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

This question already has answers here: How do I clone a list so that it doesn't change unexpectedly after assignment? (23 answers) 开发者_如何学Go Copying nested lists in Python (3 answers) Closed 4 years ago.

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)

---

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.

---

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.