Exception handling - how to handle invalid datatype in user input?

Question

Being new to programming, I am trying to input name which is a string input. If I enter anything other than a string, error should be displayed as invalid input type. How can this be achieved?

Answer 1

1. Get input from user by raw_input() (Python 2) or input() (Python 3).
2. Type of variable name is string, so we have to use string method to valid user enter string.
3. Use isalpha() string method to check user entered string is valid or not.

code:

name = raw_input("Enter your Last Name:")
if not name.isalpha():
    print "Enter only alpha values."

output:

:~/Desktop/stackoverflow$ python 5.py 
Enter your Last Name:vivek
:~/Desktop/stackoverflow$ python 5.py 
Enter your Last Name:123
Enter only alpha values.
:~/Desktop/stackoverflow$ python 5.py 
Enter your Last Name:vivek 123
Enter only alpha values.

---

Other string methods to check user string is integer or alpha or both

>>> "123".isalnum()
True
>>> "123a".isalnum()
True
>>> "123abc".isalnum()
True
>>> "123abc".isalpha()
False
>>> "123abc".isdigit()
False
>>> "123".isdigit()
True
>>> "123".isalpha()
False

---

By Type conversion and Exception method

e.g. for invalid input:

>>> a = "123a"
>>> try:
...    a = int(a)
... except ValueError:
...    print "User string is not number"
... 
User string is not number

e.g. for valid input:

>>> a = "123"
>>> try:
...    a = int(a)
... except ValueError:
...    print "User string is not number"
... 
>>> print a
123
>>> type(a)
<type 'int'>
>>>

---

Ask user to enter value again and again if user enter invalid value.

code:

while 1:
    try:
        age = int(raw_input("what is your age?: "))
        break
    except ValueError:
        print "Enter only digit."
        continue

print "age:", age

output:

vivek@vivek:~/Desktop/stackoverflow$ python 5.py 
what is your age?: test
Enter only digit.
what is your age?: 123test
Enter only digit.
what is your age?: 24
age: 24

Answer 2

You can write an if statement like this:

if not entry.isalpha():
   print ("Invalid entry!")