File Handling in Python

Introduction

File handling is a mechanism that allows Python programs to read data from disk files and write data back to disk files. Unlike standard input/output which is temporary, file handling enables permanent data storage.

Data Files

Data files can be stored in two formats:

• Text Files: Store information in ASCII/Unicode characters. Each line ends with an EOL (End of Line) character (\n, \r, or both).

• Binary Files: Store data in the same format as in memory, making them faster and more efficient but not directly readable.

Steps in File Handling

1. Opening a File: Using open(filename, mode).

2. Performing Read/Write Operations: Using functions like read(), write(), etc.

3. Closing the File: Using close() to release resources.

Opening Files

Syntax: file_object = open(filename, mode)

Modes:

• ‘r’: Read (default).
• ‘w’: Write (overwrites existing file).
• ‘a’: Append (adds to existing file).
• ‘b’: Binary mode (e.g., ‘rb’, ‘wb’).
• ‘+’: Read and write (e.g., ‘r+’, ‘w+’).

Example:

myfile = open(“story.txt”, “r”)  # Opens for reading

Closing Files

Use file_object.close() to release the file.

Example:

myfile = open(“data.txt”, “r”)
content = myfile.read()
myfile.close()

Reading from Files

Methods:

• read(size): Reads size bytes (or entire file if size is omitted).

• readline(size): Reads a line (up to size bytes).

• readlines(): Reads all lines into a list.

Example:

myfile = open(“data.txt”, “r”)
line1 = myfile.readline()  # Reads first line
lines = myfile.readlines()  # Reads all remaining lines
myfile.close()

Writing to Files

Methods:

• write(string): Writes a string to the file.

• writelines(list): Writes a list of strings to the file.

Example:

myfile = open(“output.txt”, “w”)
myfile.write(“Hello, World!\n”)
myfile.writelines([“Line 1\n”, “Line 2\n”])
myfile.close()

File Pointer

Tracks the current position for read/write operations.

Use seek(offset, whence) to move the pointer:

• whence=0: Start of file (default).
• whence=1: Current position.
• whence=2: End of file.

Example:

myfile = open(“data.txt”, “r”)
myfile.seek(10)  # Moves pointer to 10th byte

Binary File Operations

Using pickle Module:

Pickling (Writing):

import pickle
data = [1, 2, 3]
with open(“data.bin”, “wb”) as f:
    pickle.dump(data, f)

Unpickling (Reading):

import pickle
with open(“data.bin”, “rb”) as f:
    loaded_data = pickle.load(f)

CSV File Handling

Reading CSV:

import csv
with open(“data.csv”, “r”) as csvfile:
    reader = csv.reader(csvfile)
    for row in reader:
        print(row)

Writing CSV:

import csv
with open(“data.csv”, “w”) as csvfile:
    writer = csv.writer(csvfile)
    writer.writerow([“Name”, “Age”])
    writer.writerow([“Alice”, 25])

Absolute vs. Relative Paths

• Absolute Path: Full path from the root (e.g., C:\folder\file.txt).

• Relative Path: Path relative to the current directory (e.g., .\subfolder\file.txt).

Example:

import os
current_dir = os.getcwd()  # Gets current working directory

Standard Streams

• sys.stdin: Standard input (keyboard).

• sys.stdout: Standard output (monitor).

• sys.stderr: Standard error (monitor).

Example:

import sys
sys.stdout.write(“Hello, World!\n”)

Key Points

• Always close files after use or use with statements for automatic closure.

• Binary files are more efficient for structured data.

• Use pickle for serializing Python objects.

• CSV files are ideal for tabular data.