File Handling in Python

🟡 Intermediate

📖 Definition

File Handling allows Python programs to create, read, update, write, and append data files on disk storage. Python provides the open() built-in function and the with context manager (with open(...) as file:) which guarantees that files are automatically closed when operations complete, even if errors occur.

🇮🇳 Hindi

Disk par stored files ko read aur write karne ke liye File Handling ka use hota hai. Hamesha with open(...) as file: Context Manager ka use karein—isse file operations complete hone par file automatically close ho jaati hai, jisse memory/resource leaks nahi hote.

🚩 Marathi

File madhun data vachnyasathi aani lihinyasathi open() function vaparatat. Automatic file close honyasathi with open(...) as file: cha wapar kara.

📝 1. File Access Modes

Mode Meaning What It Does
'r' Read (Default) Opens file for reading. Raises FileNotFoundError if file is missing.
'w' Write Opens file for writing. Truncates (overwrites) file if it exists, creates if missing!
'a' Append Opens file for appending. Preserves existing content and appends at the end.
'r+' Read/Write Opens file for both reading and writing.
'b' Binary Appended for binary files ('rb', 'wb') like images, PDFs, or audio files.

📝 2. File Reading Methods

  • file.read(): Reads entire file content as a single string.
  • file.readline(): Reads a single line from the file.
  • file.readlines(): Reads all lines into a list of strings (['line1\n', 'line2\n']).
  • Iterating File Directly (Best Memory Practice): Streams line by line without loading the entire file into memory!
# Memory-Efficient Line-by-Line Iteration
with open("large_file.txt", "r", encoding="utf-8") as file:
    for line in file:
        print(line.strip()) # Process each line

💡 Complete Example: Writing & Reading Application Logs

import os

log_file_path = "app_activity.log"

print("=== FILE HANDLING DEMO ===")

# 1. Writing / Appending to Log File using Context Manager
log_entries = [
    "USER_LOGIN: User 'Aarav' logged in successfully.",
    "DB_QUERY: Fetched 25 records from 'users' table.",
    "USER_LOGOUT: User 'Aarav' logged out."
]

# Append entries to file
with open(log_file_path, "a", encoding="utf-8") as file:
    for entry in log_entries:
        file.write(entry + "\n")
        
print(f"Appended {len(log_entries)} log entries to '{log_file_path}'.")

# 2. Reading and Parsing Logs Line-by-Line
print("\n--- READING LOG FILE CONTENT ---")

if os.path.exists(log_file_path):
    with open(log_file_path, "r", encoding="utf-8") as file:
        for line_num, line in enumerate(file, start=1):
            print(f"Line #{line_num}: {line.strip()}")
            
    # Cleanup log file after demo
    os.remove(log_file_path)
    print(f"\nCleaned up demo file '{log_file_path}'.")

👀 Output

=== FILE HANDLING DEMO ===
Appended 3 log entries to 'app_activity.log'.

--- READING LOG FILE CONTENT ---
Line #1: USER_LOGIN: User 'Aarav' logged in successfully.
Line #2: DB_QUERY: Fetched 25 records from 'users' table.
Line #3: USER_LOGOUT: User 'Aarav' logged out.

Cleaned up demo file 'app_activity.log'.

⚠️ Common Mistakes

  • Opening files with f = open('file.txt') without using with open(), which leaves unclosed file handles open and causes resource leaks!
  • Opening an existing file in 'w' (write) mode when you intended to append ('a'), causing all previous content to be permanently wiped out!
  • Forgetting to specify encoding="utf-8", causing platform-dependent encoding crashes when reading non-ASCII characters on Windows.

🛡️ Safety / Important Notes

Always use encoding="utf-8" explicitly when calling open() to guarantee cross-platform text rendering consistency across Windows, macOS, and Linux.

🌍 Real-World Usage

Writing application log files, saving configuration settings, processing CSV/JSON data files, generating text reports, and reading database dumps.

🧪 Try It Yourself

  1. Create a script that creates a file notes.txt and writes 3 lines into it.
  2. Read notes.txt using with open() and print the total number of lines present.

🎯 Mini Challenge

Write a program that reads a text file, counts how many times the word "python" appears in the file (case-insensitive), and prints the count.

🔗 Related Topics

🧭 Navigation

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