Python Revision Tour

Tokens, variables and data types

Class XII builds on everything from Class XI. A Python program is made of tokens: keywords (if, def, return), identifiers (total), literals (25, 'CS', True, None), operators (+ == and) and punctuators (( ) [ ] : ,). Variables are names bound to objects; the type comes from the value (dynamic typing).

Data 📁 types/
├── 📁 Numbers/      int, float, complex, bool
├── 📁 Sequences/    str, list, tuple
├── 📁 Mapping/      dict
├── 📁 Set/          set
└── None

Mutable vs immutable

a = [1, 2, 3]
b = a
c = a[:]
b.append(4)
c.append(5)
print(a, b, c)
s = 'cs'
s2 = s
s2 += '12'
print(s, s2)

b = a creates an alias; c = a[:] creates a copy. Strings are immutable, so s2 += '12' makes a new string and s is unchanged.

Operators and precedence

Order from high to low: () → ** (right to left) → unary + - ~ → * / // % → + - → comparisons, in, is → not → and → or.

print(5 // 2 * 2, 2 ** 3 ** 0, not 0 or 5, 7 and 0, 'a' < 'B')

7 and 0 returns 0 (the first false value); 'a' < 'B' is False because 'a' is 97 and 'B' is 66.

Flow of control

x = 10
while x > 0:
    x -= 3
print(x)
for i in range(3):
    pass
else:
    print('loop finished, i =', i)

Strings

s = 'Board Exam 2026'
print(s[6:10], s[-4:], s.lower().count('a'))
print(s.split(), s.replace(' ', '-'))

Must-know methods: len, upper, lower, title, capitalize, count, find, index, replace, split, join, strip, startswith, endswith, isalpha, isdigit, isalnum, isupper, islower, isspace.

Lists, tuples and dictionaries

L = [5, 'two', 3.0, [7, 8]]
print(len(L), L[3][1], L[-2:])
t = (1, 2, 3) * 2
print(t, t.index(3))
d = {'a': 1}
d.setdefault('b', 2)
print(d, list(d.items()))

Comprehensions

print([i * i for i in range(6) if i % 2])
print({k: len(k) for k in ['cs', 'ip', 'maths']})

Board-style output questions

Find the output (trace word by word):

text = 'Python Is Easy To Learn'
words = text.split()
new = ''
for w in words:
    if len(w) > 2:
        new += w[0].lower() + w[1:].upper() + ' '
    else:
        new += w + ' '
print(new.strip())

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