Command line Parsing using python.

• Intention:
  • To create a python program which can be used as command line
  • To run python program we use syntax python <file.py>
  • In this case we would like to run our program on linux machine by using file.py
• Scenario:
  • Build a calculator for additions add.py 1 2 3
• Special variable in python:
  • __name__: This variable will have value of __main__ if called from command line

Parsing Arguments using sys.argv

• sys.argv will return a list with [‘filename.py’, ‘arg0’, ‘arg1’…..]
• To get only positional arguments using python list slicing sys.argv[1::]
• Sample

#!/usr/bin/python
import sys

# calc.py add/mul  10 20 30

def multiply(args):
    result = 1
    for argument in args:
        result = result * int(argument)
    return result

def addition(args):
    result = 0
    for argument in args:
        result = result + int(argument)
    return result


if __name__ == '__main__':
    operation = sys.argv[1]
    if operation == 'add':
        print(addition(sys.argv[2::]))
    elif operation == 'mul':
        print(multiply(sys.argv[2::]))
    else:
        print("invalid arguments enter calc.py add args or calc.py mul args")
        exit(1)

Classes and Objects

• Object:
  • Capabilities:
    • What it can do
  • Contents:
    • What it has
• Examples:
  • Laptop:
    • Capabilities: Browsing, LoadOs, InstallSofware
    • Contents: Keyboard, RAM, Processor …..
  • BankAccount:
    • Capabilities: Deposit, Withdraw, Open, Close
    • Contents: Accountno, Bank name, Branch…
• Capabilities will become Methods
• Contents will become Members

Exercise:

• Write a python code to set and get environment variables.

envr.py set JAVA_HOME  /opt/lib/jav/home

envr.py get PATH

envr.py get-all

• Refer python module os