Pycon US 2025
Resource
itertools is one of those modules everyone imports but most people only use 3-4 functions from. Went through all 20 and sorted them by how often you'd actually reach for each one in real code, not the order they show up in the docs.
| Function | Example | What it does |
|---|---|---|
| chain() |
chain(it1, it2)
| Concatenates iterables, flattens multiple iterables into a single iterator. |
| chain.from_iterable() |
chain.from_iterable(nested)
|
Flattens one level of nesting lazily, like chain(*nested) without unpacking.
|
| islice() |
islice(it, start, stop)
| Slices an iterator, doesn't support negative indices. |
| compress() |
compress('ABCDEF', [1,0,1,0,1,1])
|
Filters data by a boolean selector iterable, yields A, C, E, F.
|
| filterfalse() |
filterfalse(lambda x: x<5, [1,4,6,3,8])
|
Yields elements where the predicate is false, opposite of filter().
|
| starmap() |
starmap(pow, [(2,5), (3,2)])
| Applies a function with arguments unpacked from each tuple in the iterable. |
| pairwise() |
pairwise([1, 2, 3])
|
Returns overlapping pairs of consecutive elements: (1,2), (2,3) (3.10+).
|
| batched() |
batched(range(10), 3)
| Splits an iterable into fixed-size tuples, last batch may be shorter (3.12+). |
| groupby() |
groupby(items, key=func)
| Groups consecutive elements by key, input must already be sorted by that key. |
| accumulate() |
accumulate(nums, func)
| Running totals or reductions, default is addition, accepts any binary function. |
| takewhile() |
takewhile(pred, it)
| Yields elements while the predicate is true, stops at the first false. |
| dropwhile() |
dropwhile(pred, it)
| Skips elements while the predicate is true, then yields everything remaining. |
| tee() |
tee(iterable, n=2)
| Splits one iterator into n independent iterators, use before the original is consumed. |
| zip_longest() |
zip_longest(a, b, fillvalue=None)
|
Like zip() but continues to the longest input, filling shorter ones with fillvalue.
|
| combinations() |
combinations(items, r)
| All r-length combinations, order doesn't matter, no repeated elements. |
| combinations_with_replacement() |
combinations_with_replacement('AB', 2)
|
All r-length combinations allowing repeats, yields AA, AB, BB.
|
| permutations() |
permutations(items, r)
| All r-length permutations, order matters, no repeated elements. |
| product() |
product(a, b)
| Cartesian product, equivalent to nested for-loops. |
| count() |
count(start=0, step=1)
|
Infinite counter, useful with zip() or takewhile().
|
| cycle() |
cycle([1, 2, 3])
| Repeats an iterable infinitely, keeps a copy of all elements in memory. |
| repeat() |
repeat(10, times=3)
|
Repeats an object n times, infinite if times is omitted.
|
pairwise and batched are the two most people don't know about, both fixed a "why am I hand-rolling this" moment for me. Curious which one on this list you use the least, or if I'm missing one people reach for a lot.
Discussion
…with zero comments, no docstrings, variable names like df2_final_FINAL_v3 and it’s somehow still running in production.
I don’t remember writing it or deploying it. Nobody knows it exists except the one analyst who emails me if it stops working.
Please tell me I’m not alone.
