Consider the following minimal example:
class MyClass():
def __init__(self, length):
self._list = [[] for _ in range(length)]
def __setitem__(self, key, value):
self._list[key] = value.copy()
def __getitem__(self, key):
return self._list[key]
Now, the Python documentation says:
Assignment statements in Python do not copy objects, they create bindings between a target and an object.
Applying the class above would mean something like:
>>> l = [[1,2,3,4], [3,2,1,0]]
>>> C = MyClass(2)
>>> C[0] = l[0]
This is indeed an assignment statement. But the values are copies now:
>>> C[0] == l[0]
True
>>> C[0] is l[0]
False
Is that an acceptable behavior for a class in Python?
There is nothing inherently wrong with this. As long as your class is well documented, if you have a good reason to do this go for it. Consider the following class from the standard library shelve module, which essentially provides a dict-like database interface. Since using object[key] = value will write to disk, and retrieving that object will retrieve it from disk, it won't give you the same object (necessarily, there are caching options)! So:
In [1]: import shelve
In [2]: data = [[1,2,3],[4,5,6]]
In [3]: database = shelve.open('db')
In [4]: database['key'] = data[0]
In [5]: database['key']
Out[5]: [1, 2, 3]
In [6]: database['key'] is data[0]
Out[6]: False
That part of the documentation is referring to assignment statements to a variable, like this:
some_var = foo
Where indeed, it never copies. And you can't really change that (well, maybe, but that's another question). You are free to implement __setitem__ and __getitem__ however you wish.