What is a generator in Python and how does it differ from a list?

A generator is a function that uses the yield keyword to return values one at a time, pausing execution between yields and resuming from the same point when the next value is requested. It produces an iterator without building the entire result in memory.

# List builds everything in memory first
squares_list = [x**2 for x in range(1_000_000)]  # ~8 MB

# Generator yields one value at a time — constant memory
def squares_gen(n):
    for x in range(n):
        yield x ** 2

gen = squares_gen(1_000_000)
print(next(gen))   # 0
print(next(gen))   # 1
print(next(gen))   # 4

# Or use a generator expression (same thing, less code)
gen2 = (x**2 for x in range(1_000_000))

Generators are lazy — they compute the next value only when asked. This makes them ideal for: large file processing (stream lines without loading the whole file), infinite sequences, data pipelines, and any situation where you do not need all results at once.

# Stream a huge log file without loading it into memory
def error_lines(filepath):
    with open(filepath) as f:
        for line in f:
            if 'ERROR' in line:
                yield line.strip()

for line in error_lines('/var/log/app.log'):
    print(line)

Once a generator is exhausted (raises StopIteration) it cannot be reset — you must create a new generator object. This is the key difference from a list, which can be iterated multiple times.

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