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# Python - Command Line Arguments

# Accessing Command Line Arguments:

Python provides a getopt module that helps you parse command-line options and arguments.

    $ python test.py arg1 arg2 arg3

The Python sys module provides access to any command-line arguments via the sys.argv. This serves two purpose:

- sys.argv is the list of command-line arguments.

- len(sys.argv) is the number of command-line arguments.

Here sys.argv[0] is the program ie. script name.

## Example:

Consider the following script test.py:

    #!/usr/bin/python

    import sys

    print 'Number of arguments:', len(sys.argv), 'arguments.'
    print 'Argument List:', str(sys.argv)

Now run above script as follows:

    $ python test.py arg1 arg2 arg3

This will produce following result:

    Number of arguments: 4 arguments.
    Argument List: ['test.py', 'arg1', 'arg2', 'arg3']

NOTE: As mentioned above, first argument is always script name and it is also being counted in number of arguments.

# Parsing Command Line Arguments:

Python provided a getopt module that helps you parse command-line options and arguments. This module provides two functions and an exception to enable command line argument parsing. This tutorial would discuss about one method and one exception which are sufficient for your programming requirements

## getopt.getopt method:

This method parses command line options and parameter list. Following is simple syntax for this method:

    getopt.getopt(args, options[, long_options])

Here is the detail of the parameters:

- args : This is the argument list to be parsed.

- options: This is the string of option letters that the script wants to recognize, with options that require an argument should be followed by a colon (:).

- long_options: This is optional parameter and if specified, must be a list of strings with the names of the long options which should be supported. Long options which require an argument should be followed by an equal sign ('='). To accept only long options, options should be an empty string.

This method returns value consists of two elements: the first is a list of (option, value) pairs. The second is the list of program arguments left after the option list was stripped.

Each option-and-value pair returned has the option as its first element, prefixed with a hyphen for short options (e.g., '-x') or two hyphens for long options (e.g., '--long-option').

## exception getopt.GetoptError:

This is raised when an unrecognized option is found in the argument list or when an option requiring an argument is given none.

The argument to the exception is a string indicating the cause of the error. The attributes msg and opt give the error message and related option

## Example:

Consider we want to pass two file names through command line, and we also want to give an option to check the usage of the script. Usage of the script is as follows:

    usage: test.py -i <inputfile> -o <outputfile>

Here is the following script to test.py:

    #!/usr/bin/python

    import sys, getopt

    def main(argv):
       inputfile = ''
       outputfile = ''
       try:
          opts, args = getopt.getopt(argv,"hi:o:",["ifile=","ofile="])
       except getopt.GetoptError:
          print 'test.py -i <inputfile> -o <outputfile>'
          sys.exit(2)
       for opt, arg in opts:
          if opt == '-h':
             print 'test.py -i <inputfile> -o <outputfile>'
             sys.exit()
          elif opt in ("-i", "--ifile"):
             inputfile = arg
          elif opt in ("-o", "--ofile"):
             outputfile = arg
       print 'Input file is "', inputfile
       print 'Output file is "', outputfile

    if __name__ == "__main__":
       main(sys.argv[1:])

Now run above script as follows:

    $ test.py -h
    usage: test.py -i <inputfile> -o <outputfile>

    $ test.py -i BMP -o
    usage: test.py -i <inputfile> -o <outputfile>

    $ test.py -i inputfile
    Input file is " inputfile
    Output file is "

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