import sys
def add(num1=0, num2=0):
  return int(num1) + int(num2)
def sub(num1=0, num2=0):
  return int(num1) - int(num2)
def main():
  #Assuming our inputs are valid numbers
  print sys.argv
  addition = add(sys.argv[1], sys.argv[2])
  print addition
  subtraction = sub(sys.argv[1], sys.argv[2])
  print subtraction
if __name__ == '__main__':
  main()

import pdb
pdb.set_trace()

import pdb
import sys
def add(num1=0, num2=0):
  return int(num1) + int(num2)
def sub(num1=0, num2=0):
  return int(num1) - int(num2)
def main():
  #Assuming our inputs are valid numbers
  print sys.argv
  pdb.set_trace() # <-- Break point added here
  addition = add(sys.argv[1], sys.argv[2])
  print addition
  subtraction = sub(sys.argv[1], sys.argv[2])
  print subtraction
if __name__ == '__main__':
  main()

python debugger.py 1 2

['debugger.py']
> /Users/someuser/debugger.py(15)main()
-> addition = add(sys.argv[1], sys.argv[2])
(Pdb)

> /Users/someuser/debugger.py(14)main()
-> addition = add(sys.argv[1], sys.argv[2])
(Pdb) n
> /Users/someuser/debugger.py(15)main()
-> print addition

['debugger.py', '1', '2']
> /Users/someuser/debugger.py(14)main()
-> addition = add(sys.argv[1], sys.argv[2])
(Pdb)

-> addition = add(sys.argv[1], sys.argv[2])
(Pdb) p sys.argv
['debugger.py', '1', '2']
(Pdb) p sys.argv[1]
'1'
(Pdb)

(Pdb) s
--Call--
> /Users/someuser/debugger.py(4)add()
-> def add(num1=0, num2=0):
(Pdb) n
> /Users/someuser/debugger.py(5)add()
-> return int(num1) + int(num2)
(Pdb)

['debugger.py', '1', '2']
> /Users/someuser/debugger.py(15)main()
-> addition = add(sys.argv[1], sys.argv[2])
(Pdb)

-> addition = add(sys.argv[1], sys.argv[2])
(Pdb) b 18
Breakpoint 1 at /Users/someuser/debugger.py:18
(Pdb) c
We are in add--
3
> /Users/someuser/debugger.py(18)main()
-> print subtraction
(Pdb) p subtraction
-1
(Pdb)

['debugger.py', '1', '2']
> /Users/someuser/debugger.py(15)main()
-> addition = add(sys.argv[1], sys.argv[2])
(Pdb) l
 10
 11   def main():
 12     #Assuming our inputs are valid numbers
 13     print sys.argv
 14     pdb.set_trace() # <-- Break point added here
 15 ->   addition = add(sys.argv[1], sys.argv[2])
 16     print addition
 17     subtraction = sub(sys.argv[1], sys.argv[2])
 18     print subtraction

['debugger.py', '1', '2']
> /Users/someuser/debugger.py(15)main()
-> addition = add(sys.argv[1], sys.argv[2])
(Pdb) n
We are in add--
> /Users/someuser/debugger.py(16)main()
-> print addition
(Pdb) p addition
3 #<--- addition here is 3
(Pdb) addition = 'this is now string' #<--- We changed the value of additon
(Pdb) n
this is now string #<--- Now when we print it we actually gets it as a string. that we just set above.
> /Users/someuser/debugger.py(17)main()
-> subtraction = sub(sys.argv[1], sys.argv[2])

(Pdb) !n=5
(Pdb) p n
5