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Loops and Iteration

A for loop consumes an iterable. It is the default when values come from a collection, generator, file, or other iteration protocol.

for index, value in enumerate(values):
    process(index, value)

for left, right in zip(left_values, right_values, strict=True):
    compare(left, right)

Use range(start, stop, step) when the numbers themselves define the iteration, not merely to imitate indexing.

While loops​

A while loop repeats while a condition remains truthy. Make initialization, progress, and termination visible:

attempts = 0
while attempts < maximum_attempts:
    if try_operation():
        break
    attempts += 1
else:
    raise RuntimeError("operation never succeeded")

The loop else runs only when the loop finishes without break. It is often useful for search exhaustion, but a helper function with an early return may be clearer.

Control statements​

  • break exits the nearest loop.
  • continue starts its next iteration.
  • pass performs no operation; it is a syntactic placeholder.

Avoid deeply nested break logic; extract a function or maintain explicit state when several loops must terminate together.

Mutation during iteration​

Changing the size or structure of the container being traversed can skip values, repeat work, or raise an error. Iterate over a copy, build a new collection, or separate discovery from mutation:

active = [item for item in items if item.enabled]

Iterators are consumable​

Many iterators yield each value once. Converting to a list enables repeated passes but materializes all values. Preserve streaming when input may be large.

Source​