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+from collections import defaultdict, deque
+from itertools import filterfalse
+
+
+def unique_everseen(iterable, key=None):
+ "List unique elements, preserving order. Remember all elements ever seen."
+ # unique_everseen('AAAABBBCCDAABBB') --> A B C D
+ # unique_everseen('ABBCcAD', str.lower) --> A B C D
+ seen = set()
+ seen_add = seen.add
+ if key is None:
+ for element in filterfalse(seen.__contains__, iterable):
+ seen_add(element)
+ yield element
+ else:
+ for element in iterable:
+ k = key(element)
+ if k not in seen:
+ seen_add(k)
+ yield element
+
+
+# copied from more_itertools 8.8
+def always_iterable(obj, base_type=(str, bytes)):
+ """If *obj* is iterable, return an iterator over its items::
+
+ >>> obj = (1, 2, 3)
+ >>> list(always_iterable(obj))
+ [1, 2, 3]
+
+ If *obj* is not iterable, return a one-item iterable containing *obj*::
+
+ >>> obj = 1
+ >>> list(always_iterable(obj))
+ [1]
+
+ If *obj* is ``None``, return an empty iterable:
+
+ >>> obj = None
+ >>> list(always_iterable(None))
+ []
+
+ By default, binary and text strings are not considered iterable::
+
+ >>> obj = 'foo'
+ >>> list(always_iterable(obj))
+ ['foo']
+
+ If *base_type* is set, objects for which ``isinstance(obj, base_type)``
+ returns ``True`` won't be considered iterable.
+
+ >>> obj = {'a': 1}
+ >>> list(always_iterable(obj)) # Iterate over the dict's keys
+ ['a']
+ >>> list(always_iterable(obj, base_type=dict)) # Treat dicts as a unit
+ [{'a': 1}]
+
+ Set *base_type* to ``None`` to avoid any special handling and treat objects
+ Python considers iterable as iterable:
+
+ >>> obj = 'foo'
+ >>> list(always_iterable(obj, base_type=None))
+ ['f', 'o', 'o']
+ """
+ if obj is None:
+ return iter(())
+
+ if (base_type is not None) and isinstance(obj, base_type):
+ return iter((obj,))
+
+ try:
+ return iter(obj)
+ except TypeError:
+ return iter((obj,))
+
+
+# Copied from more_itertools 10.3
+class bucket:
+ """Wrap *iterable* and return an object that buckets the iterable into
+ child iterables based on a *key* function.
+
+ >>> iterable = ['a1', 'b1', 'c1', 'a2', 'b2', 'c2', 'b3']
+ >>> s = bucket(iterable, key=lambda x: x[0]) # Bucket by 1st character
+ >>> sorted(list(s)) # Get the keys
+ ['a', 'b', 'c']
+ >>> a_iterable = s['a']
+ >>> next(a_iterable)
+ 'a1'
+ >>> next(a_iterable)
+ 'a2'
+ >>> list(s['b'])
+ ['b1', 'b2', 'b3']
+
+ The original iterable will be advanced and its items will be cached until
+ they are used by the child iterables. This may require significant storage.
+
+ By default, attempting to select a bucket to which no items belong will
+ exhaust the iterable and cache all values.
+ If you specify a *validator* function, selected buckets will instead be
+ checked against it.
+
+ >>> from itertools import count
+ >>> it = count(1, 2) # Infinite sequence of odd numbers
+ >>> key = lambda x: x % 10 # Bucket by last digit
+ >>> validator = lambda x: x in {1, 3, 5, 7, 9} # Odd digits only
+ >>> s = bucket(it, key=key, validator=validator)
+ >>> 2 in s
+ False
+ >>> list(s[2])
+ []
+
+ """
+
+ def __init__(self, iterable, key, validator=None):
+ self._it = iter(iterable)
+ self._key = key
+ self._cache = defaultdict(deque)
+ self._validator = validator or (lambda x: True)
+
+ def __contains__(self, value):
+ if not self._validator(value):
+ return False
+
+ try:
+ item = next(self[value])
+ except StopIteration:
+ return False
+ else:
+ self._cache[value].appendleft(item)
+
+ return True
+
+ def _get_values(self, value):
+ """
+ Helper to yield items from the parent iterator that match *value*.
+ Items that don't match are stored in the local cache as they
+ are encountered.
+ """
+ while True:
+ # If we've cached some items that match the target value, emit
+ # the first one and evict it from the cache.
+ if self._cache[value]:
+ yield self._cache[value].popleft()
+ # Otherwise we need to advance the parent iterator to search for
+ # a matching item, caching the rest.
+ else:
+ while True:
+ try:
+ item = next(self._it)
+ except StopIteration:
+ return
+ item_value = self._key(item)
+ if item_value == value:
+ yield item
+ break
+ elif self._validator(item_value):
+ self._cache[item_value].append(item)
+
+ def __iter__(self):
+ for item in self._it:
+ item_value = self._key(item)
+ if self._validator(item_value):
+ self._cache[item_value].append(item)
+
+ yield from self._cache.keys()
+
+ def __getitem__(self, value):
+ if not self._validator(value):
+ return iter(())
+
+ return self._get_values(value)