How Do I Check If A String Is A Negative Number Before Passing It Through Int()?

I'm trying to write something that checks if a string is a number or a negative. If it's a number (positive or negative) it will passed through int(). Unfortunately isdigit() won't

Solution 1:

You can easily remove the characters from the left first, like so:

choice.lstrip('-+').isdigit()

However it would probably be better to handle exceptions from invalid input instead:

print x
whileTrue:
    choice = raw_input("> ")
    try:
        y = int(choice)
        breakexcept ValueError:
        print"Invalid input."
x += y

Solution 2:

Instead of checking if you can convert the input to a number you can just try the conversion and do something else if it fails:

choice = raw_input("> ")

try:
    y = int(choice)
    x += y
except ValueError:
    print"Invalid input."

Solution 3:

You can solve this by using float(str). float should return an ValueError if it's not a number. If you're only dealing with integers you can use int(str)

So instead of doing

if choise.isdigit():
   #operationelse:
   #operation

You can try

try:
    x = float(raw_input)
except ValueError:
    print ("What you entered is not a number")

Feel free to replace float with int, and tell me if it works! I haven't tested it myself.

EDIT: I just saw this on Python's documentation as well (2.7.11) here

Solution 4:

isn't this simpler?

def is_negative_int(value: str) -> bool:
    """
    ref:
        - https://www.kite.com/python/answers/how-to-check-if-a-string-represents-an-integer-in-python#:~:text=To%20check%20for%20positive%20integers,rest%20must%20represent%20an%20integer.
        - https://stackoverflow.com/questions/37472361/how-do-i-check-if-a-string-is-a-negative-number-before-passing-it-through-int
    """if value == "":
        return False
    is_positive_integer: bool = value.isdigit()
    if is_positive_integer:
        return True
    else:
        is_negative_integer: bool = value.startswith("-") and value[1:].isdigit()
        is_integer: bool = is_positive_integer or is_negative_integer
        return is_integer