Python IO

Reading Keyboard Input:

Python provides two built-in functions to read a line of text from standard input, which by default comes from the keyboard. These functions are:

·         raw_input

·         input

The raw_input Function:

The raw_input([prompt]) function reads one line from standard input and returns it as a string (removing the trailing newline):

#!/usr/bin/python

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

This would prompt you to enter any string and it would display same string on the screen. When I typed "Hello Python!", it output is like this:

Enter your input: Hello Python
Received input is :  Hello Python

The 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

This would produce following result against the entered input:

Enter your input: [x*5 for x in range(2,10,2)]
Recieved input is :  [10, 20, 30, 40]

Opening and Closing Files:

Until now, you have been reading and writing to the standard input and output. Now we will see how to play with actual data files.

Python provides basic functions and methods necessary to manipulate files by default. You can do your 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.

Syntax:

file object = open(file_name [, access_mode][, buffering])

Here is paramters detail:

·         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 ie. 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 will take place. If the buffering value is 1, line buffering will be performed while accessing a file. If you specify the buffering value as an integer greater than 1, then buffering action will be performed with the indicated buffer size. If negative, the buffer size is the system default(default behavior).

Here is a list of the different modes of opening a file:

The file object atrributes:

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

Here is a list of all attributes related to file object:

Example:

#!/usr/bin/python

# Open a file
fo = open("foo.txt", "wb")
print "Name of the file: ", fo.name
print "Closed or not : ", fo.closed
print "Opening mode : ", fo.mode
print "Softspace flag : ", fo.softspace

This would produce following result:

Name of the file:  foo.txt
Closed or not :  False
Opening mode :  wb
Softspace flag :  0

The 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.

Syntax:

fileObject.close();

Example:

#!/usr/bin/python

# Open a file
fo = open("foo.txt", "wb")
print "Name of the file: ", fo.name

# Close opend file
fo.close()

This would produce following result:

Name of the file:  foo.txt

Reading and Writing Files:

The file object provides a set of access methods to make our lives easier. We would see how to use read() and write() methods to read and write files.

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:

Syntax:

fileObject.write(string);

Here passed parameter is the content to be written into the opend file.

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()

The above method would create foo.txt file and would write given content in that file and finally it would close that file. If you would open this file, it would have following content

Python is a great language.
Yeah its great!!

The read() Method:

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

Syntax:

fileObject.read([count]);

Here passed parameter is the number of bytes to be read from the opend 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, may be until the end of file.

Example:

Let's take a file foo.txt which we have created above.

#!/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()

This would produce following result:

Read String is :  Python is

File Positions:

The tell() method tells you the current position within the file in other words, the next read or write will occur at that many bytes from the beginning of the file:

The seek(offset[, from]) method changes the current file position. The offset argument indicates the number of bytes to be moved. The from argument specifies the reference position from where the bytes are to be moved.

If from is set to 0, it means use the beginning of the file as the reference position and 1 means use the current position as the reference position and if it is set to 2 then the end of the file would be taken as the reference position.

Example:

Let's take a file foo.txt which we have created above.

#!/usr/bin/python

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

# Check current position
position = fo.tell();
print "Current file position : ", position

# Reposition pointer at the beginning once again
position = fo.seek(0, 0);
str = fo.read(10);
print "Again read String is : ", str
# Close opend file
fo.close()

This would produce following result:

Read String is :  Python is
Current file position :  10
Again read String is :  Python is

Renaming and Deleting Files:

Python os module provides methods that help you perform file-processing operations, such as renaming and deleting files.

To use this module you need to import it first and then you can all any related functions.

The rename() Method:

The rename() method takes two arguments, the current filename and the new filename.

Syntax:

os.rename(current_file_name, new_file_name)

Example:

Following is the example to rename an existing file test1.txt:

#!/usr/bin/python
import os

# Rename a file from test1.txt to test2.txt
os.rename( "test1.txt", "test2.txt" )

The remove() Method:

You can use the remove() method to delete files by supplying the name of the file to be deleted as the argument.

Syntax:

os.remove(file_name)

Example:

Following is the example to delete an existing file test2.txt:

#!/usr/bin/python
import os

# Delete file test2.txt
os.remove("text2.txt")

Directories in Python:

All files are contained within various directories, and Python has no problem handling these too. The os module has several methods that help you create, remove, and change directories.

The mkdir() Method:

You can use the mkdir() method of the os module to create directories in the current directory. You need to supply an argument to this method, which contains the name of the directory to be created.

Syntax:

os.mkdir("newdir")

Example:

Following is the example to create a directory test in the current directory:

#!/usr/bin/python
import os

# Create a directory "test"
os.mkdir("test")

The chdir() Method:

You can use the chdir() method to change the current directory. The chdir() method takes an argument, which is the name of the directory that you want to make the current directory.

Syntax:

os.chdir("newdir")

Example:

Following is the example to go into "/home/newdir" directory:

#!/usr/bin/python
import os

# Changing a directory to "/home/newdir"
os.chdir("/home/newdir")

The getcwd() Method:

The getcwd() method displays the current working directory.

Syntax:

os.getcwd()

Example:

Following is the example to give current directory:

#!/usr/bin/python
import os

# This would give location of the current directory
os.getcwd()

The rmdir() Method:

The rmdir() method deletes the directory, which is passed as an argument in the method.

Before removing a directory, all the contents in it should be removed.

Syntax:

os.rmdir('dirname')

Example:

Following is the example to remove "/tmp/test" directory. It is required to give fully qualified name of the directory otherwise it would search for that directory in the current directory.

#!/usr/bin/python
import os

# This would  remove "/tmp/test"  directory.
os.rmdir( "/tmp/test"  )

File & Directory Related Methods:

There are three important sources which provide a wide range of utility methods to handle and manipulate files & directories on Windows and Unix operating systems. They are as follows:

Reading and Writing Files

open() returns a file object, and is most commonly used with two arguments: open(filename, mode).

>>> 

>>> f = open('workfile', 'w')
>>> print f
<open file 'workfile', mode 'w' at 80a0960>

The first argument is a string containing the filename. The second argument is another string containing a few characters describing the way in which the file will be used. mode can be 'r' when the file will only be read, 'w' for only writing (an existing file with the same name will be erased), and 'a' opens the file for appending; any data written to the file is automatically added to the end. 'r+' opens the file for both reading and writing. The mode argument is optional; 'r' will be assumed if it’s omitted.

On Windows, 'b' appended to the mode opens the file in binary mode, so there are also modes like 'rb', 'wb', and 'r+b'. Python on Windows makes a distinction between text and binary files; the end-of-line characters in text files are automatically altered slightly when data is read or written. This behind-the-scenes modification to file data is fine for ASCII text files, but it’ll corrupt binary data like that in JPEG or EXE files. Be very careful to use binary mode when reading and writing such files. On Unix, it doesn’t hurt to append a 'b' to the mode, so you can use it platform-independently for all binary files.

Methods of File Objects

The rest of the examples in this section will assume that a file object called f has already been created.

To read a file’s contents, call f.read(size), which reads some quantity of data and returns it as a string. size is an optional numeric argument. When size is omitted or negative, the entire contents of the file will be read and returned; it’s your problem if the file is twice as large as your machine’s memory. Otherwise, at most size bytes are read and returned. If the end of the file has been reached, f.read() will return an empty string ("").

>>> 

>>> f.read()
'This is the entire file.\n'
>>> f.read()
''

f.readline() reads a single line from the file; a newline character (\n) is left at the end of the string, and is only omitted on the last line of the file if the file doesn’t end in a newline. This makes the return value unambiguous; if f.readline() returns an empty string, the end of the file has been reached, while a blank line is represented by '\n', a string containing only a single newline.

>>> 

>>> f.readline()
'This is the first line of the file.\n'
>>> f.readline()
'Second line of the file\n'
>>> f.readline()
''

For reading lines from a file, you can loop over the file object. This is memory efficient, fast, and leads to simple code:

>>> 

>>> for line in f:
        print line,

This is the first line of the file.
Second line of the file

If you want to read all the lines of a file in a list you can also use list(f) or f.readlines().

f.write(string) writes the contents of string to the file, returning None.

>>> 

>>> f.write('This is a test\n')

To write something other than a string, it needs to be converted to a string first:

>>> 

>>> value = ('the answer', 42)
>>> s = str(value)
>>> f.write(s)

f.tell() returns an integer giving the file object’s current position in the file, measured in bytes from the beginning of the file. To change the file object’s position, use f.seek(offset, from_what). The position is computed from adding offset to a reference point; the reference point is selected by thefrom_what argument. A from_what value of 0 measures from the beginning of the file, 1 uses the current file position, and 2 uses the end of the file as the reference point. from_what can be omitted and defaults to 0, using the beginning of the file as the reference point.

>>> 

>>> f = open('workfile', 'r+')
>>> f.write('0123456789abcdef')
>>> f.seek(5)     # Go to the 6th byte in the file
>>> f.read(1)
'5'
>>> f.seek(-3, 2) # Go to the 3rd byte before the end
>>> f.read(1)
'd'

When you’re done with a file, call f.close() to close it and free up any system resources taken up by the open file. After calling f.close(), attempts to use the file object will automatically fail.

>>> 

>>> f.close()
>>> f.read()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ValueError: I/O operation on closed file

It is good practice to use the with keyword when dealing with file objects. This has the advantage that the file is properly closed after its suite finishes, even if an exception is raised on the way. It is also much shorter than writing equivalent try-finally blocks:

>>> with open('workfile', 'r') as f:
...     read_data = f.read()
>>> f.closed
True

File objects have some additional methods, such as isatty() and truncate() which are less frequently used; consult the Library Reference for a complete guide to file objects.