Why are slices in Python 3 still copies and not views?

Question

As I only now noticed after commenting on this answer, slices in Python 3 return shallow copies of whatever they\'re slicing rather than views. Why is this still the case? E

Answer 1

As well, the fact that you can use assignment to slices to modify the original list, but slices are themselves copies and not views.

Hmm.. that's not quite right; although I can see how you might think that. In other languages, a slice assignment, something like:

a[b:c] = d

is equivalent to

tmp = a.operator[](slice(b, c)) # which returns some sort of reference
tmp.operator=(d)        # which has a special meaning for the reference type.

But in python, the first statement is actually converted to this:

a.__setitem__(slice(b, c), d)

Which is to say that an item assignment is actually specially recognized in python to have a special meaning, separate from item lookup and assignment; they may be unrelated. This is consistent with python as a whole, because python doesn't have concepts like the "lvalues" found in C/C++; There's no way to overload the assignment operator itself; only specific cases when the left side of the assignment is not a plain identifier.

Suppose lists did have views; And you tried to use it:

myView = myList[1:10]
yourList = [1, 2, 3, 4]
myView = yourList

In languages besides python, there might be a way to shove yourList into myList, but in python, since the name myView appears as a bare identifier, it can only mean a variable assignemnt; the view is lost.