GCSE Computer Science Revision — File Handling

Revise File Handling for GCSE Computer Science with a topic explanation, worked example and common mistakes. Check the board notes for specification differences.

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Topic explanation

What is File Handling?

File handling allows a program to read data from and write data to external files. This is essential for making data persistent, meaning it is saved even after the program has stopped running. Programs can open files in different modes, such as read mode to access existing data, write mode to create a new file or overwrite an old one, and append mode to add new data to the end of a file.

Board notes: All GCSE boards (AQA, Edexcel, OCR) cover basic file handling. You will be expected to know how to open, read from, write to, and append to simple text files (.txt) or comma-separated value files (.csv).

Step-by-step explanation

Worked examples

Worked example

To save a user's score to a file named `score.txt` in Python: `with open('score.txt', 'w') as f: f.write('150')`. The `with` statement is good practice as it automatically closes the file. To read the score back: `with open('score.txt', 'r') as f: score = f.read()`. The variable `score` would then contain the string '150'.

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Common mistakes

1Forgetting to close a file after opening it. This can lock the file and, in some cases, lead to data loss as the changes might not be saved until the file is properly closed.
2Trying to read from a file that has been opened in write mode, or write to a file opened in read mode. This will cause a runtime error.
3Confusing write mode and append mode. Write mode (`w`) will erase the entire contents of an existing file, while append mode (`a`) will simply add to the end of it.

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Frequently asked questions

What is the difference between reading a file line by line versus all at once?
Reading a file all at once with `.read()` is simple but can use a lot of memory for large files. Reading line by line, for example using a loop, is more memory-efficient as it only processes one line at a time.
How do you handle errors when a file doesn't exist?
You should use a `try...except` block. You `try` to open the file, and if a `FileNotFoundError` occurs, the `except` block can handle it gracefully, for example by printing an error message instead of crashing the program.

What is File Handling?

Step-by-step explanation

Worked examples

Worked example

Common mistakes

1Forgetting to close a file after opening it. This can lock the file and, in some cases, lead to data loss as the changes might not be saved until the file is properly closed.

2Trying to read from a file that has been opened in write mode, or write to a file opened in read mode. This will cause a runtime error.

3Confusing write mode and append mode. Write mode (`w`) will erase the entire contents of an existing file, while append mode (`a`) will simply add to the end of it.

Frequently asked questions

What is the difference between reading a file line by line versus all at once?

Reading a file all at once with `.read()` is simple but can use a lot of memory for large files. Reading line by line, for example using a loop, is more memory-efficient as it only processes one line at a time.

How do you handle errors when a file doesn't exist?

You should use a `try...except` block. You `try` to open the file, and if a `FileNotFoundError` occurs, the `except` block can handle it gracefully, for example by printing an error message instead of crashing the program.