File Handling

This lesson covers file handling for the OCR A-Level Computer Science (H446) specification. You need to understand how programs read from and write to text files, CSV files, and binary files, as well as the difference between sequential and random access.

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Why Use Files?

Data stored in variables is volatile -- it is lost when the program ends. Files provide persistent storage, allowing data to be saved and retrieved between program executions.

Storage Type	Volatile?	Example
Variables (RAM)	Yes	Program variables, arrays
Files (disk)	No	Text files, databases, images

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File Modes

When opening a file, you specify a mode that determines how the file can be accessed:

Mode	Description	Creates File?	Overwrites?
"r"	Read -- open for reading only	No (error if not found)	No
"w"	Write -- open for writing; creates file or overwrites existing	Yes	Yes
"a"	Append -- open for writing; adds to end of file	Yes	No
"r+"	Read and write	No	No (but can overwrite at position)
"rb"	Read binary	No	No
"wb"	Write binary	Yes	Yes

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Reading from Text Files

Reading the Entire File

# Using 'with' ensures the file is automatically closed
with open("students.txt", "r") as file:
    content = file.read()
    print(content)

Reading Line by Line

with open("students.txt", "r") as file:
    for line in file:
        print(line.strip())  # strip() removes trailing newline

Reading into a List

with open("students.txt", "r") as file:
    lines = file.readlines()
    # lines is a list of strings, one per line

for line in lines:
    print(line.strip())

Pseudocode

file = OPENREAD("students.txt")
WHILE NOT file.endOfFile()
    line = file.readLine()
    OUTPUT line
ENDWHILE
file.close()

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Writing to Text Files

Overwriting a File

with open("output.txt", "w") as file:
    file.write("Hello, World!\n")
    file.write("This is line 2\n")

Appending to a File

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

Pseudocode

file = OPENWRITE("output.txt")
file.writeLine("Hello, World!")
file.writeLine("This is line 2")
file.close()

Exam Tip: Always remember to close files after use. In Python, the with statement does this automatically. In pseudocode, always show file.close(). Failure to close a file can result in data loss or corruption.

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CSV Files

CSV (Comma-Separated Values) files store tabular data as plain text, with each value separated by a comma and each row on a new line.

Example CSV File (students.csv)

Name,Age,Grade
Alice,17,A
Bob,18,B
Charlie,17,C

Reading a CSV File

import csv

with open("students.csv", "r") as file:
    reader = csv.reader(file)
    header = next(reader)  # Skip header row
    for row in reader:
        name = row[0]
        age = int(row[1])
        grade = row[2]
        print(f"{name} (age {age}): Grade {grade}")

Writing a CSV File

import csv

students = [
    ["Alice", 17, "A"],
    ["Bob", 18, "B"],
    ["Charlie", 17, "C"]
]

with open("students.csv", "w", newline="") as file:
    writer = csv.writer(file)
    writer.writerow(["Name", "Age", "Grade"])  # Header
    writer.writerows(students)  # All data rows

Using DictReader and DictWriter

import csv

# Reading with DictReader
with open("students.csv", "r") as file:
    reader = csv.DictReader(file)
    for row in reader:
        print(f"{row['Name']}: {row['Grade']}")

# Writing with DictWriter
with open("output.csv", "w", newline="") as file:
    fieldnames = ["Name", "Age", "Grade"]
    writer = csv.DictWriter(file, fieldnames=fieldnames)
    writer.writeheader()
    writer.writerow({"Name": "Diana", "Age": 18, "Grade": "A*"})

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Binary Files