File Handling with os Module

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.

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.