Exercise 1

In the episode on Randomness we discussed how it is important (and saves time!) to use well-tested library routines to generate random numbers. This principle of reuse is also true for any other task you need to carry out. Python is packaged with an extensive library of modules. What module could you use to…

• parse a string containing a date or time (e.g. “Thursday, 27 May, 2010″)? (Answer)
  datetime and in particular, the function datetime.strptime()
• inspect a bunch of files in a folder or subfolders? (Answer)
  One module for doing this is the os.path. It has lots of functions for manipulating path names, and also this gem of a function: os.path.walk.
• manage command line arguments to your program? (Answer)
  argparse
• access data on the web? (Answer)
  For very basic operations like downloading files, check out: urllib

Exercise 2

In this lecture we wrote roughly the following snippet to parse the command line arguments.

import sys

def fail(message):
 print message
 sys.exit(1)

def parse_arguments(arguments):
  '''Parse strings to get controlling parameters.'''

  try:
    grid_size   = int(arguments[0])
    value_range = int(arguments[1])
    random_seed = int(arguments[2])
  except IndexError:
    fail('Expected 3 arguments, got %d' % len(arguments))
  except ValueError:
    fail('Expected int arguments, got %s' % str(arguments))

  return grid_size, value_range, random_seed

if __name__ == '__main__':
 arguments = sys.argv[1:]
 grid_size, value_range, random_seed = parse_arguments(arguments)

  # print out the arguments
  print "grid size = %i \t value range = %i \t seed = %i" % \
    (grid_size, value_range, random_seed)

As we learned in Exercise 1, python comes with a library, argparse, to do this in a way that’s easier to extend and with better error messages.  In this exercise, you’ll rework the above code to use the argparse library. You’ll need to import the argparse module, and rewrite parse_arguments to use it.
A hint on getting started.

A hint on setting up argparse.

  parser.add_argument('grid_size', type=int, help="Grid size")

A hint on accessing the parsed arguments.

  args = parser.parse_args()
  print args.foo

Our answer:

import sys, argparse

def parse_arguments(arguments):
  '''Parse strings to get controlling parameters.'''

  parser = argparse.ArgumentParser()
  parser.add_argument('grid_size',   type=int, help='Grid size')
  parser.add_argument('value_range', type=int, help='Value range')
  parser.add_argument('random_seed', type=int, help='Random seed')
  args   = parser.parse_args(arguments)

  return args.grid_size, args.value_range, args.random_seed

if __name__ == '__main__':
  arguments = sys.argv[1:]
  grid_size, value_range, random_seed = parse_arguments(arguments)
  print "grid size = %i \t value range = %i \t seed = %i" % \
    (grid_size, value_range, random_seed)