문자열의 구성이 모두 숫자인지 확인하는 메소드
isdigit()
>>> a = '123'
>>> a.isdigit()
True
>>> b = 'a123'
>>> b.isdigit()
False
>>> Ex1 = '010-1234-5678'
>>> Ex2 = '123456'
>>> Ex3 = "R4R3"
>>> print(Ex1.isdigit())
False
>>> print(Ex2.isdigit())
True
>>> print(Ex3.isdigit())
False