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What is the difference between a generator expression and a list comprehension? Generator expression और list comprehension में क्या अंतर है?

Answer

Aspect	List comprehension	Generator expression
Syntax	[x for x in ...]	(x for x in ...)
Evaluation	Eager - computes all values immediately	Lazy - computes values on demand
Memory	Stores entire result in memory	Minimal memory - one value at a time
Reusability	Can be iterated multiple times	Single-use, exhausted after one pass

import sys

# List comprehension - all values computed and stored immediately
list_comp = [x**2 for x in range(1000000)]
print(sys.getsizeof(list_comp))  # large - ~8MB+ for a million ints

# Generator expression - values computed lazily, one at a time
gen_exp = (x**2 for x in range(1000000))
print(sys.getsizeof(gen_exp))  # tiny - ~200 bytes regardless of range size

# Both can be iterated the same way
for val in gen_exp:
    if val > 20:
        break
    print(val)

# Generators are exhausted after one full iteration
gen = (x for x in range(3))
print(list(gen))  # [0, 1, 2]
print(list(gen))  # [] - already exhausted, second call gets nothing

# Lists can be iterated repeatedly
lst = [x for x in range(3)]
print(list(lst))  # [0, 1, 2]
print(list(lst))  # [0, 1, 2] - works again

# When to use which: generator expressions for large/streaming data
# processed once; list comprehensions when you need random access,
# multiple iterations, or len()
total = sum(x**2 for x in range(1000000))  # generator, memory-efficient sum

पहलू	List comprehension	Generator expression
Syntax	[x for x in ...]	(x for x in ...)
Evaluation	Eager - सारी values तुरंत	Lazy - demand पर
Memory	पूरा result memory में	बहुत कम memory
Reusability	कई बार iterate हो सकता है	एक बार use, फिर exhaust

import sys

list_comp = [x**2 for x in range(1000000)]
print(sys.getsizeof(list_comp))  # बड़ा size

gen_exp = (x**2 for x in range(1000000))
print(sys.getsizeof(gen_exp))  # बहुत छोटा size

for val in gen_exp:
    if val > 20:
        break
    print(val)

# Generator एक बार पूरा iterate होने पर exhaust हो जाता है
gen = (x for x in range(3))
print(list(gen))  # [0, 1, 2]
print(list(gen))  # [] - exhausted

# Lists बार-बार iterate हो सकती हैं
lst = [x for x in range(3)]
print(list(lst))  # [0, 1, 2]
print(list(lst))  # [0, 1, 2] - फिर काम करता है

total = sum(x**2 for x in range(1000000))  # memory-efficient sum

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