File Handling in Python

File Handling: • File handling allows Python programs to create, read, write, and manipulate files stored on disk — enabling data to persist beyond the program's execution. • Python's built-in open() function is used to open files. Syntax: file_object = open(filename, mode) File Opening Modes: 1. 'r' (Read — default): • Opens an existing file for reading only. • File pointer placed at the beginning. • Error (FileNotFoundError) if file does not exist. • Example: f = open('data.txt', 'r') 2. 'w' (Write): • Opens file for writing only. • Creates the file if it does not exist. • If the file exists, it is TRUNCATED (all content erased) before writing. • Example: f = open('output.txt', 'w') 3. 'a' (Append): • Opens file for appending — new data is added at the END of the file. • Creates the file if it does not exist. • Existing content is NOT erased. • Example: f = open('log.txt', 'a') 4. 'r+' (Read and Write): • Opens existing file for both reading and writing. • Does not truncate; file must exist. 5. 'w+' (Write and Read): • Opens file for reading and writing; truncates if exists. 6. 'a+' (Append and Read): • Opens for reading and appending. 7. 'b' (Binary mode — combined with other modes): • Opens file in binary mode (for images, audio, PDFs, etc.) • Example: 'rb' (read binary), 'wb' (write binary) Opening and Closing: # Method 1: Explicit open and close f = open('data.txt', 'r') content = f.read() f.close() # Always close to release resources # Method 2: with statement (recommended — auto-closes) with open('data.txt', 'r') as f: content = f.read() # File is automatically closed when the with block exits The with statement is preferred because it ensures the file is closed even if an exception occurs.

Reading Methods: 1. read(size=-1): • Reads the entire file content as a single string (if size not specified). • If size is specified, reads that many characters. with open('data.txt', 'r') as f: content = f.read() # entire file content = f.read(100) # first 100 chars 2. readline(): • Reads one line at a time (including the '\n' newline character). • Returns an empty string ('') when end of file is reached. with open('data.txt', 'r') as f: line = f.readline() # reads first line while line: print(line, end='') line = f.readline() 3. readlines(): • Reads all lines and returns them as a list of strings. with open('data.txt', 'r') as f: lines = f.readlines() # e.g., ['Hello\n', 'World\n'] 4. Iterating directly (most Pythonic): with open('data.txt', 'r') as f: for line in f: print(line, end='') Writing Methods: 1. write(string): • Writes a string to the file. • Does NOT add a newline automatically — you must include '\n' if needed. • Returns the number of characters written. with open('output.txt', 'w') as f: f.write('Hello, World!\n') f.write('Second line\n') 2. writelines(list_of_strings): • Writes a list of strings to the file. • Does NOT add newlines — include '\n' in each string if needed. lines = ['Line 1\n', 'Line 2\n', 'Line 3\n'] with open('output.txt', 'w') as f: f.writelines(lines) File Pointer Methods: • tell(): Returns current position of the file pointer (byte offset from start). • seek(offset, from_what=0): Moves the file pointer. - seek(0) — move to beginning - seek(0, 2) — move to end

CSV (Comma-Separated Values): • A CSV file is a plain text file that stores tabular data — each row is a line of text, and values in each row are separated by commas (or another delimiter). • CSV files are widely used for data exchange between applications (spreadsheets, databases, data science tools). • Example CSV content (students.csv): RollNo,Name,Marks 1,Priya,92 2,Arjun,85 3,Meena,78 Python's csv Module: • Python's standard library includes the csv module for easy reading/writing of CSV files. Writing a CSV File: import csv data = [ ['RollNo', 'Name', 'Marks'], [1, 'Priya', 92], [2, 'Arjun', 85], [3, 'Meena', 78] ] with open('students.csv', 'w', newline='') as f: writer = csv.writer(f) writer.writerows(data) # writes all rows at once # OR: writer.writerow(row) to write one row at a time # Note: newline='' prevents extra blank lines on Windows. Reading a CSV File: import csv with open('students.csv', 'r') as f: reader = csv.reader(f) for row in reader: print(row) # each row is a list of strings # Output: # ['RollNo', 'Name', 'Marks'] # ['1', 'Priya', '92'] # ['2', 'Arjun', '85'] Using DictReader and DictWriter (more readable): # DictReader — reads rows as dictionaries (keys = header column names) with open('students.csv', 'r') as f: reader = csv.DictReader(f) for row in reader: print(row['Name'], row['Marks']) # Output: Priya 92, Arjun 85 ... # DictWriter — writes from list of dictionaries import csv fields = ['RollNo', 'Name', 'Marks'] rows = [{'RollNo': 1, 'Name': 'Priya', 'Marks': 92}] with open('students.csv', 'w', newline='') as f: writer = csv.DictWriter(f, fieldnames=fields) writer.writeheader() # writes the header row writer.writerows(rows)