Can someone explain why the example with integers results in different values for x and y and the example with the list results in x and y being the same object?
x = 42 y = x x = x + 1 print x # 43 print y # 42 x = [ 1, 2, 3 ] y = x x[0] = 4 print x # [4, 2, 3] print y # [4, 2, 3] x is y # True Because integers are immutable, while list are mutable. You can see from the syntax. In x = x + 1 you are actually assigning a new value to x (it is alone on the LHS). In x[0] = 4, you're calling the index operator on the list and giving it a parameter - it's actually equivalent to x.__setitem__(0, 4), which is obviously changing the original object, not creating a new one.
x + 1. After the assignment (that's what it is called in the Python documentation) x refers to the new value. And there's a concept of rvalue/lvalue in Python. It's not the same as in other languages, but it is there - that's why the left side of the = has to be a (usually singular) sequence of identifiers, not an expression, in the basic assignment statement. See: docs.python.org/reference/…If you do y = x, y and x are the reference to the same object. But integers are immutable and when you do x + 1, the new integer is created:
>>> x = 1 >>> id(x) 135720760 >>> x += 1 >>> id(x) 135720748 >>> x -= 1 >>> id(x) 135720760 When you have a mutable object (e.g. list, classes defined by yourself), x is changed whenever y is changed, because they point to a single object.
That's because when you have a list or a tuple in python you create a reference to an object. When you say that y = x you reference to the same object with y as x does. So when you edit the object of x y changes with it.
As the previous answers said the code you wrote assigns the same object to different names such aliases. If you want to assign a copy of the original list to the new variable (object actually) use this solution:
>>> x=[1,2,3] >>> y=x[:] #this makes a new list >>> x [1, 2, 3] >>> y [1, 2, 3] >>> x[0]=4 >>> x [4, 2, 3] >>> y [1, 2, 3] 