File Handling with os Module


File Handling with os Module



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In addition to File object returned by open() function, file IO operations can also be performed using Python”s built-in library has os module that provides useful operating system dependent functions. These functions perform low level read/write operations on file.

The open() function from os module is similar to the built-in open(). However, it doesn”t return a file object but a file descriptor, a unique integer corresponding to file opened. File descriptor”s values 0, 1 and 2 represent stdin, stdout, and stderr streams. Other files will be given incremental file descriptor from 2 onwards.

As in case of open() built-in function, os.open() function also needs to specify file access mode. Following table lists various modes as defined in os module.











Sr.No. Os Module & Description
1

os.O_RDONLY

Open for reading only

2

os.O_WRONLY

Open for writing only

3

os.O_RDWR

Open for reading and writing

4

os.O_NONBLOCK

Do not block on open

5

os.O_APPEND

Append on each write

6

os.O_CREAT

Create file if it does not exist

7

os.O_TRUNC

Truncate size to 0

8

os.O_EXCL

Error if create and file exists

To open a new file for writing data in it, specify O_WRONLY as well as O_CREAT modes by inserting pipe (|) operator. The os.open() function returns a file descriptor.


f=os.open("test.dat", os.O_WRONLY|os.O_CREAT)

Note that, data is written to disk file in the form of byte string. Hence, a normal string is converted to byte string by using encode() function as earlier.


data="Hello World".encode(''utf-8'')

The write() function in os module accepts this byte string and file descriptor.


os.write(f,data)

Don’t forget to close the file using close() function.


os.close(f)

To read contents of a file using os.read() function, use following statements:


f=os.open("test.dat", os.O_RDONLY)
data=os.read(f,20)
print (data.decode(''utf-8''))

Note that, the os.read() function needs file descriptor and number of bytes to be read (length of byte string).

If you want to open a file for simultaneous read/write operations, use O_RDWR mode. Following table shows important file operation related functions in os module.







Sr.No Functions & Description
1

os.close(fd)

Close the file descriptor.

2

os.open(file, flags[, mode])

Open the file and set various flags according to flags and possibly its mode according to mode.

3

os.read(fd, n)

Read at most n bytes from file descriptor fd. Return a string containing the bytes read. If the end of the file referred to by fd has been reached, an empty string is returned.

4

os.write(fd, str)

Write the string str to file descriptor fd. Return the number of bytes actually written.

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