Idiomatic loops

Looping in general

data = ["John", "Doe", "was", "here"]

Don't do it like this. While loops are actually really rarely needed.

idx = 0
while idx < len(data):
    print(data[idx])
    idx += 1

John
Doe
was
here

Don't do like this either.

for idx in range(len(data)):
    print(data[idx])

John
Doe
was
here

Do it like this!

for item in data:
    print(item)

John
Doe
was
here

If you need the index as well, you can use enumerate.

for idx, val in enumerate(data):
    print(f"{idx}: {val}")

0: John
1: Doe
2: was
3: here

Looping over a range of numbers

Don't do this.

i = 0
while i < 6:
    print(i)
    i += 1

0
1
2
3
4
5

Don't do this either.

for val in [0, 1, 2, 3, 4, 5]:
    print(val)

0
1
2
3
4
5

Do it like this!

for val in range(6):
    print(val)

0
1
2
3
4
5

Reversed looping

data = ["first", "to", "last", "from"]

This is no good.

i = len(data) - 1
while i >= 0:
    print(data[i])
    i -= 1

from
last
to
first

Do it like this!

for item in reversed(data):
    print(item)

from
last
to
first

Looping over n collections simultaneously

collection1 = ["a", "b", "c"]
collection2 = (10, 20, 30, 40, 50)
collection3 = ["John", "Doe", True]

Oh boy, not like this.

shortest = len(collection1)
if len(collection2) < shortest:
    shortest = len(collection2)
if len(collection3) < shortest:
    shortest = len(collection3)

i = 0
while i < shortest:
    print(collection1[i], collection2[i], collection3[i])
    i += 1

a 10 John
b 20 Doe
c 30 True

This is getting better but there's even a better way!

shortest = min(len(collection1), len(collection2), len(collection3))
for i in range(shortest):
    print(collection1[i], collection2[i], collection3[i])

a 10 John
b 20 Doe
c 30 True

Do it like this!

for first, second, third in zip(collection1, collection2, collection3):
    print(first, second, third)

a 10 John
b 20 Doe
c 30 True

You can also create a dict out of two collections!

my_dict = dict(zip(collection1, collection2))
print(my_dict)

{'a': 10, 'b': 20, 'c': 30}

for - else - Checking for a match in a collection

Let's say we want to verify a certain condition is met by at least one element in a collection. Let's consider the following relatively naive example where we want to verify that at least one item is "python" (case insensitive) in data. If not, we'll raise a ValueError.

data = [1, 2, 3, "This", "is", "just", "a", "random", "Python", "list"]

Don't do it like this

found = False
for val in data:
    if str(val).lower() == "python":
        found = True
        break
if not found:
    raise ValueError("Nope, couldn't find.")

Do it like this!

for val in data:
    if str(val).lower() == "python":
        break
else:
    raise ValueError("Nope, couldn't find.")