Sorry to bother, I've occured a newbiw question.

[perfringo]

I typed "a = int(input())" and press "Enter" to make a new statement, the prompt didn't show up.

Are you sure that prompt didn't show up? As you have no text for input to display it can be easily missed (empty row without >>> at the beginning).

To make it more prominent you should use text:

>>> a = int(input('Please enter integer: '))
Please enter integer: 

As of ValueError while copying: Python interactive interpreter interprets code line by line. This means that after line a = int(input()) it displayed input prompt and then entered into it next line b = int(input()). You can observe it yourself (note line without starting >>>)

>>> a = int()
>>> b = int()
>>> a = int(input())
b = int(input())                      # value entered into prompt
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'b = int(input())'