52. The os and sys module

OS Module

The 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. This os and os.path modules include many functions to interact with the file system.

Following are some functions in OS module:
1. os.name

This function gives the name of the operating system dependent module imported. The following names have currently been registered: ‘posix’, ‘nt’, ‘os2’, ‘ce’, ‘java’ and ‘riscos’

>>>import os

>>>os.name

'nt'

2.os.getcwd() 

Function os.getcwd(), returns the Current Working Directory(CWD) of the file used to execute the code, can vary from system to system.

>>> os.getcwd()

'C:\\Users\\binuvp\\AppData\\Local\\Programs\\Python\\Python38-32'

3.os.listdir('.').

To print files and directories in the current directory  on your system 

>>> os.listdir('.')

['are.py', 'DLLs', 'Doc', 'include', 'Lib', 'libs', 'LICENSE.txt', 'mymodule.py', 'NEWS.txt', 'polygon.py', 'python.exe', 'python3.dll', 'python38.dll', 'pythonw.exe', 's.py', 'Scripts', 'student.py', 't.py', 'tcl', 'test.py', 'Tools', 'vcruntime140.dll', '__pycache__']

4.os.chdir('..')

This function is used to change the CWD

>>> os.getcwd()

'C:\\Users\\binuvp\\AppData\\Local\\Programs\\Python\\Python38-32'

>>> os.chdir('..')

>>> os.getcwd()

'C:\\Users\\binuvp\\AppData\\Local\\Programs\\Python'

>>> os.chdir('Python38-32')

>>> os.getcwd()

'C:\\Users\\binuvp\\AppData\\Local\\Programs\\Python\\Python38-32'

5.os.mkdir(path)

This will create a test directory in C drive

6.os.rmdir(path)

Remove the directory temp

7.os.remove(path)

Remove a file

>>> os.listdir()

['are.py', 'DLLs', 'Doc', 'include', 'Lib', 'libs', 'LICENSE.txt', 'mymodule.py', 'NEWS.txt', 'polygon.py', 'python.exe', 'python3.dll', 'python38.dll', 'pythonw.exe', 's.py', 'Scripts', 'student.py', 't.py', 'tcl', 'test.py', 'Tools', 'vcruntime140.dll', '__pycache__']

>>> os.remove('s.py')

>>> os.listdir()

['are.py', 'DLLs', 'Doc', 'include', 'Lib', 'libs', 'LICENSE.txt', 'mymodule.py', 'NEWS.txt', 'polygon.py', 'python.exe', 'python3.dll', 'python38.dll', 'pythonw.exe', 'Scripts', 'student.py', 't.py', 'tcl', 'test.py', 'Tools', 'vcruntime140.dll', '__pycache__']

8.os.rename(old.new)

Renames the file or directory named old to new

8.os.walk

The method walk() generates the file names in a directory tree by walking the tree either top-down or bottom-up.

For each directory in the tree rooted at directory top (including top itself), it yields a 3-tuple (dirpath, dirnames, filenames).

root : Prints out directories only from what you specified.
dirs : Prints out sub-directories from root.
files : Prints out all files from root and directories.

The following code will print all files in the root directory c:\\bvp and its sub directories 

( University question)

import os

os.chdir('c:\\bvp')

for (root,dirs,files) in os.walk('.', topdown=True): 

        print(root )

        print(dirs )

        print(files)

        print('--------------------------------')

os.path module functions

1.os.path.isfile(path)

This function is used to test whether the argument specified is a file

>>> os.path.isfile('t.py')

True

>>> os.path.isfile('x.py')

False

2.os.path.isdir(path)

return True if path names a directory

3.os.path.getsize(path)

Return the size of the file in bytes

>>> os.path.getsize('c:\\users\\binuvp\\t.py')

111

4.os.path.exists(path)

Returns true if the file exists

>>> os.path.exists('t.py')

True

>>> os.path.exists('s.py')

False

When working with files in Python, there may be times when you need to check whether a file exists or not.Confirming the existence of a specific file comes in handy when you want to perform particular operations, such as opening, reading from, or writing to that file.

If you attempt to perform any of the operations mentioned above and the file doesn't exist, you will come across bugs and your program will end up crashing.

So, to perform operations and prevent your program from crashing, it is a helpful first step to check if a file exists on a given path.

Example:

import os.path 

path = './example.txt' 

check_file = os.path.isfile(path) 

print(check_file) # output  True if file exist

OR 

import os.path 

path = './example.txt' 

check_file = os.path.exists(path) 

print(check_file) # output # True

Summary

Sample programs ( University Questions)

The following program will count the number of files in the current directory.

import os

def countfiles(path):

    count=0

    lst=os.listdir(path)

    for f in lst:

        if os.path.isfile(f):

            count=count+1

        else:

            os.chdir(f)

            count=count+countfiles(os.getcwd())

            os.chdir('..')

    return count

c=countfiles('.')

print("number of files...=",c)

Print the names of the files in the current directory having ".py" extension.

import os

path=os.getcwd()

lst=os.listdir(path)

for f in lst:

     if '.py' in f:

           print(f)

Write a Python function that takes a filename as input and checks if the file exists in the current directory. The function should return True if the file exists and False otherwise. Use the os module for this task.

import os

def file_exists(filename):

   if  os.path.isfile(filename):

    return True

   else:

    return False

filename = input("Enter the filename to check: ")

if file_exists(filename):

    print("File exists.")

else:

    print("File does not exist.")

Write a Python function using recursion that prints the names of all .py files in a given directory and its sub directories.(university question)

Here's a Python function that uses recursion to print the names of all .py files in a given directory and its subdirectories:

import os

def list_py_files(directory):
    # List all entries in the directory
    for entry in os.listdir(directory):
        full_path = os.path.join(directory, entry)

        # If it's a directory, recurse into it
        if os.path.isdir(full_path):
            list_py_files(full_path)
        # If it's a .py file, print it
        elif entry.endswith(".py"):
            print(full_path)

---

🧪 Example Usage:

list_py_files("/path/to/your/project")

If you run this with the current working directory:

list_py_files(".")

🔍 How it Works:

• os.listdir(directory) → Lists all files and subdirectories.

• os.path.isdir(full_path) → Checks if the entry is a directory.

• entry.endswith(".py") → Checks if the file has a .py extension.

• os.path.join(...) → Ensures portable path construction across operating systems.

Enhance the above function to count the number of .py files and write their paths to a log file.Use the os module and demonstrate how exceptions(e.g.,permission errors)are handled gracefully.(university question)

import os

def list_and_log_py_files(directory, log_filename="py_file_log.txt"):

    count = 0  # Counter for .py files

    def recurse(dir_path):

        nonlocal count  # Allows updating the outer 'count' variable

        try:

            for entry in os.listdir(dir_path):

                full_path = os.path.join(dir_path, entry)

                if os.path.isdir(full_path):

                    recurse(full_path)  # Recurse into subdirectory

                elif entry.endswith(".py"):

                    with open(log_filename, "a") as log_file:

                        log_file.write(full_path + "\n")

                    print(full_path)

                    count += 1

        except PermissionError:

            print(f"Permission denied: {dir_path}")

        except FileNotFoundError:

            print(f"Directory not found: {dir_path}")

        except Exception as e:

            print(f"Error accessing {dir_path}: {e}")

    # Clear the log file at the start

    with open(log_filename, "w") as log_file:

        log_file.write("Python File Log\n")

    # Start recursion

    recurse(directory)

    print(f"\nTotal .py files found: {count}")

list_and_log_py_files(".")

sys Module

The sys module provides functions and variables used to manipulate different parts of the Python runtime environment. The sys module provides information about constants, functions and methods of the Python interpreter.

The current version number of Python

>>> import sys

>>> sys.version

'3.8.5 (tags/v3.8.5:580fbb0, Jul 20 2020, 15:43:08) [MSC v.1926 32 bit (Intel)]'

>>> sys.version_info

sys.version_info(major=3, minor=8, micro=5, releaselevel='final', serial=0)

recursion limit

>>> sys.getrecursionlimit()

1000

search path for all Python modules

>>> sys.path

['C:\\Users\\binuvp\\AppData\\Local\\Programs\\Python\\Python38-32\\Lib\\idlelib', 'C:\\Users\\binuvp\\AppData\\Local\\Programs\\Python\\Python38-32\\python38.zip', 'C:\\Users\\binuvp\\AppData\\Local\\Programs\\Python\\Python38-32\\DLLs', 'C:\\Users\\binuvp\\AppData\\Local\\Programs\\Python\\Python38-32\\lib', 'C:\\Users\\binuvp\\AppData\\Local\\Programs\\Python\\Python38-32', 'C:\\Users\\binuvp\\AppData\\Local\\Programs\\Python\\Python38-32\\lib\\site-packages']

sys.exit()

This causes the script to exit back to either the Python console or the command prompt. This is generally used to safely exit from the program in case of generation of an exception.

Name of the platform on which Python is running, e.g. "linux2" for Linux and "win32" for Windows

>>> sys.platform

'win32'

A string containing the name of the executable binary (path and executable file name) for the Python interpreter.

>>> sys.executable

'C:\\Users\\binuvp\\AppData\\Local\\Programs\\Python\\Python38-32\\pythonw.exe'

Command-line arguments

Lots of scripts need access to the arguments passed to the script, when the script was started.  sys.argv is a list, which contains the command-line arguments passed to the script. The first item of this list contains the name of the script itself. The arguments follow the script name.

The following script iterates over the sys.argv list :

 import sys 

 # it's easy to print this list of course: 

print (sys.argv )

 # or it can be iterated via a for loop: 

for i in range(len(sys.argv)): 

     if i == 0: 

         print("Function name: " , sys.argv[0] )

    else: 

         print("%d. argument: %s" % (i,sys.argv[i]) )

We save this script as arguments.py. If we call it, we get the following output::

$ python arguments.py arg1 arg2 

['arguments.py', 'arg1', 'arg2'] 

Function name: arguments.py 

1. argument: arg1 

2. argument: arg2