Beginner, Python, if statement

Question

num=float(raw_input('Enter a number: '))
if num <0:
    print "-"
elif num>0:
    print "+"
else:
    print "0"

this is a simple example from one book. It works fine, but I have a question, how to make that this program would see that I'm entering a letter instead of a number? It works just fine if I'm using numbers, but if I'm entering a letter, the program crashes. I understand that when a program gets a raw input, it needs to check if it is a number or another symbol, but I just don't know what the simplest solution would be?

Answer 1

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Method 1:

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Try to check if input is a number. This will work for both negative and positive numbers, but not for numbers like 1.1, 1.2, etc:

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

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Method 2:

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Using an if-statement alone you could use the isdigit() function. However, this will not work for negative numbers as for example -1 will return False.

if userInput.isdigit():
    #your code here
else:
    Print("Not a digit")

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Method 3:

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Here is another method to check if given string is a number:

import re
num_format = re.compile("^[\-]?[1-9][0-9]*\.?[0-9]+$")
isnumber = re.match(num_format,givennumber)
if isnumber:
    print("Given string is number")
else:
    print("Given string is not a number")

Also, try to include a log with the error next time, and add some more tags for visibility like integer, string and if-statement. :)

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Source: How to check if string input is a number?

Answer 2

num=raw_input('Enter a number: ')
if num is float:
    num = float(num)
    if num <0:
        print "-"
    elif num>0:
        print "+"
    else:
        print "0"
else:
    #do something

use 'is' to judge whether input value is float or not

Answer 3

Use some simple string methods like below:

>>> s = '1234'
>>> s.isdigit()
True
>>>s = 'abd'
>>> s.isdigit()
False
>>> s = 'abc123'
>>> s.isdigit()
False