20.2 File Processing and Exception Handling

Introduction

File processing is a fundamental task in computer science, allowing programs to read data from and write data to persistent storage. This section explores the core concepts of file operations, including opening, reading, writing, and closing files. It also covers the crucial aspect of exception handling to gracefully manage potential errors during file operations.

File Modes

When opening a file, we specify a mode that determines how the file will be accessed. Common file modes include:

• 'r': Read mode. Opens the file for reading. The file must exist.
• 'w': Write mode. Opens the file for writing. If the file exists, its contents are overwritten. If the file does not exist, a new file is created.
• 'a': Append mode. Opens the file for writing. New data is added to the end of the file. If the file does not exist, a new file is created.
• 'x': Exclusive creation mode. Creates a new file. If the file already exists, the operation fails.
• 'r+': Read and write mode. Opens the file for both reading and writing.
• 'w+': Read and write mode. Opens the file for both reading and writing. If the file exists, its contents are truncated. If the file does not exist, a new file is created.
• 'a+': Read and append mode. Opens the file for both reading and appending. New data is added to the end of the file. If the file does not exist, a new file is created.

File Operations

The following are the fundamental file operations:

1. Opening a file: Using the open() function with the appropriate mode.
2. Reading from a file: Using functions like read(), readline(), or iterating through the file object.
3. Writing to a file: Using functions like write() or writelines().
4. Closing a file: Using the close() function to release system resources. It's crucial to close files after use.

Example Code (Python)

The following Python code demonstrates basic file processing operations:

def process_file(filename):    try:        file = open(filename, 'r')        contents = file.read()        print(\"File Contents:\", contents)        file.close()    except FileNotFoundError:        print(f\"Error: File '{filename}' not found.\")    except Exception as e:        print(f\"An error occurred: {e}\")process_file(\"my_file.txt\")    

Exception Handling

Exception handling is essential for robust file processing. File operations can fail due to various reasons, such as the file not existing, insufficient permissions, or disk errors. try and except blocks are used to handle these exceptions.

Common exceptions encountered during file operations include:

• FileNotFoundError: Raised when the specified file does not exist.
• PermissionError: Raised when the program does not have the necessary permissions to access the file.
• IOError: A general exception for input/output errors.

Example Code (Python) with Exception Handling

The following Python code demonstrates exception handling during file operations:

def read_file_safe(filename):    try:        file = open(filename, 'r')        data = file.read()        file.close()        return data    except FileNotFoundError:        print(f\"Error: File '{filename}' not found.\")        return None    except PermissionError:        print(f\"Error: Permission denied to access '{filename}'.\")        return None    except Exception as e:        print(f\"An unexpected error occurred: {e}\")        return Nonefile_content = read_file_safe(\"data.txt\")if file_content:    print(\"File content read successfully:\", file_content)    

Table of File Modes

Mode	Description	Behavior if file exists	Behavior if file does not exist
'r'	Read	File must exist	Error
'w'	Write	Contents are overwritten	New file is created
'a'	Append	New data is added to the end	New file is created
'x'	Exclusive creation	Error	New file is created
'r+'	Read and Write	File must exist	Error
'w+'	Read and Write	Contents are truncated	New file is created
'a+'	Read and Append	New data is added to the end	New file is created

Further Considerations

In real-world applications, file processing often involves handling large files efficiently, using buffered I/O, and implementing robust error handling mechanisms. Understanding file modes and exception handling is crucial for writing reliable and robust file processing programs.