How to check if string input is a number?

Apparently this will not work for negative values, but it will for positive numbers.

Use isdigit()

if userinput.isdigit():
    #do stuff

Simply try converting it to an int and then bailing out if it doesn't work.

try:
    val = int(userInput)
except ValueError:
    print("That's not an int!")

See Handling Exceptions in the official tutorial.

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