কম্প্রিহেনশন (Comprehensions)

# The old way (loop)
squares_loop = []
for n in range(1, 6):
    squares_loop.append(n ** 2)
print("Loop:", squares_loop)

# The comprehension way — same result, one line!
squares_comp = [n ** 2 for n in range(1, 6)]
print("Comp:", squares_comp)

# Transform strings
names = ["anika", "rafi", "sadia"]
shouting = [name.upper() for name in names]
print(shouting)

# Extract data
prices = [{"item": "coffee", "price": 4.50},
          {"item": "muffin", "price": 3.25},
          {"item": "juice", "price": 5.00}]

item_names = [p["item"] for p in prices]
print(item_names)

Loop: [1, 4, 9, 16, 25]
Comp: [1, 4, 9, 16, 25]
['ALICE', 'BOB', 'CHARLIE']
['coffee', 'muffin', 'juice']

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

# Filter: only even numbers
evens = [n for n in numbers if n % 2 == 0]
print("Evens:", evens)

# Filter: words longer than 4 characters
words = ["hi", "python", "is", "absolutely", "cool"]
long_words = [w for w in words if len(w) > 4]
print("Long words:", long_words)

# Conditional expression (if/else in the expression)
labels = ["even" if n % 2 == 0 else "odd" for n in range(1, 6)]
print("Labels:", labels)

# Real-world: clean and validate emails
raw_emails = ["  [email protected] ", "", "[email protected]", "  ", "[email protected]"]
clean = [e.strip().lower() for e in raw_emails if e.strip()]
print("Valid emails:", clean)

Evens: [2, 4, 6, 8, 10]
Long words: ['python', 'absolutely']
Labels: ['odd', 'even', 'odd', 'even', 'odd']
Valid emails: ['[email protected]', '[email protected]', '[email protected]']

# Dict comprehension — word lengths
words = ["cat", "elephant", "dog", "hippopotamus"]
word_lengths = {w: len(w) for w in words}
print(word_lengths)

# Dict comprehension — flip keys and values
original = {"a": 1, "b": 2, "c": 3}
flipped = {v: k for k, v in original.items()}
print(flipped)

# Dict from two lists using zip()
countries = ["US", "UK", "JP"]
capitals = ["Washington", "London", "Tokyo"]
country_map = {c: cap for c, cap in zip(countries, capitals)}
print(country_map)

# Set comprehension — unique first letters
animals = ["ant", "bear", "cat", "ant", "butterfly", "cobra"]
first_letters = {a[0] for a in animals}
print(f"Unique first letters: {first_letters}")

{'cat': 3, 'elephant': 8, 'dog': 3, 'hippopotamus': 12}
{1: 'a', 2: 'b', 3: 'c'}
{'US': 'Washington', 'UK': 'London', 'JP': 'Tokyo'}
Unique first letters: {'a', 'c', 'b'}

# Generator expression — lazy evaluation
gen = (n ** 2 for n in range(1, 6))
print(type(gen))   # It's a generator, not a list!

# Consume one at a time
print(next(gen))   # 1
print(next(gen))   # 4

# Use directly in functions — no extra brackets needed!
total = sum(n ** 2 for n in range(1, 11))
print(f"Sum of squares 1-10: {total}")

# Memory comparison
import sys
big_list = [n for n in range(10_000)]
big_gen = (n for n in range(10_000))

print(f"List size: {sys.getsizeof(big_list):,} bytes")
print(f"Generator size: {sys.getsizeof(big_gen)} bytes")

<class 'generator'>
1
4
Sum of squares 1-10: 385
List size: 85,176 bytes
Generator size: 200 bytes