File Handling: Making Data Persistent

What might confuse you right now

"I have data in variables. Why write to a file?"

Memory gets wiped when the program ends. Files are how you save and share data long-term.

One-line definition

File handling is reading, writing, and appending data to disk using different modes.

Real-life analogy

Variables are like whiteboard notes -- gone when the power's off. Files are like notebooks -- they stick around.

Minimal runnable example

with open("demo.txt", "w", encoding="utf-8") as f:
    f.write("Hello Python\n")

with open("demo.txt", "r", encoding="utf-8") as f:
    print(f.read())

Mode selection

• r: read
• w: overwrite
• a: append

Quick quiz (5 min)

1. Write three lines of text, then read them back.
2. Append one more line using a mode.
3. Count the total number of lines in the file.

Quiz answer guidelines & grading criteria

• Answer direction: working code that covers core conditions and edge inputs from the prompt.
• Criterion 1 (Correctness): Main flow produces correct results, key branches execute.
• Criterion 2 (Readability): Clear variable names, no excessive nesting.
• Criterion 3 (Robustness): Basic protection against null values, type errors, or unexpected input.

Transfer task (homework)

Build a "log summarizer": read log files, output a summary file.

Acceptance criteria

You can independently:

• Read and write files using with open()
• Choose the right r/w/a mode
• Handle encoding issues

Common errors & debugging steps (beginner edition)

• Can't understand the error: read the last line for the error type (e.g., TypeError, NameError), then trace back to the relevant code line.
• Not sure about a variable's value: temporarily add print(variable, type(variable)) at key points to verify data matches expectations.
• Code changes aren't taking effect: confirm the file is saved, you're running the right file, and your terminal environment (venv) is correct.

Common misconceptions

• Misconception: Forgetting to specify encoding.
• Reality: Always explicitly use utf-8 for text files.