To put it a different way "id()" is not what you care about. You want to know if the variable name can be modified without harming the source variable name.
>>> a = 'hello'
>>> b = a[:]
>>> c = a
>>> b += ' world'
>>> c += ', bye'
>>> a
'hello'
>>> b
'hello world'
>>> c
'hello, bye'
If you're used to C, then these are like pointer variables except you can't de-reference them to modify what they point at, but id() will tell you where they currently point.
The problem for python programmers comes when you consider deeper structures like lists or dicts:
>>> o={'a': 10}
>>> x=o
>>> y=o.copy()
>>> x['a'] = 20
>>> y['a'] = 30
>>> o
{'a': 20}
>>> x
{'a': 20}
>>> y
{'a': 30}
Here o and x refer to the same dict o['a'] and x['a'], and that dict is "mutable" in the sense that you can change the value for key 'a'. That's why "y" needs to be a copy and y['a'] can refer to something else.