Thread by @treyhunner

Python's "for" loops are single-purposed: they can loop over an iterable item-by-item. That's it.

The "for" loop itself can't loop in reverse or loop over multiple iterables.

For this reason, looping helpers are a VERY big deal in #Python.

Let's talk about looping helpers. 🧵

Want to loop over an iterable in the reverse direction? If it's a reversible iterable (lists, tuples, dictionaries, etc), you can use the reversed helper:

>>> colors = ["pink", "blue", "green"]
>>> for color in reversed(colors):
... print(color)
...
green
blue
pink

Note that the "for" loop isn't reversing here: reversed is!

The built-in reversed function accepts an iterable and returns a lazy iterable that provides each item from the original iterable in the reverse order.

>>> r = reversed(colors)
>>> next(r)
'green'
>>> next(r)
'blue'

Want to loop while counting upward? Python's enumerate function can help!

>>> for n, color in enumerate(colors, start=1):
... print(n, color)
...
1 pink
2 blue
3 green

Again the "for" loop isn't doing the work: enumerate is! The enumerate function returns a lazy iterable.

But what about that magic "n, color" thing?

That's tuple unpacking.

enumerate returns an iterable of tuples:

>>> enumerate(colors)
<enumerate object at 0x7efe54e6c6c0>
>>> list(enumerate(colors))
[(0, 'pink'), (1, 'blue'), (2, 'green')]

And we're unpacking them as we loop!

Need to loop over multiple iterables at once? That's what zip is for!

>>> colors = ["pink", "blue", "green"]
>>> animals = ["duck", "walrus", "monkey"]
>>> for color, animal in zip(colors, animals):
... print(color, animal)
...
pink duck
blue walrus
green monkey

The built-in zip function accepts any number of iterables and returns a lazy iterable that provides tuples of the n-th item from each of the given iterables (1st item from each, second from each, and so on).

Like enumerate, zip also pairs nicely with tuple unpacking.

What if you don't even have an "iterable" and you just need to increment or decrement numbers?

Python's range function can return an iterable that'll do that for you!

>>> for n in range(50, 300, 50):
... print(n)
...
50
100
150
200
250

These are just some of Python's many looping helpers.

Here are more examples from the itertools module:

• itertools.islice: stop after N items or skip the first N items
• itertools.chain: lazily "chain" iterables together
• itertools.takewhile: loop while a condition is true

You can also use generators & iterators to invent your own lazy looping helpers in Python.

This one acts like enumerate, but it counts downward:

def denumerate(iterable, start=0):
n = start
for item in iterable:
yield n, item
n -= 1

More on some of the built-in looping helpers:

➡️ enumerate: pym.dev/looping-with-i…
➡️ zip: pym.dev/looping-over-m…
➡️ reversed: trey.io/reversed
➡️ range: trey.io/range

On making your own looping helpers: trey.io/making-looping…

Want some practice with looping helpers? Try out @PythonMorsels! Lots of Python Morsels exercises involve both using and inventing own looping helpers.

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1. Follow me, @treyhunner, for more of these
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