Introduction to File Operations

What is a File?

Files are used to store data persistently. Example:

Purpose of Files

As the saying goes, " palest ink is better than the best memory." Computers, like humans, can forget data. For instance, a program might spend significant effort calculating a result; without saving it, the data would be lost after a reboot.

Files allow us to store data so that it can be reused later without recalculating or recreating it.

Opening and Closing Files

Consider the process of writing a resume in Word:

1. Open Word and create a new file.
2. Write the resume content.
3. Save the file.
4. Close Word.

Similarly, file operations follow a similar pattern:

1. Open (or create) a file.
2. Read or write data.
3. Close the file.

Opening Files

In Python, the open() function is used to open an existing file or create a new one.

open(filename, mode)

Example:

f = open('test.txt', 'w')

File Access Modes

Mode	Description
r	Read-only; file pointer at start (default).
w	Write-only; overwrites existing file or creates new one.
a	Append; writes at end; creates new file if it doesn't exist.
rb	Binary read-only.
wb	Binary write-only; overwrites or creates.
ab	Binary append; creates new file if missing.
r+	Read/write; file pointer at start.
w+	Read/write; overwrites or creates.
a+	Read/write; append mode (end); creates new file if missing.
rb+	Binary read/write; start.
wb+	Binary read/write; overwrites or creates.
ab+	Binary read/write; append; creates new file if missing.

Closing Files

Always close a file after use with the close() method.

f = open('test.txt', 'w')
f.close()

Reading and Writing Files

Writing Data: write()

Use write() to write data to a file.

f = open('test.txt', 'w')
f.write('hello world, i am here!')
f.close()

Output:

Note: If the file exists, it is overwritten (cleared first). If it does not exist, a new file is created.

Reading Data: read()

read(num) reads up to num bytes. Without argumants, it reads the entire file.

f = open('test.txt', 'r')

content = f.read(5)
print(content)

print('-' * 30)

content = f.read()
print(content)

f.close()

Note:

• If you open a file in read mode, you can omit the mode: open('test.txt').
• Subsequent reads continue from where the previous read stopped.

Reading Lines: readlines()

readlines() reads all lines and returns them as a list, each line being a string element (including newline characters).

f = open('test.txt', 'r')
content = f.readlines()

print(type(content))

for i, line in enumerate(content, 1):
    print(f'{i}:{line}')

f.close()

Reading Single Line: readline()

readline() reads one line at a time.

f = open('test.txt', 'r')

print('1:' + f.readline())
print('2:' + f.readline())

f.close()

Handling Large Files

For large files (e.g., 5 GB), reading the entire file into memory is inefficient. Use iteration or read in chunks.

Application: File Backup

Task

Input a file name, and the program automatically creates a backup copy with a suffix like [backup].

Code

old_filename = input('Enter the file name to copy: ')

with open(old_filename, 'r') as old_file:
    dot_index = old_filename.rfind('.')
    if dot_index > 0:
        extension = old_filename[dot_index:]
        new_filename = old_filename[:dot_index] + '[backup]' + extension
    else:
        new_filename = old_filename + '[backup]'

    with open(new_filename, 'w') as new_file:
        for line in old_file:
            new_file.write(line)

Random Access: tell() and seek()

Getting Current Position: tell()

f = open('test.txt', 'r')
data = f.read(3)
print(f'Read data: {data}')
position = f.tell()
print(f'Current position: {position}')

data = f.read(3)
print(f'Read data: {data}')
position = f.tell()
print(f'Current position: {position}')

f.close()

Seeking to a Position: seek(offset, whence)

• offset: number of bytes to move.
• whence: reference point.
  • 0: beginning of file
  • 1: current position
  • 2: end of file

Example: Seek from beginning

f = open('test.txt', 'r')
data = f.read(30)
print(f'Read data: {data}')

position = f.tell()
print(f'Current position: {position}')

f.seek(5, 0)  # Move to 5th byte from beginning

position = f.tell()
print(f'Current position: {position}')

f.close()

Example: Seek from end

f = open('test.txt', 'r')

position = f.tell()
print(f'Current position: {position}')

f.seek(-3, 2)  # Move to 3 bytes before end

data = f.read()
print(f'Read data: {data}')

f.close()

Renaming and Deleting Files

Use the os module for file system operations.

Renaming a File

import os
os.rename('thesis.txt', 'thesis_final.txt')

Deleting a File

import os
os.remove('thesis.txt')

Folder Operations

The os module also handles directories (folders).

Creating a Folder

import os
os.mkdir('new_folder')

Getting the Current Directory

import os
current_dir = os.getcwd()

Changing the Current Directory

import os
os.chdir('../')

Listing Directory Contents

import os
items = os.listdir('./')

Removing a Folder

import os
os.rmdir('new_folder')

Application: Batch Rename Files

Before and After

Before:

After:

Code: Add or Remove a Prefix

import os

# 1: add prefix, 2: remove prefix
operation = 1
prefix = '[MyBrand]-'
folder = './renameDir/'

file_list = os.listdir(folder)

for name in file_list:
    if operation == 1:
        new_name = prefix + name
    elif operation == 2:
        new_name = name[len(prefix):]

    os.rename(os.path.join(folder, name), os.path.join(folder, new_name))