Is there an expression for an infinite iterator?

for x in iter(int, 1): pass

• Two-argument iter = zero-argument callable + sentinel value
• int() always returns 0

Therefore, iter(int, 1) is an infinite iterator. There are obviously a huge number of variations on this particular theme (especially once you add lambda into the mix). One variant of particular note is iter(f, object()), as using a freshly created object as the sentinel value almost guarantees an infinite iterator regardless of the callable used as the first argument.

itertools provides three infinite iterators:

• count(start=0, step=1): 0, 1, 2, 3, 4, ...

• cycle(p): p[0], p[1], ..., p[-1], p[0], ...

• repeat(x, times=∞): x, x, x, x, ...

I don't know of any others in the standard library.

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Since you asked for a one-liner:

__import__("itertools").count()