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"""Classes and functions for implementing self-referential iterators.""" |
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import typing |
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Input = typing.TypeVar("Input") |
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Output = typing.TypeVar("Output") |
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class Action(typing.Generic[Input, Output]): |
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"""Abstract base class for reified stack iteration actions.""" |
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def handle(self, to_process: typing.List[Input]) -> typing.Iterator[Output]: |
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"""Yield a value or mutate the stack.""" |
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raise NotImplementedError |
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class Yield(Action[Input, Output]): |
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"""Reified action for yielding an output value.""" |
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def __init__(self, item: Output) -> None: |
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self.item = item |
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def handle(self, to_process: typing.List[Input]) -> typing.Iterator[Output]: |
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"""Yield out ``self.item``""" |
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del to_process |
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yield self.item |
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class Extend(Action[Input, Output]): |
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"""Reified action for pushing to the stack.""" |
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def __init__(self, iterable: typing.Iterable[Input]) -> None: |
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self.iterable = iterable |
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def handle(self, to_process: typing.List[Input]) -> typing.Iterator[Output]: |
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"""Extend the process list with ``iterable``, and yield nothing.""" |
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to_process.extend(self.iterable) |
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yield from () |
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def handle( |
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action: typing.Optional[Action[Input, Output]], to_process: typing.List[Input] |
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) -> typing.Iterator[Output]: |
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"""If ``action`` is an ``Action``, delegate to its ``handle`` method.""" |
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if action is not None: |
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yield from action.handle(to_process) |
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def stack_iter( |
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first: Input, |
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process: typing.Callable[[Input], typing.Optional[Action[Input, Output]]], |
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) -> typing.Iterator[Output]: |
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"""Stack iterate over the initial value using the processing function. |
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To "stack iterate" is to build a stack, starting with the initial value. |
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Then, as long as the stack is non-empty, pop the top value, pass it to the |
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processing function, and use the ``handle`` helper to ignore ``None`` |
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values. If the processing function returns a value, it will be a subclass |
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of Action, and its ``handle`` method can yield values or mutate the stack |
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arbitrarily. |
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The point of this setup is to write very little code in the service of |
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desctructuring nested data without relying on unbounded recursion. |
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""" |
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to_process = [first] |
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while to_process: |
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yield from handle(process(to_process.pop()), to_process) |
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mwchase /
python-structured-data
