has no constant references, I\'m wondering what coding techniques might help me avoid making this mista

Answer 1

You can use a metaclass as follows:

import copy, new
class MakeACopyOfConstructorArguments(type):

    def __new__(cls, name, bases, dct):
        rv = type.__new__(cls, name, bases, dct)

        old_init = dct.get("__init__")
        if old_init is not None:
            cls.__old_init = old_init
            def new_init(self, *a, **kw):
                a = copy.deepcopy(a)
                kw = copy.deepcopy(kw)
                cls.__old_init(self, *a, **kw)

        rv.__init__ = new.instancemethod(new_init, rv, cls)
        return rv

class Test(object):
    __metaclass__ = MakeACopyOfConstructorArguments

    def __init__(self, li):
        li[0]=3
        print li


li = range(3)
print li
t = Test(li)
print li