Iterators and next in Python

The protocol under every for loop: iter, next, exhaustion.

colors = iter(["red", "green", "blue"])

first = next(colors)
second = next(colors)
rest = list(colors)
done = next(colors, "exhausted")

letters = iter("hi")
pairs = zip([1, 2, 3], "abc")
zipped = list(pairs)

numbered = list(enumerate(["a", "b"], start=1))

How it works

1. iter turns a list into a one-way stream.
2. next pulls items; a default avoids StopIteration.
3. zip and enumerate build richer iterators.

Keywords and builtins used here

• enumerate
• iter
• list
• next
• zip

The run, in numbers

Lines

12

Characters to type

258

Tokens

97

Three-star pace

95 tpm

At the three-star pace of 95 tokens a minute, this run takes about 61 seconds.

Type this snippet

Step 1 of 7 in Iterators, errors & files, step 44 of 72 in Language basics.

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