File Processing and Streams

This lesson covers file processing — reading from and writing to files — which is an essential skill for A-Level Computer Science. Programs need to persist data beyond their execution time, and files provide a way to store and retrieve data permanently.

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Why Use Files?

Data stored in variables is volatile — it is lost when the program terminates. Files provide persistent storage, allowing data to be:

• Saved for later use
• Shared between programs
• Backed up and archived
• Processed in batches

Storage Type	Volatile?	Example
Variables (RAM)	Yes — lost when program ends	A score counter in a game
Files (Disk)	No — persists after program ends	A saved game file, a CSV of student records
Database	No — persists and supports queries	A relational database of customers

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File Operations

The basic file operations are:

Operation	Description
Open	Prepares the file for reading or writing.
Read	Retrieves data from the file.
Write	Stores data to the file (overwriting existing content).
Append	Adds data to the end of the file without overwriting.
Close	Releases the file, ensuring all data is saved.

File Modes

Mode	Description
"r"	Read — opens for reading (file must exist).
"w"	Write — opens for writing (creates file or overwrites existing).
"a"	Append — opens for appending (creates file if it does not exist).
"r+"	Read and write — file must exist.

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Reading Files

Pseudocode

file = OPEN("students.txt", "r")
WHILE NOT file.endOfFile()
    line = file.readLine()
    OUTPUT line
END WHILE
file.close()

Python

# Method 1: Read all content at once
with open("students.txt", "r") as file:
    content = file.read()
    print(content)

# Method 2: Read line by line
with open("students.txt", "r") as file:
    for line in file:
        print(line.strip())

# Method 3: Read all lines into a list
with open("students.txt", "r") as file:
    lines = file.readlines()
    for line in lines:
        print(line.strip())

The with Statement

The with statement in Python automatically closes the file when the block ends, even if an exception occurs. This is the recommended approach.

# Without 'with' — you must remember to close
file = open("data.txt", "r")
content = file.read()
file.close()

# With 'with' — file is closed automatically
with open("data.txt", "r") as file:
    content = file.read()
# File is automatically closed here

Exam Tip: Always mention closing files in exam answers. If you use Python's with statement, explain that it handles closing automatically. Forgetting to close a file can lead to data loss or corruption.

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Writing Files

Pseudocode

file = OPEN("output.txt", "w")
file.writeLine("Name,Score")
file.writeLine("Alice,85")
file.writeLine("Bob,72")
file.close()

Python

with open("output.txt", "w") as file:
    file.write("Name,Score\n")
    file.write("Alice,85\n")
    file.write("Bob,72\n")

Appending to Files

with open("log.txt", "a") as file:
    file.write("New log entry\n")

Exam Tip: Be careful with the difference between "w" (write) and "a" (append). Using "w" will overwrite all existing content. Using "a" adds to the end. This is a common source of data loss.

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Processing CSV Files