All variables in python are pointers
When you assign a value to a variable, you point the variable to a new value
Alternatively, you can modify the underlying object pointed to by the variable
y = x sets y to point to the same object as x
If you modify the underlying object pointed to by y, this also modifies the object pointed to by x
... that's because they're both pointing to the same object!
>>> x = ["a"] >>> y = x >>> y[0] = "b" >>> print x ['b'] >>> print y ['b'] # Since both x and y point to the same object, modifying the object pointed # to by y also modifies the value pointed to by x
If you set y to point to a new object, this does not modify x.
>>> x = ["a"] >>> y = x >>> y = ["b"] >>> print x ['a'] >>> print y ['b'] # As you saw above, assigning y to point to a new object does not # modify the object pointed to by x
Lists are mutable
Thus, you may modify the underlying object if it is a list
You can also set the variable to point to a new list
Strings are immutable
The underlying object cannot be modified if it is a string
>>> x = "abcdefg" >>> y = x >>> y[0] = "h" Traceback (most recent call last): File "", line 1, in ? TypeError: object doesn't support item assignment
But you can still set the variable to point to a new string
>>> x = "abcdefg" >>> y = "h" + y[1:] >>> print x abcdefg >>> print y hbcdefg
You can always set a variable to point to a value of any type
The same concept applies to elements of lists
>>> x = "abcdefg" >>> print x abcdefg >>> x = ["a"] >>> print x ["a"] >>> x[0] = [["b"]] >>> print x [[["b"]]] >>> x[0] = "c" >>> print x ["c"]