Python Read a Text File Line by Line

This guide explains read a text file line by line through a focused Python example. You will learn the behavior to expect, why it matters in a real program, and how to recognize the common mistakes before applying it in your own project.

What You Will Learn

Iterating directly over an open text file yields its lines without loading the entire file into a list. This approach is suitable for large logs and CSV-like text streams.

When and Why to Use It

The following example focuses on a practical use of Read a Text File Line by Line.

Step-by-Step Example

from io import StringIO

sample_file = StringIO("first\nsecond\nthird\n")
for line_number, line in enumerate(sample_file, start=1):
    print(line_number, line.rstrip("\n"))

Understanding the Example

The StringIO object behaves like a text stream, producing three numbered lines. rstrip removes the demonstrated newline for clean display.

Expected Output

1 first
2 second
3 third

Important Behavior and Edge Cases

rstrip() with no arguments removes all trailing whitespace, which may corrupt significant spaces in data; specify which characters you intend to remove.

Try It Yourself

Read a small local text file line by line and print only nonempty lines.

For file examples, use a dedicated practice directory or a temporary directory. Make sure you understand whether a call reads, creates, overwrites or appends before using it on important files.

Common Beginner Mistakes

• rstrip() with no arguments removes all trailing whitespace, which may corrupt significant spaces in data; specify which characters you intend to remove.
• Assuming the current working directory, encoding, filesystem contents or file permissions will be identical on every computer.
• Skipping the practice step and assuming the example covers every input. Test at least one normal case and one edge case relevant to this topic.

Key Takeaways

• Iterating directly over an open text file yields its lines without loading the entire file into a list.
• The StringIO object behaves like a text stream, producing three numbered lines.
• Apply the technique deliberately, checking its documented input requirements and failure behavior before integrating it into a larger project.