Gaddis Python 6e Chapter 06

Chapter 6: Files and Exceptions

Introduction to File Input and Output

• Purpose of Files:

  • Allows retention of data between program runs.

  • Files store data on the computer's disk for future retrieval.

Concepts

• Writing Data to a File:

  • Refers to saving data in a file.

• Input File:

  • A file from which data is read.

• Output File:

  • A file to which data is written.

Three Steps in File Processing

1. Open the File:

   • Establish a connection to the file.

2. Process the File:

   • Read from or write to the file.

3. Close the File:

   • End the connection to release resources.

Types of Files

• Text File:

  • Contains encoded text data.

• Binary File:

  • Contains data not in text format, stored in binary.

File Access Methods

• Sequential Access:

  • Reads data linearly from start to end.

• Direct Access:

  • Allows jumping to specific points in the file for data retrieval.

Filenames and File Objects

• Filenames:

  • May include extensions indicating data type (e.g., .txt, .bin).

• File Object:

  • Represents a specific file and facilitates file operations.

Opening a File

• open() Function:

  • Creates a file object associated with a specified file.

  • Syntax: file_object = open(filename, mode).

• Modes:

  • r - Reading (default mode).

  • w - Writing (overwrites if the file exists).

  • a - Appending (adds data to the end of the file).

Writing Data to a File

• Use the write() Method:

  • Syntax: file_variable.write(string) to save data.

Closing a File

• File Closure:

  • Use file_variable.close() to free resources.

Reading Data From a File

• read() Method:

  • Reads the entire content of the file at once.

• readline() Method:

  • Reads a single line at a time including newline characters.

• End of File Detection:

  • Can utilize an empty string as a sentinel: while line != '':.

Appending Data to a File

• Appending Data:

  • Mode should be set to append to avoid overwriting existing data.

Writing and Reading Numeric Data

• Conversion for Writing:

  • Numbers must be converted to strings using str().

• Conversion for Reading:

  • Convert back to numeric types using int() or float().

Using Loops to Process Files

• Loops in File Processing:

  • Essential for processing large datasets.

  • for Loop Example:

    • for line in file_object: handles end-of-file conditions automatically.

Using the with Statement to Open Files

• with Statement:

  • Simplifies file handling, automatically closing the file when done.

  • General Format: with open('filename', 'mode') as file_variable:.

Processing Records

• Record Definition:

  • A collection of related data items.

• Field Definition:

  • A single unit of data within a record.

• Record Operations:

  • Include adding, displaying, searching, modifying, and deleting records.

Exceptions

• Definition of Exceptions:

  • Errors occurring during program execution.

• Traceback Information:

  • Provides details about errors and their locations in the code.

• Handling Exceptions:

  • Use try/except blocks to manage potential errors.

  • Syntax Example:

    try:
        # code that may raise an exception
    except ExceptionType:
        # handling code

• Handling Multiple Exceptions:

  • Write separate clauses for different exception types; general exception handler should be last.

The else Clause in Exception Handling

• Else Clause:

  • Optional clause that executes if no exception occurs in the try block.

The finally Clause

• Finally Clause:

  • Always executes, regardless of exceptions, for cleanup operations.

What If an Exception Is Not Handled?

• Unhandled Exceptions:

  • Can terminate the program.

• Documentation Reference:

  • Python documentation provides information on potential exceptions for functions.

Summary

• Chapter Highlights:

  • Types of files and file access methods.

  • Handling filenames and file objects.

  • Methods for writing and reading data.

  • Processing records and handling exceptions.