python - Pass list to function by value

Question

I want to pass a list into function by value. By default, lists and other complex objects passed to function by reference. Here is some desision:

def add_at_rank(ad, rank):
    result_ = copy.copy(ad)
    .. do something with result_
    return result_

Can this be written shorter? In other words, I wanna not to change ad.

Answer 1

You can use [:], but for list containing lists(or other mutable objects) you should go for copy.deepcopy():

lis[:] is equivalent to list(lis) or copy.copy(lis), and returns a shallow copy of the list.

In [33]: def func(lis):
    print id(lis)
   ....:     

In [34]: lis = [1,2,3]

In [35]: id(lis)
Out[35]: 158354604

In [36]: func(lis[:])
158065836

When to use deepcopy():

In [41]: lis = [range(3), list('abc')]

In [42]: id(lis)
Out[42]: 158066124

In [44]: lis1=lis[:]

In [45]: id(lis1)
Out[45]: 158499244  # different than lis, but the inner lists are still same

In [46]: [id(x) for x in lis1] = =[id(y) for y in lis]
Out[46]: True

In [47]: lis2 = copy.deepcopy(lis)  

In [48]: [id(x) for x in lis2] == [id(y) for y in lis]  
Out[48]: False