Python | os.pwrite() method

OS module in Python provides functions for interacting with the operating system. OS comes under Python’s standard utility modules. This module provides a portable way of using operating system dependent functionality.

os.pwrite() method in Python is used to write the specified bytestring to the file associated with the specified file descriptor at the specified position. Any existing value will be overwritten at the specified position.

A file descriptor is small integer value that corresponds to a file that has been opened by the current process. It is used to perform various lower level I/O operations like read, write, send etc.

Note: os.pwrite() method is intended for low-level operation and should be applied to a file descriptor as returned by os.open() or os.pipe() method.

Syntax: os.pwrite(fd, str, offset)

Parameters:
fd: A file descriptor representing the file to be written.
str: A bytestring representing the content to be written in the file
offset: An integer value denoting the starting position. File writing will start from this offset value.

Return Type: This method returns an integer value representing the number of bytes actually written. .

Consider the below text as the content of the file named file.txt.

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Code: Use of os.pwrite() method

# Python program to explain os.pwrite() method

# Importing os module
import os

# Filename
filename = "file.txt"

# Open the file and get the
# file descriptor associated
# with it using os.open method
fd = os.open(filename, os.O_RDWR)

# String to be written in the file
s = "Python, "

# converting string to bytestring
text = s.encode("utf-8")

# Position from where
# file writing will start
offset = 0

# As offset is 0, bytestring
# will be written in the
# beginning of the file

# Write the bytestring
# to the file indicated by
# file descriptor at
# specified position
bytesWritten = os.pwrite(fd, text, offset)
print("Number of bytes actually written:", bytesWritten)

# Print the content of the file
with open(filename) as f:
    print(f.read())

# String to be written in the file
s = "Javascript, "

# converting string to bytestring
text = s.encode("utf-8")

# Position from where
# file writing will start
# os.stat(fd).st_size will return
# file size in bytes
# so bytestring will be written
# at the end of the file
offset = os.stat(fd).st_size

# Write the bytestring
# to the file indicated by
# file descriptor at
# specified position
bytesWritten = os.pwrite(fd, text, offset)
print("\nNumber of bytes actually written:", bytesWritten)

# Print the content of the file
with open(filename) as f:
    print(f.read())

# String to be written in the file
s = "R Programming, "

# converting string to bytestring
text = s.encode("utf-8")

# Position from where
# file writing will start
offset = 10

# Write the bytestring
# to the file indicated by
# file descriptor at
# specified position
bytesWritten = os.pwrite(fd, text, offset)
print("\nNumber of bytes actually written:", bytesWritten)

# Print the content of the file
with open(filename) as f:
    print(f.read())

Output:

Number of bytes actually written: 8
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Number of bytes actually written: 12
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Javascript, 

Number of bytes actually written: 15
Python, JaR Programming, ascript,