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class Cycle(object): """This is like iterating a circular list: when you reach the end, go back to the beginning and continue forever. But this class also handles a linear progression of cycle -> sub-cycle -> sub-sub-cycle, to any depth; e.g., for one-to-many relationships. One-level usage is like this: <% my_cycle = Cycle(["even", "odd"]) %> % for r in c.records: <tr class="${my_cycle.next()}"> ... % endfor Multi-level could be used to display a one-to-many relationship, where the child objects are listed indented after their parent. The parent here alternates between "white" and "blue" classes for its table rows, while the child alternates between "evenchild" and "oddchild". The child cycle is reset to the beginning for each group of child records. <% my_cycle = Cycle(["white", "blue"], ["evenchild", "oddchild"]) %> <table> % for parent_record in c.records: <tr class="${my_cycle.next()}"> ... <% my_cycle.cycle.reset() %> <table> % for child_record in parent_record.children: <tr class="${my_cycle.cycle.next()}"> ... </tr> % endfor </table> </tr> % endfor </table> """ def __init__(self, choices, *descendant_choices): self.choices = list(choices) self.index = 0 # Index of next choice to produce. self.cycle = None if descendant_choices: self.cycle = Cycle(*descendant_choices) def next(self): if self.index >= len(self.choices): self.index = 0 element = self.choices[self.index] self.index += 1 return element # Cycle is endless, no need for StopIteration. def reset(self): self.index = 0