CBSE Class 12 Python: functions, files, stack and SQL

def area(length, breadth=None):
    """Area of a rectangle; a square if breadth is not given."""
    if breadth is None:
        breadth = length
    return length * breadth

def stats(marks):
    return max(marks), min(marks), sum(marks) / len(marks)   # returns a tuple

count = 0                      # global variable
def add_student(name):
    global count               # needed only to CHANGE a global inside a function
    count += 1
    print("Added", name, "-> total", count)

print(area(5, 3), area(4))               # positional, then the default
print(area(breadth=2, length=6))         # keyword arguments, any order
hi, lo, avg = stats([88, 72, 95])
print(hi, lo, round(avg, 2))
add_student("Neha")
add_student("Arjun")
print(len("abc"), abs(-4), max(3, 9))    # built-in functions
import math
print(math.gcd(12, 18))                  # a function defined in a module

15 16
12
95 72 85.0
Added Neha -> total 1
Added Arjun -> total 2
3 4 9
6

def divide():
    try:
        a = int(input("Enter numerator: "))
        b = int(input("Enter denominator: "))
        print("Quotient =", a / b)
    except ValueError:
        print("Error: enter whole numbers only")
    except ZeroDivisionError:
        print("Error: cannot divide by zero")
    else:
        print("No error occurred")
    finally:
        print("finally runs every time")

divide()
divide()
divide()

try:
    marks = int(input("Enter marks: "))
    if not 0 <= marks <= 100:
        raise ValueError("marks must be between 0 and 100")
except ValueError as e:
    print("Invalid:", e)

try:
    open("missing.txt")
except FileNotFoundError as e:
    print(type(e).__name__)

Enter numerator: Enter denominator: Quotient = 5.0
No error occurred
finally runs every time
Enter numerator: Enter denominator: Error: cannot divide by zero
finally runs every time
Enter numerator: Error: enter whole numbers only
finally runs every time
Enter marks: Invalid: marks must be between 0 and 100
FileNotFoundError

The online compiler cannot read typed input yet: input() raises an error there. To run a program that asks for input, replace each input(...) with the value shown under "Typed in" — e.g. n = int("29") — then press Run. The output on this page was produced with those same values on Python 3.12.

with open("story.txt", "w") as f:                 # "w" creates / overwrites
    f.write("Python makes file handling simple.\n")
    f.writelines(["Every line ends with a newline.\n",
                  "CBSE asks for readlines().\n"])

with open("story.txt", "a") as f:                 # "a" appends at the end
    f.write("This line was appended.\n")

with open("story.txt", "r") as f:
    print(f.read())                               # whole file as one string

with open("story.txt") as f:                      # mode defaults to "r"
    print(f.readline(), end="")                   # one line, keeps its "\n"
    print(f.readline().split()[0])                # first word of line 2

with open("story.txt") as f:
    lines = f.readlines()                         # list of lines
    print(len(lines), "lines")
    words = sum(len(line.split()) for line in lines)
    chars = sum(len(line) for line in lines)
    print("Words:", words, "Characters:", chars)
    f.seek(0)                                     # back to the start
    print(f.read(6))                              # first 6 characters
    print("Position now:", f.tell())

# Copy only the lines that start with "C" into a second file
with open("story.txt") as src, open("copy.txt", "w") as dst:
    for line in src:                              # a file object is iterable
        if line.startswith("C"):
            dst.write(line)
print(open("copy.txt").read(), end="")

f = open("story.txt", "r+")                       # read and write, no truncation
f.seek(0)
f.write("PYTHON")
f.close()
print(open("story.txt").readline(), end="")

Python makes file handling simple.
Every line ends with a newline.
CBSE asks for readlines().
This line was appended.

Python makes file handling simple.
Every
4 lines
Words: 19 Characters: 118
Python
Position now: 6
CBSE asks for readlines().
PYTHON makes file handling simple.

import pickle

students = [
    {"roll": 1, "name": "Aman", "marks": 88},
    {"roll": 2, "name": "Bela", "marks": 92},
]
with open("students.dat", "wb") as f:              # create / overwrite
    for s in students:
        pickle.dump(s, f)                         # one record per dump

with open("students.dat", "ab") as f:              # append one more record
    pickle.dump({"roll": 3, "name": "Chirag", "marks": 79}, f)

def show_all():
    with open("students.dat", "rb") as f:
        try:
            while True:                           # read until end of file
                rec = pickle.load(f)
                print(rec)
        except EOFError:
            pass

show_all()

roll = int(input("Enter roll number to search: "))
with open("students.dat", "rb") as f:
    try:
        while True:
            rec = pickle.load(f)
            if rec["roll"] == roll:
                print("Found:", rec["name"], rec["marks"])
                break
    except EOFError:
        print("Record not found")

# Update: read everything into a list, change it, write it all back
data = []
with open("students.dat", "rb") as f:
    try:
        while True:
            data.append(pickle.load(f))
    except EOFError:
        pass
for rec in data:
    if rec["roll"] == 2:
        rec["marks"] = 95
with open("students.dat", "wb") as f:
    for rec in data:
        pickle.dump(rec, f)
print("After update:")
show_all()

{'roll': 1, 'name': 'Aman', 'marks': 88}
{'roll': 2, 'name': 'Bela', 'marks': 92}
{'roll': 3, 'name': 'Chirag', 'marks': 79}
Enter roll number to search: Found: Bela 92
After update:
{'roll': 1, 'name': 'Aman', 'marks': 88}
{'roll': 2, 'name': 'Bela', 'marks': 95}
{'roll': 3, 'name': 'Chirag', 'marks': 79}

The online compiler cannot read typed input yet: input() raises an error there. To run a program that asks for input, replace each input(...) with the value shown under "Typed in" — e.g. n = int("29") — then press Run. The output on this page was produced with those same values on Python 3.12.

import csv

with open("marks.csv", "w", newline="") as f:     # newline="" avoids blank lines
    writer = csv.writer(f)
    writer.writerow(["Roll", "Name", "Marks"])      # header
    writer.writerows([[1, "Aman", 88], [2, "Bela", 92], [3, "Chirag", 79]])

with open("marks.csv", "a", newline="") as f:     # append one record
    csv.writer(f).writerow([4, "Diya", 85])

with open("marks.csv", "r", newline="") as f:
    reader = csv.reader(f)
    header = next(reader)                          # skip / keep the header row
    print(header)
    total = 0
    count = 0
    for row in reader:                             # each row is a list of str
        print(row[1], "scored", row[2])
        total += int(row[2])                       # convert before arithmetic
        count += 1
    print("Class average:", total / count)

with open("marks.csv") as src, open("toppers.csv", "w", newline="") as dst:
    reader = csv.reader(src)
    writer = csv.writer(dst)
    writer.writerow(next(reader))
    for row in reader:
        if int(row[2]) >= 85:
            writer.writerow(row)
print(open("toppers.csv").read())

['Roll', 'Name', 'Marks']
Aman scored 88
Bela scored 92
Chirag scored 79
Diya scored 85
Class average: 86.0
Roll,Name,Marks
1,Aman,88
2,Bela,92
4,Diya,85

# push = append(), pop = pop(), top = stack[-1]
def is_empty(stack):
    return len(stack) == 0

def push(stack, item):
    stack.append(item)

def pop(stack):
    if is_empty(stack):
        return "Underflow"
    return stack.pop()

def peek(stack):
    if is_empty(stack):
        return "Underflow"
    return stack[-1]

def display(stack):
    if is_empty(stack):
        print("Stack is empty")
    else:
        for item in reversed(stack):      # top first
            print(item)

stack = []
push(stack, 10)
push(stack, 20)
push(stack, 30)
display(stack)
print("Popped:", pop(stack))
print("Top:", peek(stack))
pop(stack)
pop(stack)
print(pop(stack))                         # Underflow

# Board pattern: push the names of students scoring above 85
students = [("Aman", 88), ("Bela", 92), ("Chirag", 79)]
toppers = []
for name, marks in students:
    if marks > 85:
        push(toppers, name)
display(toppers)

def balanced(expr):                       # classic stack application
    st = []
    pairs = {")": "(", "]": "[", "}": "{"}
    for ch in expr:
        if ch in "([{":
            st.append(ch)
        elif ch in pairs:
            if not st or st.pop() != pairs[ch]:
                return False
    return not st

print(balanced("{[()]}"), balanced("{[(])}"))

30
20
10
Popped: 30
Top: 20
Underflow
Bela
Aman
True False

import sqlite3
con = sqlite3.connect(":memory:")   # MySQL in the exam; SQLite here, same SQL
cur = con.cursor()
cur.executescript("""
CREATE TABLE STUDENT (
    RollNo INTEGER PRIMARY KEY,
    Name   VARCHAR(30) NOT NULL,
    Class  INTEGER,
    Marks  DECIMAL(5,2)
);
INSERT INTO STUDENT VALUES (1, 'Aman', 12, 88.5);
INSERT INTO STUDENT VALUES (2, 'Bela', 12, 92.0);
INSERT INTO STUDENT VALUES (3, 'Chirag', 11, 79.0);
INSERT INTO STUDENT VALUES (4, 'Diya', 11, 85.0);
INSERT INTO STUDENT (RollNo, Name, Class) VALUES (5, 'Esha', 12);
UPDATE STUDENT SET Marks = 90 WHERE RollNo = 1;
DELETE FROM STUDENT WHERE Marks < 80;
CREATE TABLE SPORTS (RollNo INTEGER, Game VARCHAR(20));
INSERT INTO SPORTS VALUES (1, 'Cricket'), (2, 'Chess'), (4, 'Kho-Kho');
""")
queries = [
    "SELECT * FROM STUDENT ORDER BY Marks DESC",
    "SELECT Name, Marks FROM STUDENT WHERE Class = 12 AND Marks > 85",
    "SELECT Name FROM STUDENT WHERE Marks BETWEEN 85 AND 90 OR Marks IS NULL",
    "SELECT Name FROM STUDENT WHERE Name LIKE 'A%' OR Name LIKE '%a'",
    "SELECT DISTINCT Class FROM STUDENT",
    "SELECT Class, COUNT(*), AVG(Marks), MAX(Marks) FROM STUDENT GROUP BY Class HAVING COUNT(*) > 1",
    "SELECT S.Name, P.Game FROM STUDENT S, SPORTS P WHERE S.RollNo = P.RollNo",
    "SELECT S.Name, P.Game FROM STUDENT S JOIN SPORTS P ON S.RollNo = P.RollNo ORDER BY P.Game",
]
for sql in queries:
    print(sql)
    for row in cur.execute(sql):
        print("   ", row)
con.close()

SELECT * FROM STUDENT ORDER BY Marks DESC
    (2, 'Bela', 12, 92)
    (1, 'Aman', 12, 90)
    (4, 'Diya', 11, 85)
    (5, 'Esha', 12, None)
SELECT Name, Marks FROM STUDENT WHERE Class = 12 AND Marks > 85
    ('Aman', 90)
    ('Bela', 92)
SELECT Name FROM STUDENT WHERE Marks BETWEEN 85 AND 90 OR Marks IS NULL
    ('Aman',)
    ('Diya',)
    ('Esha',)
SELECT Name FROM STUDENT WHERE Name LIKE 'A%' OR Name LIKE '%a'
    ('Aman',)
    ('Bela',)
    ('Diya',)
    ('Esha',)
SELECT DISTINCT Class FROM STUDENT
    (12,)
    (11,)
SELECT Class, COUNT(*), AVG(Marks), MAX(Marks) FROM STUDENT GROUP BY Class HAVING COUNT(*) > 1
    (12, 3, 91.0, 92)
SELECT S.Name, P.Game FROM STUDENT S, SPORTS P WHERE S.RollNo = P.RollNo
    ('Aman', 'Cricket')
    ('Bela', 'Chess')
    ('Diya', 'Kho-Kho')
SELECT S.Name, P.Game FROM STUDENT S JOIN SPORTS P ON S.RollNo = P.RollNo ORDER BY P.Game
    ('Bela', 'Chess')
    ('Aman', 'Cricket')
    ('Diya', 'Kho-Kho')

# In the exam:  import mysql.connector as sql
#               con = sql.connect(host="localhost", user="root",
#                                 passwd="1234", database="school")
#               placeholders are %s
# Here (no MySQL server in a browser): the same flow with sqlite3, placeholders ?
import sqlite3
con = sqlite3.connect("school.db")
cur = con.cursor()
cur.execute("CREATE TABLE IF NOT EXISTS STUDENT (RollNo INTEGER, Name TEXT, Marks REAL)")
cur.execute("DELETE FROM STUDENT")
cur.execute("INSERT INTO STUDENT VALUES (?, ?, ?)", (1, "Aman", 88.5))
data = [(2, "Bela", 92.0), (3, "Chirag", 79.0)]
cur.executemany("INSERT INTO STUDENT VALUES (?, ?, ?)", data)
print("Rows inserted:", cur.rowcount)        # rows affected by the last execute
con.commit()                                 # save the changes

cur.execute("SELECT * FROM STUDENT WHERE Marks > 80")
print(cur.fetchone())                        # one row as a tuple
print(cur.fetchall())                        # the remaining rows
cur.execute("SELECT * FROM STUDENT")
print(cur.fetchmany(2))                      # the next two rows
name = input("Enter name to update: ")
cur.execute("UPDATE STUDENT SET Marks = ? WHERE Name = ?", (81.0, name))
con.commit()
print(cur.rowcount, "row updated")
con.close()

Rows inserted: 2
(1, 'Aman', 88.5)
[(2, 'Bela', 92.0)]
[(1, 'Aman', 88.5), (2, 'Bela', 92.0)]
Enter name to update: 1 row updated

The online compiler cannot read typed input yet: input() raises an error there. To run a program that asks for input, replace each input(...) with the value shown under "Typed in" — e.g. n = int("29") — then press Run. The output on this page was produced with those same values on Python 3.12.