Reading & Writing With File Handling

Almost every real Python application reads or writes files — logs, configs, datasets, reports. Python provides a clean, OS-independent file API with the built-in open() function and the with statement. This chapter covers everything from text files and binary files to reading large logs efficiently.

• Open: open("file.txt", mode, encoding="utf-8").
• Modes: r read, w write (overwrite), a append, x create-new, b binary, + read/write.
• Always use the with statement for automatic cleanup.
• Use read(), readline(), readlines(), or iterate the file directly.

# write
with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("Line 1\nLine 2\n")

# read whole file
with open("notes.txt", encoding="utf-8") as f:
    content = f.read()

# read line by line (memory efficient)
with open("big.log", encoding="utf-8") as f:
    for line in f:
        process(line)

• File modes: r reads (default), w writes (truncates first), a appends, x creates new (fails if exists). Add b for binary or + for read/write.
• The with statement: with open(...) as f: is the cleanest pattern. The file is closed when the block exits, even on errors — preventing resource leaks.
• Reading: read() loads everything into memory — fine for small files. For large files iterate line-by-line. readlines() returns a list of all lines.
• Writing: Use write() for a string or writelines() for a list of strings. write() does NOT add a newline — append \n yourself.
• Encoding: Always specify encoding="utf-8" for text files. Without it, Python uses the platform default, which can cause bugs.
• Binary files: Open with "rb" or "wb" for images, audio, PDFs. Read/write bytes objects, not strings.

with open("hello.txt", "w", encoding="utf-8") as f:
    f.write("Hello, Python!\n")
    f.write("Welcome to PBA Institute.\n")
print("Written.")

with open("hello.txt", encoding="utf-8") as f:
    print(f.read())

with open("hello.txt", encoding="utf-8") as f:
    for line in f:
        print(line.rstrip())

with open("hello.txt", "a", encoding="utf-8") as f:
    f.write("Another line.\n")

with open("notes.txt", encoding="utf-8") as f:
    text = f.read()
print("Words:", len(text.split()))

with open("logo.png", "rb") as src, open("logo_copy.png", "wb") as dst:
    dst.write(src.read())
print("Copied")

• Always use with open(...) — it auto-closes files.
• Specify encoding="utf-8" for text files to avoid platform issues.
• Iterate line-by-line for large files; never load gigabytes into memory.
• Use "a" mode for appending — "w" overwrites the file.
• Use os.path or pathlib.Path for cross-platform paths.
• For structured data, prefer json, csv, or pandas modules.

• Forgetting to close: file handles leak; always use with.
• Wrong mode: opening with "w" erases the existing content.
• Encoding mismatch: reading a UTF-8 file as ASCII raises UnicodeDecodeError.
• Reading whole files needlessly: stream large files line by line.
• Mixing text and binary: write bytes to a binary file, strings to a text file.
• Hard-coded paths: use pathlib or join with os.path.join for portability.

• Problem 1: Write the first 10 squares to a file squares.txt.
• Problem 2: Read a file and count the number of lines, words, and characters.
• Problem 3: Copy the contents of one file into another using Python.
• Problem 4: Read a log file and print only lines containing the word 'ERROR'.
• Problem 5: Append the current date and a user message to a file each time the program runs.
• Problem 6: Read a binary image file and save a copy with a new name.

• Q1. What is the role of the with statement in file handling?
• Q2. What are the different file modes in Python?
• Q3. What's the difference between read() and readline()?
• Q4. Why is specifying encoding important?
• Q5. How would you process a 10 GB file in Python?
• Q6. What is the difference between text mode and binary mode?

• Q1: How do I open a file?
  Use open(path, mode). Combine with the with statement to ensure it is closed automatically.
• Q2: What does 'a' mode do?
  It opens the file for appending. New writes go at the end without erasing existing content.
• Q3: Why use encoding='utf-8'?
  UTF-8 is the de-facto standard for text. Without specifying it, Python uses the OS default which differs across systems.
• Q4: How do I check if a file exists?
  Use pathlib.Path('file').exists() or os.path.exists('file') before opening.
• Q5: Can I read a file in chunks?
  Yes — call f.read(chunk_size) or iterate line by line for streaming.
• Q6: What's the difference between text and binary mode?
  Text mode returns str and applies encoding; binary mode returns bytes with no transformation.