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File Handling

Opening and Closing Files

open() function, file modes, and proper cleanup

Interview: Essential — tests understanding of file I/O, resource management, and the with statement

Last Updated: June 12, 2026 • 6 min read

File handling is a fundamental skill in Python. The open() function provides access to files on disk, and understanding modes, encoding, and proper cleanup is essential for robust applications. Always use the with statement to ensure files are closed properly, even when exceptions occur.

The open() Function

  • open(file, mode, encoding) — returns a file object
  • Mode: 'r' (read), 'w' (write), 'a' (append), 'b' (binary)
  • Always specify encoding (encoding='utf-8') for text files
  • Files must be closed with close() or via with statement

Why with Statement?

The with statement is a context manager that guarantees close() is called when the block exits — even if an exception occurs. This prevents resource leaks and is considered the standard way to handle files in Python.

Common Pitfall

Opening files without with or explicit close() can lead to resource leaks. On Windows, unclosed files can prevent deletion. On Linux, you can run out of file descriptors.

Use Cases

Reading configuration files at application startup

Writing log files for application monitoring

Processing data files (CSV, JSON, XML)

Reading/writing user-uploaded files

Template file processing and report generation

Common Mistakes

Not using with statement — files stay open causing resource leaks

Not specifying encoding — may fail on non-ASCII characters on some systems

Opening for reading without handling FileNotFoundError

Using open() in write mode ('w') when you meant append ('a') — overwrites file!

Forgetting that file paths are relative to the current working directory