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File Handling

Reading Files

read, readline, readlines, and iteration methods

Interview: File input — tests understanding of memory-efficient reading and different reading strategies

Last Updated: June 12, 2026 • 5 min read

Python provides multiple ways to read file content. The right method depends on file size and what you need to do with the data. For large files, iterate line by line; for small files, read all at once.

Reading Methods

  • f.read(): Read entire file as one string
  • f.read(n): Read n characters/bytes
  • f.readline(): Read one line at a time
  • f.readlines(): Read all lines into a list
  • for line in f: Iterate lines (most memory-efficient)

Interview Tip

Know when to use each method. for line in f is the most Pythonic for line-by-line processing and uses minimal memory regardless of file size.

Use Cases

Processing log files line by line (memory efficient)

Reading configuration files at startup

Parsing CSV/JSON data files

Reading large files in chunks (binary data, media files)

Implementing file-based search (grep-like functionality)

Common Mistakes

Using readlines() on large files — loads everything into memory

Forgetting to strip newlines: line.strip() or line.rstrip()

Not handling encoding errors with errors= parameter

Using read() when readline() would suffice (reads entire file)

Forgetting that after read(), the file pointer is at the end (need seek(0))