File Handling and Exception Handling

File Handling in Python

Introduction

  • File: A named location on disk for storing related information.

  • Used for permanent data storage in non-volatile memory (e.g. hard disk).

  • Random Access Memory (RAM) is volatile; files are essential for data persistence.

File Operations

  1. Open a file

  2. Read or write (perform operations)

  3. Close the file

  • Use the open() function to open files.

Opening a File

  • Syntax:

    f = open("test.txt")

  • Returns a file object (handle) to read/modify the file.

Closing a File

  • Use the close() method when done to free resources.

  • Syntax:

    f.close()

Writing to a File

  • Open in modes:

    • 'w': Write, overwrites existing data.

    • 'a': Append, adds to existing data.

    • 'x': Exclusive creation, fails if file exists.

  • Caution: 'w' mode will erase existing content.

Reading from a File

  • Open in reading mode.

  • Methods to read:

    • read(size): read up to size bytes; if size is not specified, it reads to EOF.

  • Syntax:

    file_object = open(file_name [, access_mode])

  • Access Modes:

    • r: Read only (default).

    • rb: Read in binary format.

    • r+: Read and write.

    • w: Write only (overwrites existing).

    • wb: Write in binary format.

    • a: Append.

    • ab: Append in binary format.

    • a+: Append and read.

Reading Methods

read() Method

  • Syntax:

    fileObject.read([count])

  • Reads a specified number of bytes; if no count, reads until EOF.

  • Example:

    fo = open("sample.txt", "r+")
str = fo.read(10)
print("Read String is: ", str)
fo.close()

readlines() Method

  • Reads until EOF and returns a list of lines.

  • Returns an empty string when EOF is immediately encountered.

  • Syntax:

    fileObject.readlines(sizehint)

readline() Method

  • Reads a single line from the file.

File Positions

  • tell(): Returns current position in the file.

  • seek(offset[, from]): Changes the file position based on offset and reference point.

  • Reference points:

    • 0: Beginning of file

    • 1: Current position

    • 2: End of file

Writing Data

write() Method

  • Writes a string to an open file without newline character.

  • Syntax:

    fileObject.write(string)

  • Example:

    fo = open("sample.txt", "w")
fo.write("Good morning.\nWe are studying File Handling! !\n")
fo.close()

File Renaming and Removal

rename() Method

  • Syntax:

    os.rename(current_file_name, new_file_name)

remove() Method

  • Deletes a file.

  • Syntax:

    os.remove(file_name)

Working with Directories

Directory Operations

  • Creation: Use mkdir() method to create a directory.

    os.mkdir("newdir")

  • Change Directory: Use chdir() method to change the current directory.

    os.chdir("newdir")

  • Current Directory: Use getcwd() to get the current directory.

Removing Directories

  • rmdir(): Deletes a directory (must be empty).

    os.rmdir('dirname')

Exception Handling

Exception Overview

  • Exception: An event that disrupts the normal flow of a program.

  • Represents an error in Python.

Handling Exceptions

  • Use try and except blocks to handle exceptions:

    try:
    # Suspicious operations
except ExceptionType:
    # Handle exception

Syntax of Try-Except

  • Structure:

    try:
    # Operations
except Exception1:
    # Handle Exception1
except Exception2:
    # Handle Exception2
else:
    # Execute if no exceptions occur

Finally Clause

  • Code block always executed after try block finishes:

    try:
    # Code
finally:
    # Code always runs

Raising Exceptions

  • Use the raise statement to raise exceptions manually.

    raise ExceptionType("message")

Custom Exceptions

  • Define custom exceptions by creating a new class inherited from Exception.

  • Example:

    class CustomError(Exception):
    pass

  • Raises using the class just like built-in exceptions.