Loops

A for statement iterates over the elements of a sequence, binding each element to a name in turn and executing the body once per element. Python's for is a sequence iteration construct rather than a counter, so it applies directly to lists, strings, dictionaries and files without an index variable.

range

range produces a sequence of integers. range(stop) counts from 0; range(start, stop) counts from start; range(start, stop, step) advances by step. The stop value is excluded, consistent with slicing, so range(3) yields 0, 1 and 2.

Stepping by three walks a coding sequence codon by codon. Combined with slicing, this extracts each codon without an index arithmetic error.

enumerate and zip

When both the element and its position are required, enumerate supplies both and avoids maintaining a separate counter. Its start argument sets the first index, which is useful when reporting one-based line numbers.

zip iterates over several sequences in parallel, stopping when the shortest is exhausted.

Controlling the loop

break terminates the loop immediately. continue skips the remainder of the current iteration and proceeds to the next element. A loop may also carry an else block, which executes only if the loop completed without encountering break. This structure is useful when searching for an element and needing to distinguish between finding a target and exhausting the sequence.

while

A while statement repeats for as long as its condition holds. Use it when the number of iterations is not known in advance. The condition must be able to become false, and something in the body must affect it, or the loop will not terminate.

Modifying while iterating

Adding to or removing from a list while iterating over it produces skipped elements or a runtime error, because the iterator's position no longer matches the list. Build a new list instead, or iterate over a copy.

Task

Write codons(sequence). Split the sequence into three-character codons and return them as a list. Any trailing one or two characters that do not form a complete codon are discarded.

A slice that extends past the end of a string is silently truncated rather than raising, so the length of each slice must be checked before it is kept.