User input and command line arguments

Question

How do I have a Python script that a) can accept user input and how do I make it b) read in arguments if run from the command line?

Answer 1

Careful not to use the input function, unless you know what you're doing. Unlike raw_input, input will accept any python expression, so it's kinda like eval

Answer 2

This simple program helps you in understanding how to feed the user input from command line and to show help on passing invalid argument.

import argparse
import sys

try:
     parser = argparse.ArgumentParser()
     parser.add_argument("square", help="display a square of a given number",
                type=int)
    args = parser.parse_args()

    #print the square of user input from cmd line.
    print args.square**2

    #print all the sys argument passed from cmd line including the program name.
    print sys.argv

    #print the second argument passed from cmd line; Note it starts from ZERO
    print sys.argv[1]
except:
    e = sys.exc_info()[0]
    print e

1) To find the square root of 5

C:\Users\Desktop>python -i emp.py 5
25
['emp.py', '5']
5

2) Passing invalid argument other than number

C:\Users\bgh37516\Desktop>python -i emp.py five
usage: emp.py [-h] square
emp.py: error: argument square: invalid int value: 'five'
<type 'exceptions.SystemExit'>

Answer 3

raw_input is no longer available in Python 3.x. But raw_input was renamed input, so the same functionality exists.

input_var = input("Enter something: ")
print ("you entered " + input_var) 

Documentation of the change

Answer 4

To read user input you can try the cmd module for easily creating a mini-command line interpreter (with help texts and autocompletion) and raw_input (input for Python 3+) for reading a line of text from the user.

text = raw_input("prompt")  # Python 2
text = input("prompt")  # Python 3

Command line inputs are in sys.argv. Try this in your script:

import sys
print (sys.argv)

There are two modules for parsing command line options: optparse (deprecated since Python 2.7, use argparse instead) and getopt. If you just want to input files to your script, behold the power of fileinput.

The Python library reference is your friend.

Answer 5

If you are running Python <2.7, you need optparse, which as the doc explains will create an interface to the command line arguments that are called when your application is run.

However, in Python ≥2.7, optparse has been deprecated, and was replaced with the argparse as shown above. A quick example from the docs...

The following code is a Python program that takes a list of integers and produces either the sum or the max:

import argparse

parser = argparse.ArgumentParser(description='Process some integers.')
parser.add_argument('integers', metavar='N', type=int, nargs='+',
                   help='an integer for the accumulator')
parser.add_argument('--sum', dest='accumulate', action='store_const',
                   const=sum, default=max,
                   help='sum the integers (default: find the max)')

args = parser.parse_args()
print args.accumulate(args.integers)

Answer 6

var = raw_input("Please enter something: ")
print "you entered", var

Or for Python 3:

var = input("Please enter something: ")
print("You entered " + str(var))