I/O (Input/Output) File operation in Python

This are all the basic I/O functions available in Python. For more functions, please refer to standard Python documentation.

Printing to the Screen

The simplest way to produce output is using the print statement where you can pass zero or more expressions separated by commas. This function converts the expressions you pass into a string and writes the result to standard output as follows:

#!/usr/bin/python

print "Python is really a great language,", "isn't it?"

Reading from keybord - input Function

The input([prompt]) function is equivalent to raw_input, except that it assumes the input is a valid Python expression and returns the evaluated result to you.

#!/usr/bin/python

str = input("Enter your input: ");
print "Received input is : ", str

Opening and Closing Files

Now, we will see how to use actual data files. Python provides basic functions and methods necessary to manipulate files by default. You can do most of the file manipulation using a file object.

The open Function

Before you can read or write a file, you have to open it using Python's built-in open() function. This function creates a file object, which would be utilized to call other support methods associated with it.

file object = open(file_name [, access_mode][, buffering])
  • file_name − The file_name argument is a string value that contains the name of the file that you want to access.

  • access_mode − The access_mode determines the mode in which the file has to be opened, i.e., read, write, append, etc. A complete list of possible values is given below in the table. This is optional parameter and the default file access mode is read (r).

  • buffering − If the buffering value is set to 0, no buffering takes place. If the buffering value is 1, line buffering is performed while accessing a file. If you specify the buffering value as an integer greater than 1, then buffering action is performed with the indicated buffer size. If negative, the buffer size is the system default(default behavior).

List of the different modes of opening a file:

r

Opens a file for reading only. The file pointer is placed at the beginning of the file. This is the default mode.

rb

Opens a file for reading only in binary format. The file pointer is placed at the beginning of the file. This is the default mode.

r+

Opens a file for both reading and writing. The file pointer placed at the beginning of the file.

rb+

Opens a file for both reading and writing in binary format. The file pointer placed at the beginning of the file.

w

Opens a file for writing only. Overwrites the file if the file exists. If the file does not exist, creates a new file for writing.

wb

Opens a file for writing only in binary format. Overwrites the file if the file exists. If the file does not exist, creates a new file for writing.

w+

Opens a file for both writing and reading. Overwrites the existing file if the file exists. If the file does not exist, creates a new file for reading and writing.

wb+

Opens a file for both writing and reading in binary format. Overwrites the existing file if the file exists. If the file does not exist, creates a new file for reading and writing.

a

Opens a file for appending. The file pointer is at the end of the file if the file exists. That is, the file is in the append mode. If the file does not exist, it creates a new file for writing.

ab

Opens a file for appending in binary format. The file pointer is at the end of the file if the file exists. That is, the file is in the append mode. If the file does not exist, it creates a new file for writing.

a+

Opens a file for both appending and reading. The file pointer is at the end of the file if the file exists. The file opens in the append mode. If the file does not exist, it creates a new file for reading and writing.

ab+

Opens a file for both appending and reading in binary format. The file pointer is at the end of the file if the file exists. The file opens in the append mode. If the file does not exist, it creates a new file for reading and writing.

file Object Attributes

When a file is opened and you have one file object, you can get various information related to that file.

A list of all attributes related to file object:

file.closed

Returns true if file is closed, false otherwise.

file.mode

Returns access mode with which file was opened.

file.name

Returns name of the file.

close() Method

The close() method of a file object flushes any unwritten information and closes the file object, after which no more writing can be done.

Python automatically closes a file when the reference object of a file is reassigned to another file. It is a good practice to use the close() method to close a file.

fileObject.close();

The write() Method

The write() method writes any string to an open file. It is important to note that Python strings can have binary data and not just text.

The write() method does not add a newline character ('\n') to the end of the string:

fileObject.write(string);

Used in an example:

#!/usr/bin/python

# Open a file
fo = open("foo.txt", "wb")
fo.write( "Python is a great language.\nYeah its great!!\n");

# Close opend file
fo.close()

read() Method

The read() method reads a string from an open file. It is important to note that Python strings can have binary data. apart from text data.

fileObject.read([count]);

Here, passed parameter is the number of bytes to be read from the opened file. This method starts reading from the beginning of the file and if count is missing, then it tries to read as much as possible, maybe until the end of file.

Used in an example:

#!/usr/bin/python

# Open a file
fo = open("foo.txt", "r+")
str = fo.read(10);
print "Read String is : ", str
# Close opend file
fo.close()
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