Question

In this part of the Python programming tutorial, we will talk about files.

Everything in Python is an object. Everything in UNIX is a file.

Standard I/O

There are three basic I/O connections. Standard input, standard output and standard error. Standard input is the data that goes to the program. The standard input comes from a keyboard. Standard output is where we print our data with the print keyword. Unless redirected, it is the terminal console. The standard error is a stream where programs write their error messages. It is usually the text terminal.

Conforming to the UNIX philosophy, the standard I/O streams are file objects.

Standard input

Standard input is the data that goes to the program.

#!/usr/bin/python

# name.py

import sys


print 'Enter your name: ',

name = ''

while True:
   c = sys.stdin.read(1)
   if c == '\n':
    break
   name = name + c

print 'Your name is:', name

In order to work with standard I/O streams, we must import the sys module. The read() method reads one character from the standard input. In our example we get a prompt saying "Enter your name". We enter our name and press Enter. The Enter key generates the new line character: \n .

$ ./name.py 
Enter your name: Jan
 Your name is: Jan

For getting input we can use higher level functions: input() and raw_input() .

The input() function prints a prompt if it is given, reads input and evaluates it.

#!/usr/bin/python

# input.py


data = input('Enter expression: ')

print 'You have entered:', data

$ ./input.py 
Enter expression: 3*3
You have entered: 9

The raw_input() prints a prompt if it is present. The function then reads a line from input, converts it to a string (stripping a trailing newline), and returns that.

#!/usr/bin/python

# rawinput.py


name = raw_input('Enter your name: ')

print 'You have entered:', name

Standard output

The standard output is where we print our data.

#!/usr/bin/python

# stdout.py

import sys

sys.stdout.write('Honore de Balzac, Father Goriot\n')
sys.stdout.write('Honore de Balzac, Lost Illusions\n')

In the example, we write some text to the standard output. This is in our case the terminal console. We use the write() method.

$ ./stdout.py 
Honore de Balzac, Father Goriot
Honore de Balzac, Lost Illusions

The print keyword puts some text into the sys.stdout by default.

#!/usr/bin/python

# printkeyw.py

print 'Honore de Balzac, The Splendors and Miseries of Courtesans'
print 'Honore de Balzac, Gobseck'

We do not provide the sys.stdout explicitly. It is done behind the scenes. There is another form of the print keyword. If we want, we can use it to write some data into a regular file.

#!/usr/bin/python

# works.py

f = open('works', 'w')

print >> f, 'Beatrix'
print >> f, 'Honorine'
print >> f, 'The firm of Nucingen'

f.close()

We open a file and write three titles of Balzac books into it. This form is also called print chevron.

$ cat works
Beatrix
Honorine
The firm of Nucingen

Redirection

File objects can be redirected. In the following example, we redirect the standard output to a regular file.

#!/usr/bin/python

# redirect.py

import sys


f = open('output', 'w')
sys.stdout = f

print 'Lucien'
sys.stdout.write('Rastignac\n')
sys.stdout.writelines(['Camusot\n', 'Collin\n'])

sys.stdout = sys.__stdout__

print 'Bianchon'
sys.stdout.write('Lambert\n')

In the redirect.py script, we redirect a standard output to a regular file called output. Then we restore the original standard output back. The original value of the std.output is kept in a special sys.__stdout__ variable.

$ ./redirect.py 
Bianchon
Lambert
$ cat output 
Lucien
Rastignac
Camusot
Collin

The open function

The open() function is used to open files in our system.

open(filename, [mode='r'], [bufsize])

The filename is the name of the file to be opened. The mode indicates, how the file is going to be opened. For reading, writing, appending etc. The bufsize is an optional buffer. It specifies the file's desired buffer size: 0 means unbuffered, 1 means line buffered, any other positive value means use a buffer of that size. A negative bufsize means to use the system default. If omitted, the system default is used.

The file modes:

Mode	Meaning
r	Reading
w	Writing
a	Appending
b	Binary data
+	Updating

The following little script prints the contents of a file.

#!/usr/bin/python

# wantme.py

f = open('ifyouwantme', 'r')

for line in f:
   print line,

f.close()

We open a file in read mode. We traverse the contents of the file with the for loop. In the end, we close the file object.

$ ./wantme.py 
Are you really here or am I dreaming
I can't tell dreams from truth 
for it's been so long since I have seen you
I can hardly remember your face anymore 

When I get really lonely
and the distance causes our silence
I think of you smiling 
with pride in your eyes a lover that sighs 

...

In the next example, we do the same. Now we use the readline() method.

#!/usr/bin/python

# wantme2.py

f = open('ifyouwantme', 'r')

while True:
   line = f.readline()
   if not line: break
   else: print line,

f.close()

There is yet another simple way, how we can print the contents of a file. Using the readlines() method. The readlines() method reads all the contents of a file into the memory. This is not applicable for very large files, though.

#!/usr/bin/python

# wantme3.py

f = open('ifyouwantme', 'r')

contents = f.readlines()

for i in contents:
   print i,

f.close()

In the next example, we will demonstrate writing to a file. We will write a simple strophe into a regular file.

#!/usr/bin/python

# strophe.py

text = """Incompatible, it don't matter though
'cos someone's bound to hear my cry
Speak out if you do
You're not easy to find\n"""


f = open('strophe', 'w')
f.write(text)

f.close()

This time we open the file in a w mode, so that we can write into it. If the file does not exist, it is created. If it exists, it is overwritten.

$ cat strophe
Incompatible, it don't matter though
'cos someone's bound to hear my cry
Speak out if you do
You're not easy to find

The pickle module

So far, we have been working with simple textual data. What if we are working with objects rather than simple text? For such situations, we can use the pickle module. This module serializes Python objects. The Python objects are converted into byte streams and written to text files. This process is called pickling. The inverse operation, reading from a file and reconstructing objects is called deserializing or unpickling.

#!/usr/bin/python

# pickle.py

import pickle

class Person:
   def __init__(self, name, age):
    self.name = name
    self.age = age

   def getName(self):
    return self.name

   def getAge(self):
    return self.age


person = Person('Monica', 15)
print person.getName()
print person.getAge()

f = open('monica', 'w')
pickle.dump(person, f)
f.close()

f = open('monica', 'r')
monica = pickle.load(f)
f.close()

print monica.getName()
print monica.getAge()

In our script, we define a Person class. We create one person. We pickle the object using the dump() method. We close the file, open it again for reading. Unpickle the object using the load() method.

$ ./monica.py 
Monica
15
Monica
15

$ file monica
monica: ASCII text

The file to which we write is a simple text file.

In this part of the Python tutorial, we covered files.