📁 Files in Python (Improved Notes)

Python allows us to work with files stored on the operating system (OS) using built-in functions.

1. Opening Files

To open a file, we use:

filehandler = open("filename_or_path", "mode")

📍 File Path Rules

• If the file is in the same folder, just use:

  open("file.txt")

• If it’s in another location, use the full path:

  open("D:/folder/file.txt")

⚠️ ✅ Correction:

• ❌ “Python can’t use \” → Not fully correct
• ✅ Correct: Python can use \, but it must be escaped (\\) or replaced with /

✔ Examples:

"D:\\folder\\file.txt"   # correct
"D:/folder/file.txt"     # also correct

2. File Modes

3. Reading Files

filehandler = open("myInfo.txt", "r")
print(filehandler.read())

🔹 Read specific number of characters:

filehandler.read(20)

⚠️ Important Concept:

• Python uses a cursor (pointer) while reading files.
• After reading once, the cursor moves forward.

4. seek() Function

Moves the cursor to a specific position.

filehandler.seek(0)

⚠️ ✅ Correction:

• ❌ “seek() takes starting and ending value” → Incorrect
• ✅ Correct: seek() takes only one required argument (position)

✔ Correct usage:

filehandler.seek(0)   # move to beginning

5. tell() Function

Returns current cursor position:

filehandler.tell()

6. Writing to Files

filehandler = open("file.txt", "w")
filehandler.write("Hello")

⚠️ Important Behavior:

• w mode clears file ONLY ONCE when opened
• After that, multiple write() calls append in same session

⚠️ ✅ Correction:

• ❌ “After closing, write() wipes file again” → Incorrect explanation
• ✅ Correct: You cannot write after closing the file

✔ Correct:

filehandler.close()
# filehandler.write("data") ❌ ERROR

7. Appending

filehandler = open("file.txt", "a")
filehandler.write("New Data")

✔ Never deletes old content

⭐ Best Practice (NEW - IMPORTANT)

Instead of manually closing files:

with open("file.txt", "r") as f:
    print(f.read())

✅ Automatically closes file ✅ Safer (important for exams + real coding)

🖥️ OS Module

To work with OS:

import os

Common OS Functions

1. Create Folder

os.mkdir("NewFolder")

2. Remove Folder

os.rmdir("NewFolder")

⚠️ Only works if folder is empty

3. Current Directory

os.getcwd()

4. Change Directory

os.chdir("D:/folder")

⭐ Additional Methods (ADDED)

These were used in your class but missing in notes:

os.rename("old.txt", "new.txt")   # rename file
os.remove("file.txt")             # delete file

🔁 Control Statements in Python

1. if Statement

if condition:
    # code

2. if-else

if condition:
    # code
else:
    # code

3. if-elif-else

if condition:
    # code
elif condition:
    # code
else:
    # code

4. Nested if

if condition:
    if condition:
        # code

🔁 Loops in Python

1. for Loop

for i in range(10):
    print(i)

✔ Used when iterations are known

2. while Loop

i = 1
while i <= 10:
    print(i)
    i += 1

⚠️ ✅ Correction:

• ❌ Your example used:

  while i >= 10

• ✅ Correct condition: i <= 10

3. break and continue

• break → stops loop
• continue → skips current iteration

⚠️ for-else Concept (IMPORTANT FIX)

• ❌ Your note: “else runs when loop is stopped by break”
• ✅ Correct:

✔ else runs ONLY when loop finishes normally (NO break)

for i in range(5):
    print(i)
else:
    print("Finished")

🧪 Important Observations from Your Class Code

✔ Reading twice:

print(filehandler.read())
print(filehandler.read())  # empty because cursor is at end

✔ Writing:

fh3 = open("abc.txt","w")
fh3.write("text")

✔ Appending:

fh4 = open("abc.txt","a")

✔ Cursor:

fh5.read(20)
fh5.tell()
fh5.seek(0)

✔ OS Operations Used:

os.rename()
os.remove()
os.mkdir()
os.rmdir()
os.getcwd()
os.chdir()

✅ Final Summary (Exam Focus)

You should understand:

✔ File modes (r, w, a, etc.) ✔ Cursor behavior (read, seek, tell) ✔ Difference between w and a ✔ OS functions (mkdir, remove, etc.) ✔ Control statements (if, elif, else) ✔ Loops (for, while, break, continue) ✔ for-else concept (very important) ✔ with open() (best practice)