Question

>>> x = 10
>>> y = x
>>> y = 100
>>> y # Changes on y will not be transferred to x
100
>>> x
10

>>> x2 = 'hello'
>>> y2 = x2
>>> y2 = 'world'  # Changes on y2 will not be transferred to x2
>>> y2
'world'
>>> x2
'hello'    

>>> a = [1, 2]
>>> b = a
>>> b.append(3) # Changes on b will be transferred to a
>>> b
[1, 2, 3]
>>> a
[1, 2, 3]

>>> x1 = {}
>>> y1 = x1
>>> y1['h'] = 'hhh'  # Changes on y1 will be transferred to x1
>>> y1
{'h': 'hhh'}
>>> x1
{'h': 'hhh'}

Question> In which cases, the value of the assigned variables will affect the value
of original variables?

Python version:　Python 3.1.2

Answer 1

You should consider that all variables in python are indeed pointers to objects.

Case 1

When you write

x = 10

the variable x is a pointer to a number object with value 10.

y = x

the variable y is a pointer to the same object currently pointed by x.

y = 100

now the variable y is instead pointing to another number object with value 100. This clearly has no effect on the object that x is pointing to.

Case 2

When you write

x = [1, 2]

x is pointing to an array object that contains two pointers to number objects with value 1 and 2.

y = x

y is now pointing to the same array as x

y.append(3)

this doesn’t affect the variable y (this is the key point!), but alter the object it is pointing to (the array) by adding another element. Because x is also pointing to the same object the change will be visible from x too.