Day 15: Python Programming

File Handling

File handling is one of the MOST important concepts in Python.
It allows your program to create, read, update, and delete files.

Python supports different file types:

• Text files (.txt)
• CSV files (.csv)
• JSON files (.json)
• Binary files (.bin)
• Others (images, pdf, etc.)

1. Types of File Modes

Mode	Meaning
"r"	Read mode (file must exist)
"w"	Write mode (creates new file, deletes old content)
"a"	Append mode (adds data at end)
"r+"	Read + Write
"w+"	Write + Read (deletes old content)
"a+"	Append + Read
"b"	Binary mode (images, videos)

You can combine modes:
"rb" → read binary
"wb" → write binary

2. Reading a File
Example file: data.txt
Hello Python
Welcome to File Handling
Read entire file
f = open("data.txt", "r")
print(f.read())
f.close()
Read line by line
f = open("data.txt", "r")
for line in f:
print(line)
f.close()
Read first 10 characters
f = open("data.txt", "r")
print(f.read(10))
f.close()

3. Writing to a File
Rewrite the file (old content removed)
f = open("data.txt", "w")
f.write("This is day 22 content.\nLearning Python!")
f.close()

4. Append to a File
Add new lines without deleting old data
f = open("data.txt", "a")
f.write("\nAdding new content at the end.")
f.close()

5. Using with (Best Practice)
The file automatically closes — no need to call close().
Reading using with
with open("data.txt", "r") as f:
print(f.read())
Writing using with
with open("data.txt", "w") as f:
f.write("Hello from with block!")

6.Read File Into a List

Each line becomes list element.
with open("data.txt", "r") as f:
lines = f.readlines()
print(lines)

Example output:
['Hello Python\n', 'Welcome to File Handling']

7. Writing a List to a File
lines = ["Apple\n", "Banana\n", "Cherry\n"]
with open("fruits.txt", "w") as f:
f.writelines(lines)

8.Checking File Exists
Using os module:
import os
if os.path.exists("data.txt"):
print("File exists")
else:
print("File not found")

9. Delete a File
import os
os.remove("data.txt")
this permanently deletes the file

10. File Pointer Methods

Method	Use
tell()	Shows current cursor position
seek()	Move cursor to a position

Example
f = open("data.txt", "r")
print(f.tell()) # Position
print(f.read(5)) # Reads 5 chars
f.seek(0) # Move to start
print(f.read())
f.close()

11.Handling Errors in File Operations

Try–Except
try:
f = open("abc.txt")
print(f.read())
except FileNotFoundError:
print("File does not exist!")

12.File Handling Mini-Programs

Program 1: Count number of lines
with open("data.txt", "r") as f:
count = len(f.readlines())
print("Total lines:", count)

Program 2: Count words in file
with open("data.txt", "r") as f:
text = f.read()
words = text.split()
print("Total words:", len(words))

Program 3: Copy one file to another
with open("data.txt", "r") as f1:
with open("copy.txt", "w") as f2:
f2.write(f1.read())