>>> import re
>>> 
>>> def is_number(s):
...     return re.search(r'^\d+$', s) is not None
... 
>>> def f():
...     s = input()
...     if is_number(s):
...         print('ok')
...     else:
...         print('fail')
... 
>>> f()
123
ok
>>> f()
123a
fail
>>>