114 lines
3.5 KiB
Python
114 lines
3.5 KiB
Python
from collections import deque
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_marker = object()
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class Peekable(object):
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"""Peekable version of a normal python generator.
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Useful when you don't want to evaluate the entire iterable to look at
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a specific item at a given idx.
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"""
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def __init__(self, iterable):
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self._it = iter(iterable)
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self._cache = deque()
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def __iter__(self):
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return self
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def __bool__(self):
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try:
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self.peek()
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except StopIteration:
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return False
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return True
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def __nonzero__(self):
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# For Python 2 compatibility
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return self.__bool__()
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def peek(self, default=_marker):
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"""Return the item that will be next returned from ``next()``.
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Return ``default`` if there are no items left. If ``default`` is not
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provided, raise ``StopIteration``.
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"""
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if not self._cache:
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try:
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self._cache.append(next(self._it))
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except StopIteration:
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if default is _marker:
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raise
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return default
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return self._cache[0]
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def prepend(self, *items):
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"""Stack up items to be the next ones returned from ``next()`` or
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``self.peek()``. The items will be returned in
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first in, first out order::
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>>> p = peekable([1, 2, 3])
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>>> p.prepend(10, 11, 12)
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>>> next(p)
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10
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>>> list(p)
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[11, 12, 1, 2, 3]
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It is possible, by prepending items, to "resurrect" a peekable that
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previously raised ``StopIteration``.
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>>> p = peekable([])
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>>> next(p)
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Traceback (most recent call last):
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...
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StopIteration
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>>> p.prepend(1)
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>>> next(p)
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1
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>>> next(p)
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Traceback (most recent call last):
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...
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StopIteration
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"""
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self._cache.extendleft(reversed(items))
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def __next__(self):
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if self._cache:
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return self._cache.popleft()
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return next(self._it)
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next = __next__ # For Python 2 compatibility
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def _get_slice(self, index):
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# Normalize the slice's arguments
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step = 1 if (index.step is None) else index.step
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if step > 0:
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start = 0 if (index.start is None) else index.start
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stop = maxsize if (index.stop is None) else index.stop
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elif step < 0:
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start = -1 if (index.start is None) else index.start
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stop = (-maxsize - 1) if (index.stop is None) else index.stop
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else:
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raise ValueError('slice step cannot be zero')
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# If either the start or stop index is negative, we'll need to cache
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# the rest of the iterable in order to slice from the right side.
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if (start < 0) or (stop < 0):
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self._cache.extend(self._it)
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# Otherwise we'll need to find the rightmost index and cache to that
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# point.
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else:
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n = min(max(start, stop) + 1, maxsize)
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cache_len = len(self._cache)
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if n >= cache_len:
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self._cache.extend(islice(self._it, n - cache_len))
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return list(self._cache)[index]
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def __getitem__(self, index):
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if isinstance(index, slice):
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return self._get_slice(index)
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cache_len = len(self._cache)
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if index < 0:
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self._cache.extend(self._it)
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elif index >= cache_len:
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self._cache.extend(islice(self._it, index + 1 - cache_len))
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return self._cache[index]
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