itertools_chain.py

# itertools.chain — combine multiple iterables into a single sequence.
#
# chain() lets you iterate through several iterables one after another
# as if they were a single iterable, without building a new list in memory.

from itertools import chain


# --- 1. Chaining different iterables ---

list1 = [1, 2, 3]
list2 = ['a', 'b', 'c']
list3 = [True, False]

print('Chained sequence:')
for item in chain(list1, list2, list3):
    print(item, end=' ')
print()


# --- 2. Flattening a list of lists ---
# chain(*list_of_lists) unpacks the outer list and chains the inner lists.

list_of_lists = [[1, 2, 3], [4, 5], [6, 7, 8]]
flattened = list(chain(*list_of_lists))

print('\nFlattened:', flattened)    # [1, 2, 3, 4, 5, 6, 7, 8]


# --- 3. Memory efficiency ---
# chain() returns a lazy iterator — it does not build a new list.
# Useful when working with large sequences.

large1 = range(1_000_000)
large2 = range(1_000_000)

combined = chain(large1, large2)    # no memory allocated for the combined sequence
print('\nFirst 5 of combined large ranges:')
for i, x in enumerate(combined):
    if i >= 5:
        break
    print(x, end=' ')
print()