File Reading and Writing Basics in Python

 File I/O (Input/Output) allows programs to interact with external files on disk, like saving data to a text file or loading configurations. Python makes this straightforward using the built-in open() function. Always use the context manager (with statement) to automatically close files and handle resources safely.

Key Concepts

• Opening a File: file = open('filename.txt', 'mode')
  • Modes control access: read-only, write, etc.
• Reading: Pull data from the file into memory (as strings or lists).
• Writing: Push data from memory to the file.
• Closing: Frees resources; with handles this automatically.
• Error Handling: Use try-except for issues like "file not found."
• Binary Files: For images/PDFs, add 'b' to mode (e.g., 'rb').

Common Modes

Mode	Description	Use Case	Overwrites?
'r'	Read (default)	Load existing file	No
'w'	Write	Create/overwrite text file	Yes
'a'	Append	Add to end of file	No
'r+'	Read + Write	Modify existing file	No
'rb'/'wb'	Binary Read/Write	Images, executables	Varies

Reading Files

1. Read Entire File:
   python

   with open('example.txt', 'r') as file:
       content = file.read()  # Returns full string
   print(content)

2. Read Line by Line:
   python

   with open('example.txt', 'r') as file:
       line = file.readline()  # First line as string
       # Or loop:
       for line in file:  # Iterates over lines
           print(line.strip())  # strip() removes \n

3. Read All Lines into List:
   python

   with open('example.txt', 'r') as file:
       lines = file.readlines()  # List of strings
   print(lines[0])  # First line

Writing Files

1. Write String:
   python

   with open('output.txt', 'w') as file:
       file.write("Hello, World!\n")  # \n for new line

2. Append Text:
   python

   with open('output.txt', 'a') as file:
       file.write("More text!\n")

3. Write List of Lines:
   python

   lines = ["Line 1\n", "Line 2\n"]
   with open('output.txt', 'w') as file:
       file.writelines(lines)

Full Example: Read, Process, Write

python

# Read from input.txt
with open('input.txt', 'r') as infile:
    data = infile.read().upper()  # Convert to uppercase

# Write processed to output.txt
with open('output.txt', 'w') as outfile:
    outfile.write(data)

print("File processed!")

Tips and Best Practices

• Encoding: For non-ASCII text, add encoding='utf-8' to open().
• Exceptions:
  python

  try:
      with open('missing.txt', 'r') as file:
          print(file.read())
  except FileNotFoundError:
      print("File not found!")

• CSV/JSON: Use csv or json modules for structured data (e.g., import csv; csv.reader()).
• Performance: For large files, read in chunks: file.read(1024) (bytes at a time).

This covers the essentials for text files. For binary, databases, or another language (e.g., Java's FileWriter), let me know for more!