Python: File Input/Output Operations

Last updated: 2026-09-22

So far, our program data has only existed in memory — close the program, and the data is lost. File I/O allows programs to save data to disk and read it back on the next startup. This is a fundamental capability for any practical application.

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1. Opening and Closing Files

The basic file operation has three steps: open -> read/write -> close.

▶ Example: Manually Opening and Closing a File

Output:

# Executed successfully

Warning: Always remember close()! Open files consume system resources. Forgetting to close can lead to file corruption or resource exhaustion. But don't worry — the next section has a better approach.

2. The with Statement (Context Manager)

The with statement automatically closes the file at the end of the code block, without needing to call close():

▶ Example: Safe File I/O with with

Output:

# Executed successfully

Tip: Always use with for file operations. It's the safest, most Pythonic way — the file is correctly closed whether or not an exception occurs in the code block.

3. File Open Modes

The second parameter of open() is the mode:

▶ Example: Different File Open Modes

Output:

First line
Second line
Third line

4. Ways to Read Files

▶ Example: Three Ways to Read a File

Output:

# Executed successfully

▶ Example: File Backup Tool (Difficulty: Star-Star)

Output:

# Function defined successfully

5. Reading and Writing CSV Files

CSV (Comma-Separated Values) is the most common data exchange format. Python has a built-in csv module:

▶ Example: CSV File I/O

Output:

['Name', 'Age', 'City']
['Zhang San', '25', 'Beijing']
['Li Si', '30', 'Shanghai']
['Wang Wu', '22', 'Guangzhou']

(1) Using DictReader/DictWriter (More Intuitive)

▶ Example: DictReader/DictWriter for CSV

Output:

# Executed successfully

6. Path Handling

Use os.path or pathlib for file path handling — safer than string concatenation:

▶ Example: os.path Path Handling

Output:

# Executed successfully

(1) Recommended: pathlib (Python 3.4+)

▶ Example: pathlib Path Operations

Output:

# Executed successfully

Tip: pathlib is the recommended way to handle paths in Python 3.4+. It's more object-oriented and intuitive than os.path. For new projects, use pathlib.

7. Common Use Cases

• Data persistence: Save program data to files, restore on next startup.
• Logging: Append program runtime information to log files.
• Configuration management: Read configuration files (.ini, .json, .env).
• Data import/export: Exchange data with Excel/databases using CSV format.
• Batch file processing: Traverse directories, batch modify file content or rename files.

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❓ FAQ

📖 Summary

• File operations in three steps: open() -> read/write -> close()
• The with statement manages file closure automatically — the safest approach
• Modes: "r" read, "w" write, "a" append
• Reading methods: read() full content, readlines() as list, for line in f line by line
• The csv module for CSV file I/O; supports DictReader/DictWriter
• os.path and pathlib for file path handling; pathlib is more modern
• Always specify encoding="utf-8" for Chinese text file operations

📝 Exercises

1. Beginner (Difficulty: Star): Write a program that accepts a user's name and age and saves them to a users.txt file (one user per line, format: name,age). Append new records each run without overwriting existing data.

2. Intermediate (Difficulty: Star-Star): Write a program that reads the users.txt file from the previous exercise, parses each line, calculates and outputs the average age of users. Hint: Use split(",") to parse, int() to convert age.

3. Advanced (Difficulty: Star-Star-Star): Write a "simple notebook" program. Features: add:title|content to add a note (append to notes.csv), list to list all notes, search:keyword to search notes. Store in CSV format (fields: title, content, creation time). Hint: Use datetime.now() for the current timestamp.

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