How do I stop a Python function from modifying its inputs?

Question

I've asked almost this just before now, but the fix doesn't work for x = [[]], which I'm guessing is because it is a nested list, which is what I will be working with.

def myfunc(w):
 y = w[:]
 y[0].append('What do I need to do to get this to work here?')
 y[0].append('When I search for the manual, I get pointed to python.org, but I can\'t find the answer there.')
 return y

x = [[]]
z = myfunc(x)
print(x)

Please, see http://stackoverflow.com/questions/2541865 – jweyrich Jul 04 '10 at 06:06 — jweyrich, Jul 04 '10 at 06:06

Ben Hayden · Accepted Answer · 2010-07-04T06:10:08.507

Here's how you could fix your problem:

def myfunc(w):
    y = [el[:] for el in w]
    y[0].append('What do I need to do to get this to work here?')
    y[0].append('When I search for the manual, I get pointed to python.org, but I can\'t find the answer there.')
    return y

x = [[]]
z = myfunc(x)
print(x)

The thing about [:] is that it's a shallow copy. You could also import deepcopy from the copy module to achieve the correct result.

score 1 · Answer 2 · answered Jul 04 '10 at 10:32

Use the copy module and to create deep copies of the inputs with the deepcopy function. Modify the copies and not the original inputs.

import copy

def myfunc(w):
    y = copy.deepcopy(w)
    y[0].append('What do I need to do to get this to work here?')
    y[0].append('When I search for the manual, I get pointed to python.org, but I can\'t find the answer there.')
    return y
x = [[]]
z = myfunc(x)
print(x)

Before you use this method, read up about the problems with deepcopy (check the links above) and make sure it's safe.

How do I stop a Python function from modifying its inputs?

2 Answers2