Python handling the name error exception

Question

I wrote a small code and tried to handle the name error exception. I want to print a custom message even if there is an exception, but it is showing the complete the trace back.

#!/usr/bin/python -tt

import logging

def equaldigits(a, b):
    logging.basicConfig(filename='run.log',level=logging.INFO)
    try:
      c = a - b
      logging.info('%s is the difference between both the digits', str(c))
      print c
      return c
    except NameError as e:
      c = 'Unable to successfully complete execution'
      logging.info(c)
      print c
      #return c

def main():
  print '\n1st call'
  equaldigits(10, 10)
  print '\n2nd call'
  equaldigits(1, 0)
  print '\nException call'
  equaldigits(a, 0)

# Standard boilerplate to call the main() function.
if __name__ == '__main__':
  main()

This is the console output

1st call
0

2nd call
1

Exception call
Traceback (most recent call last):
  File "./sut.py", line 28, in <module>
    main()
  File "./sut.py", line 24, in main
    equaldigits(a, 0)
NameError: global name 'a' is not defined

Answer 1

In your attempt to catch an exception, you wrote equaldigits(a, 0). The interpreter sees the a and thinks it is a variable that isn't there, thus throwing the uncaught exception. In order to test your try/catch, you need to pass the letter a, like so

equaldigits('a', 0)
            ^ ^ note the quotes

Answer 2

The problem isn't happening within your equaldigits function where you have your logging information.

Its happening in your main function when the interpreter tries to pass the value of a to equaldigits. The variable a doesn't exist within the local scope of main, thus Python tries to find a global variable named a. Since it doesn't see one, it throws a NameError.

Answer 3

Your error is caused by the fact that a is not defined when you call equaldigits, the execution doesn't get to the try/except clause inside the function.

when you change

a - b 

to

a - d

inside the function you'll see that your try/except works fine