Lesson 5 - I/O

To obtain command line arguments for a Python script use sys.argv

sys.argv is a list where sys.argv[0] is the script name

#!/usr/bin/env python

import sys

for argument in sys.argv:
    print(argument)

$ ./test.py arg1 arg2 "string with spaces"
./test.py
arg1
arg2
string with spaces

To write Python scripts which take more complicated options, use the argparse module

• argparse supports short (-o) or long (--option) options
• Automatically generates help output
• Checks for required arguments and mutually-exclusive arguments
• Highly configurable

import argparse
parser = argparse.ArgumentParser(description='Best program, ever!')
parser.add_argument('-n', '--number', metavar='NUMBER', type=int,
                    required=True, help='Number of times to shout')

options = parser.parse_args()
print('Shout! ' * options.number)

To get user input, use raw_input() (Python 2) or input() (Python 3)

Python 2

name = raw_input("What is your name? ")
print "Hi, %s" % name

Python 3

name = input("What is your name? ")
print("Hi, %s" % name)

Result

What is your name? Avram
Hi, Avram

For version-agnostic user input

• Standard Library only

  import sys
  if sys.version_info[0] > 2:
      INPUT = input
  else:
      INPUT = raw_input
  
  name = INPUT("What is your name? ")
  

• Using the six module

  from six.moves import input
  name = input("What is your name? ")
  

It is important to make sure file are closed when you are finished with them

One method of doing this is a try-finally statement

• Ensures a file is closed even if there is an issue

fileObject = open('filename')
try:
    pass # Do many fancy file things
finally:
    fileObject.close()

In Python 2.5 and later, files can be opened using a with statement

The file will automatically be closed when the with statement completes

with open('filename') as fileObject:
    pass # Do many fancy file things

Open a file in text mode and read contents as a single string

>>> with open('/etc/redhat-release') as releaseFile:
...     releaseFile.read()
...
'Fedora release 24 (Twenty Four)\n'

Open a file in text mode and read contents line by line

>>> with open('/etc/group') as groupFile:
...     groupFile.readline()  # Read the first line
...     for line in groupFile:  # Read the rest of the lines
...         print(line.strip())
...
'root:x:0:\n'
bin:x:1:
daemon:x:2:
sys:x:3:

When writing to a file object, newline characters must be explicitly added

Open a file and write a series of random numbers

>>> import random
>>> randomGen = random.SystemRandom()
>>> with open('/tmp/random', 'w') as randomFile:
...     for num in range(1, 100):
...             randomNum = randomGen.randint(0, 100000)
...             randomFile.write('%d\n' % randomNum)
...

$ cat /tmp/random
42380
30569
43790
76564
...

Open multiple files at once

>>> with open('/etc/group') as groupFile, open('/tmp/group', 'w') as newFile:
...     for line in groupFile:
...             groupname = line.split(':')[0]
...             newFile.write('%s\n' % groupname)
...

NOTE:

When using write() on the Python 3 console, a series of numbers will be printed to the screen. This is the number of bytes written.

The sys module includes three special file objects

• sys.stdin - Standard In
• sys.stdout - Standard Out
• sys.stderr - Standard Error

Write to stdout

• Use instead of print() when Python <=2.5 must be supported

>>> sys.stdout.write("Hello, from sunny standard out\n")
Hello, from sunny standard out

Write to standard error

>>> sys.stderr.write("Hello, from the cold depths of standard error\n")
Hello, from the cold depths of standard error

Read from standard in

#!/usr/bin/env python

import sys
input = sys.stdin.readlines()
input.sort()
for line in input:
    sys.stdout.write(line.upper())

$ cat /etc/group | ./capitalize.py
ABRT:X:173:
ADM:X:4:
AUDIO:X:63:
AVAHI-AUTOIPD:X:170:
AVAHI:X:70:
...

A Python script will exit with a returncode of 0 if no unhandled exceptions occur

To exit explicitly, use sys.exit()

• No arguments or None, returncode is 0

sys.exit()

• If an integer is given, it will be used as the returncode

sys.exit(11)

• If another object type is given, it is printed using str to standard error

  • Returncode will be 1

if not len(sys.argv) > 1:
    sys.exit("Where are your arguments?")