Checking whether the user input is a str in Python

Question

I checked various questions on Stack Overflow but one thing every logic lacks. Let me demonstrate using Python:

while True:
    user_input = raw_input()
    if type(user_input) == str:
        print 'ERROR'
    else:
        print 'BINGO'

Also, we cannot use input() in place of raw_input() as it gives the error:Traceback (most recent call last):

File ".\test.py", line 3, in <module>
    user_input = int(input())
  File "<string>", line 1, in <module>
NameError: name 'asdf' is not defined

The problem here is that raw_input converts the user input into string so it always prints 'ERROR' and if I change the second line to

user_input = int(raw_input)

then, it gives an error:

Traceback (most recent call last):
  File ".\test.py", line 3, in <module>
    user_input = int(raw_input())
ValueError: invalid literal for int() with base 10: 'asdf'

I tried this with try and except but it shall work fine to check integer but not a string.

Hmmm, how exactly do you want raw_input to know that the next keystrokes by the user will be an integer? Conversely, how do you expect the int function to cast "asdf" into an integer? — logc
– logc, Commented May 27, 2014 at 13:34

Idan Arye · Accepted Answer · 2014-05-27 13:38:31Z

3

You got it all backwards - raw_input does not "convert the user input into string" - the user input was a string to begin with! It's input that converts it to other things.

If you want to use raw_input, you can assume you'll get a string and you need to convert it to int yourself. As you have seen, this fails if the string does not represent an integer, but you can easily catch that error:

while True:
    user_input = raw_input()
    try:
        user_input_as_integer = int(user_input)
        print 'BINGO'
    except ValueError:
        print 'ERROR'

answered May 27, 2014 at 13:38

Idan Arye

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2 Comments

Sahil Babbar Over a year ago

You got one write out of two. Go through this docs.python.org/2/library/functions.html#raw_input

Idan Arye Over a year ago

OK, so it says that raw_input converts the input into a string - but that conversion is not in the way implied from your question. When the user enters the input 10, for example, raw_input does not convert it from the int 10 to the string '10' - it was never an int in the first place. What the documentation means is that raw_input converts the input from a byte array or a buffer or something like that to a Python string object.

Ciasto piekarz · Accepted Answer · 2014-05-28 04:33:12Z

-1

# first store the value to a variable
user_input = raw_input('>')
try:
    if int(user_input):
        print "User entered %d is integer" % int(user_input)
except ValueError: 
    # check whether the user entered string or not
    print "User entered %s is a string" % user_input

The raw_input always stores string whether you enter an integer.

edited May 28, 2014 at 4:33

answered May 27, 2014 at 16:37

Ciasto piekarz

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