dynamic type variable

[J. Peng]

Python's variable is dynamic type,is it?
But why this can't work?

>>> 3 + 'a'
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: unsupported operand type(s) for +: 'int' and 'str'


So I see the number 3 can't be converted to string type automacially.