Identifying the data type of an input

Question

I am trying to print the data type of a user input and produce a table like following:

ABCDEFGH = String
1.09 = float
0 = int
true = bool

etc.

I'm using Python 3.2.3 and I know I could use type() to get the type of the data but in Python all user inputs are taken as strings and I don't know how to determine whether the input is a string or Boolean or integer or float. Here is that part of the code:

user_var = input("Please enter something: ")
print("you entered " + user_var)
print(type(user_var))

which always returns str for string.

Faunie

Answer 1

from ast import literal_eval

def get_type(input_data):
    try:
        return type(literal_eval(input_data))
    except (ValueError, SyntaxError):
        # A string, so return str
        return str

print(get_type("1"))        # <class 'int'>
print(get_type("1.2354"))   # <class 'float'>
print(get_type("True"))     # <class 'bool'>
print(get_type("abcd"))     # <class 'str'>

Bartz

only Boolean values are still taken as strings. Any Idea? – Faunie

@ApeironKambyses Do the boolean values have initial capital letters? – Bartz

Answer 2

input() will always return a string. If you want to see if it is possible to be converted to an integer, you should do:

try:
    int_user_var = int(user_var)
except ValueError:
    pass # this is not an integer

You could write a function like this:

def try_convert(s):
    try:
        return int(s)
    except ValueError:
        try:
            return float(s)
        except ValueError:
            try:
                return bool(s)
            except ValueError:
                return s

However, as mentioned in the other answers, using ast.literal_eval would be a more concise solution.

Syd

do I have to use "if statements" to determine every single type and try to convert them to Booleans, floats etc? – Faunie

Yes, you will have to do that. – Syd

bool(s) is True for every non-empty string. Without literal_eval you'd have to use a string comparison for "True" and "False". – Ellingson

Answer 3

Input will always return a string. You need to evaluate the string to get some Python value:

>>> type(eval(raw_input()))
23423
<type 'int'>
>>> type(eval(raw_input()))
"asdas"
<type 'str'>
>>> type(eval(raw_input()))
1.09
<type 'float'>
>>> type(eval(raw_input()))
True
<type 'bool'>

If you want safety (here user can execute arbitrary code), you should use ast.literal_eval:

>>> import ast
>>> type(ast.literal_eval(raw_input()))
342
<type 'int'>
>>> type(ast.literal_eval(raw_input()))
"asd"
<type 'str'>

Ponzo

From the question I'm using python 3.2.3 – Bartz

This is very dangerous if the user input is import os; os.system("rm -rf *"). – Syd

@TomLeese no it's not, you are probably thinking of exec and not eval – Nablus

Ok, you are correct that you would not be able to import os, but you could still run os.system("rm -rf *") if it was already imported. – Syd

@Tom Leese: see my edit. I didn't write it at first because I didn't remember in with module the literal_eval function was. – Ponzo

Thanks everybody. You all have been very helpful – Faunie

eval('__import__("os").listdir(".")'). Don't use eval. Even if you use CPython's ability to set __builtins__, someone can always find a way back to builtins. – Ellingson

Answer 4

The problem here is that any input is taken a 'string'. So we need to treat 'string' as a special case, and keep it separate from everything else.

x = input("Enter something: ")

try:
    if type(eval(x)) == float:
        print(x, " is floating point number")
    elif type(eval(x)) == int:
        print(x, " is interger number")    
    elif type(eval(x)) == bool:
        print(x, " is a boolean")      
except:
    print("That is a string")

Here the input is first evaluated. If it's anything other than a string, the eval function shall show the type. If it's a string, it is considered as an "error", and the error message is given as "That is a string".

Aten