File I/O: Types and Access Methods

Introduction to File Input and Output

• Saving data to a file allows a program to retain data between runs.
• Data is saved to a file, typically on a computer disk, and can be retrieved later.
• Writing data to a file involves saving data on a file, and an output file is one where data is written.

Introduction to File Input and Output (cont'd.)

• Reading data from a file is the process of retrieving data from a file.
• An input file is a file from which data is read.
• Using a file in a program involves three steps: opening the file, processing the file, and closing the file.

Types of Files and File Access Methods

• There are generally two types of files: text files and binary files.
• A text file contains data encoded as text.
• A binary file contains data that has not been converted to text.
• Two ways to access data stored in a file include Sequential and Direct access.
• Sequential access involves reading a file from beginning to end, without skipping ahead.
• Direct access allows jumping directly to any piece of data in the file.
• In text files, each line is terminated with a special character called EOL (End of Line), represented as '\n' in Python.
• In binary files, there isn't a terminator for a line, and the data is stored after converting it into machine-understandable binary language.

Filenames and File Objects

• Filename extensions are short sequences of characters at the end of a filename preceded by a period, and indicate the type of data stored in the file.
• A file object is associated with a specific file, providing a way for a program to work with the file as a file object referenced by a variable.

Opening a File

• The open function is used to open a file, creating a file object and associating it with a file on the disk.
• The general format is file_object = open(filename, mode).
• Mode is a string specifying how the file will be opened, such as reading only ('r'), writing ('w'), or appending ('a').

Specifying the Location of a File

• If the open function receives a filename without a path, it assumes the file is in the same directory as the program.
• If a program is running and a file is created, it is created in the same directory as the program.
• An alternative path and file name can be specified in the open function argument.
• Prefix the path string literal with the letter r.

Writing Data to a File

• A method is a function that belongs to an object, performing operations using that object.
• The file object's write method is used to write data to the file, with the format file_variable.write(string).
• The file should be closed using the file object's close method, with the format file_variable.close().

Reading Data From a File

• The read method, a file object method, reads the entire file contents into memory.
• It only works if the file has been opened for reading, and the contents are returned as a string.
• The readline method, also a file object method, reads a line from the file and returns it as a string, including \n.
• The read position marks the location of the next item to be read from a file.

Concatenating a Newline to and Stripping it From a String

• Concatenating \n to data before writing it and removing \n from strings after reading is often necessary.
• Concatenation is carried out using the + operator in the argument of the write method.
• The rstrip method, a string method, strips specific characters from the end of the string.

Appending Data to an Existing File

• When opening a file with 'w' mode, if the file already exists, it is overwritten.
• 'a' mode is used to append data to a file.
• If the file exists, it is not erased, and if it does not exist, it is created.
• Data is written to the file at the end of the current contents.

Writing and Reading Numeric Data

• Numbers must be converted to strings before they are written to a file.
• The str function converts a value to a string.
• Numbers are read from a text file as strings and must be converted to a numeric type (int or float) in order to perform mathematical operations.

Using Loops to Process Files

• Files typically hold large amounts of data, and loops are commonly used in reading from and writing to a file.
• The readline method uses an empty string as a sentinel when the end of the file is reached.

Using Python's for Loop to Read Lines

• Python allows a for loop to automatically read lines in a file and to stops when the end of file is reached.
• The format is for line in file_object: statements.
• The loop iterates once over each line in the file.

Processing Records

• A record is a set of data that describes one item; a field is a single piece of data within a record.
• Records can be written to a sequential access file by writing the fields one after the other.
• Records can be read from a sequential access file by reading each field until the record is complete.

Records and fields using Excel as example

• A record is a single row of data in programs, which could be a list or python object,.
• Each row represents a unique set of information or a single entity.
• A field is typically one column header, ex. 'Model' or 'Color'.

Processing Records (cont'd.)

• Working with records involves adding records, displaying records, searching for a specific record, modifying records, and deleting records.

Exceptions

• An exception is an error that occurs while a program is running, usually causing the program to abruptly halt.
• A traceback is an error message giving information regarding line numbers that caused the exception, and indicates the type of exception with a brief description.

Exceptions (cont'd.)

• Many exceptions can be prevented by careful coding, such as input validation and simple decision constructs.
• Some exceptions, such as trying to convert a non-numeric string to an integer or opening a non-existent file for reading, cannot be avoided by careful coding.

Exceptions (cont'd.)

• An exception handler is code that responds when exceptions are raised and prevents the program from crashing.
• In Python, exception handlers are written as try/except statements.
• The try suite contains statements that can potentially raise an exception.
• The handler contains statements which are contained in except block.

Exceptions (cont'd.)

• If a statement in the try suite raises an exception and the exception type is specified in the except clause, the handler immediately following the except clause executes, and the program continues after the try/except statement.
• If other exceptions are raised, the program halts with a traceback error message.
• If no exception is raised, handlers are skipped.

Handling Multiple Exceptions

• The code in the try suite can throw more than one type of exception; thus, we need to write except clauses for each type of exception that needs to be handled.
• An except clause that does not list a specific exception will handle any exception that is raised in the try suite and should always be last in a series of except clauses.

Displaying an Exception's Default Error Message

• An exception object is created in memory when an exception is thrown.
• It usually contains a default error message pertaining to the exception.
• The exception object can be assigned to a variable in an except clause. (e.g., except ValueError as err:).
• The exception object variable can be passed to a print function to display the default error message.

The else Clause

• A try/except statement may include an optional else clause, which appears after all the except clauses.
• It is aligned with the try and except clauses and has a syntax similar to the else clause in a decision structure.
• The else suite is a block of statements executed after the statements in the try suite, only if no exceptions were raised.
• If an exception was raised, the else suite is skipped.

The finally Clause

• A try/except statement may include an optional finally clause, which appears after all the except clauses.
• It is aligned with the try and except clauses, and follows this format: finally: statements.
• The finally suiteis a block of statements after the finally clause that executes whether an exception occurs or not.
• The purpose is to perform cleanup before exiting and closing file resources.

Error Handling

• try and except blocks allow for handling errors or exceptional situations gracefully.
• The code within the else block is executed if no exceptions occur.
• The finally block is always executed, regardless of whether an exception was raised or not.
• Three cases of error handling are demonstrated with a valid division, division by zero, and dividing a string by a number (which raises a TypeError).

What If an Exception Is Not Handled?

• Exceptions can go unhandled if there is no except clause specifying the exception of the right type or if an exception is raised outside a try suite.
• Exceptions cause halt program.
• Python documentation provides information about exceptions that can be raised by different functions.