Is there a way to append to a list WITHOUT modifying the parameter passed?

[volcano63]

If you don't want to modify the original why not tuple?

>>> t = ('one',)
>>> t += ('two',)
>>> t
('one', 'two')
>>> 

Hm! This actually modifies it

No, it creates a new t object

Output:
In [28]: t = ('one',)

In [29]: id(t)
Out[29]: 140190907161512

In [30]: >>> t += ('two',)

In [31]: id(t)
Out[31]: 140190903830408

With lists, += will be equivalent to append (or extend)

Output:
In [34]: id(t)
Out[34]: 140190903120520

In [35]:  t = [1]

In [36]: id(t)
Out[36]: 140190903091912

In [37]: t += [2]

In [38]: id(t)
Out[38]: 140190903091912

In [39]: t
Out[39]: [1, 2]