What are itertools and the most useful functions?

Question

Accepted Answer

**itertools** provides **iterator building blocks** for efficient looping.

```python
import itertools

# chain — join iterables
result = list(itertools.chain([1, 2], [3, 4]))
# [1, 2, 3, 4]

# product — cartesian product
pairs = list(itertools.product("AB", "12"))
# [("A","1"), ("A","2"), ("B","1"), ("B","2")]

# combinations / permutations
comb = list(itertools.combinations([1, 2, 3], 2))
# [(1,2), (1,3), (2,3)]

perm = list(itertools.permutations([1, 2, 3], 2))
# [(1,2), (1,3), (2,1), (2,3), (3,1), (3,2)]

# groupby — group consecutive elements
data = [("A", 1), ("A", 2), ("B", 3)]
for key, group in itertools.groupby(data, key=lambda x: x[0]):
    print(key, list(group))

# islice — slice an iterator
first_three = list(itertools.islice(range(100), 3))
# [0, 1, 2]

# takewhile / dropwhile
result = list(itertools.takewhile(lambda x: x < 5, range(10)))
# [0, 1, 2, 3, 4]
```

**All functions return iterators** — memory efficient.

Python: What are itertools and the most useful functions?