Python references

Question

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?

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Answer 1

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.