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File Handling

Writing Files

write, writelines, and file output patterns

Interview: File output — tests understanding of write modes, buffering, and efficient output

Last Updated: June 12, 2026 • 5 min read

Writing files in Python uses write() and writelines() methods. Understanding the difference between write mode ('w', overwrites) and append mode ('a', adds to end) is critical to avoid data loss.

Writing Methods

  • f.write(string): Write a string, returns number of characters written
  • f.writelines(list): Write a list of strings (no separators added)
  • print(..., file=f): Use print function to write to file
  • f.flush(): Force write buffer to disk immediately

Common Pitfall

writelines() does NOT add newlines between lines. You must include them: f.writelines([line + '\n' for line in lines]).

Use Cases

Writing log files for application monitoring

Generating reports and output files

Saving application state and configuration

Creating data export files (CSV, JSON)

Template-based file generation

Common Mistakes

Using write mode ('w') when you meant append ('a') — overwrites existing data!

Forgetting that writelines() doesn't add newlines between items

Not specifying encoding for text files

Not flushing or closing before reading the file back

Writing binary data in text mode (use 'wb' for bytes)