annotate env/lib/python3.7/site-packages/boltons/queueutils.py @ 3:758bc20232e8 draft

"planemo upload commit 2a0fe2cc28b09e101d37293e53e82f61762262ec"
author shellac
date Thu, 14 May 2020 16:20:52 -0400
parents 26e78fe6e8c4
children
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1 # -*- coding: utf-8 -*-
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2 """Python comes with a many great data structures, from :class:`dict`
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3 to :class:`collections.deque`, and no shortage of serviceable
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4 algorithm implementations, from :func:`sorted` to :mod:`bisect`. But
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5 priority queues are curiously relegated to an example documented in
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6 :mod:`heapq`. Even there, the approach presented is not full-featured
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7 and object-oriented. There is a built-in priority queue,
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8 :class:`Queue.PriorityQueue`, but in addition to its austere API, it
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9 carries the double-edged sword of threadsafety, making it fine for
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10 multi-threaded, multi-consumer applications, but high-overhead for
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11 cooperative/single-threaded use cases.
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12
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13 The ``queueutils`` module currently provides two Queue
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14 implementations: :class:`HeapPriorityQueue`, based on a heap, and
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15 :class:`SortedPriorityQueue`, based on a sorted list. Both use a
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16 unified API based on :class:`BasePriorityQueue` to facilitate testing
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17 the slightly different performance characteristics on various
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18 application use cases.
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19
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20 >>> pq = PriorityQueue()
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21 >>> pq.add('low priority task', 0)
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22 >>> pq.add('high priority task', 2)
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23 >>> pq.add('medium priority task 1', 1)
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24 >>> pq.add('medium priority task 2', 1)
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25 >>> len(pq)
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26 4
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27 >>> pq.pop()
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28 'high priority task'
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29 >>> pq.peek()
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30 'medium priority task 1'
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31 >>> len(pq)
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32 3
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33
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34 """
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35
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36
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37 from heapq import heappush, heappop
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38 from bisect import insort
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39 import itertools
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40
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41 try:
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42 from typeutils import make_sentinel
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43 _REMOVED = make_sentinel(var_name='_REMOVED')
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44 except ImportError:
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45 _REMOVED = object()
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46
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47 try:
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48 from listutils import BList
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49 # see BarrelList docstring for notes
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50 except ImportError:
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51 BList = list
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52
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53
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54 __all__ = ['PriorityQueue', 'BasePriorityQueue',
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55 'HeapPriorityQueue', 'SortedPriorityQueue']
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56
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57
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58 # TODO: make Base a real abstract class
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59 # TODO: add uniqueification
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60
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61
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62 class BasePriorityQueue(object):
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63 """The abstract base class for the other PriorityQueues in this
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64 module. Override the ``_backend_type`` class attribute, as well as
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65 the :meth:`_push_entry` and :meth:`_pop_entry` staticmethods for
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66 custom subclass behavior. (Don't forget to use
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67 :func:`staticmethod`).
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68
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69 Args:
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70 priority_key (callable): A function that takes *priority* as
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71 passed in by :meth:`add` and returns a real number
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72 representing the effective priority.
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73
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74 """
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75 # negating priority means larger numbers = higher priority
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76 _default_priority_key = staticmethod(lambda p: -float(p or 0))
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77 _backend_type = list
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78
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79 def __init__(self, **kw):
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80 self._pq = self._backend_type()
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81 self._entry_map = {}
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82 self._counter = itertools.count()
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83 self._get_priority = kw.pop('priority_key', self._default_priority_key)
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84 if kw:
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85 raise TypeError('unexpected keyword arguments: %r' % kw.keys())
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86
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87 @staticmethod
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88 def _push_entry(backend, entry):
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89 pass # abstract
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90
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91 @staticmethod
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92 def _pop_entry(backend):
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93 pass # abstract
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94
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95 def add(self, task, priority=None):
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96 """
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97 Add a task to the queue, or change the *task*'s priority if *task*
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98 is already in the queue. *task* can be any hashable object,
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99 and *priority* defaults to ``0``. Higher values representing
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100 higher priority, but this behavior can be controlled by
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101 setting *priority_key* in the constructor.
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102 """
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103 priority = self._get_priority(priority)
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104 if task in self._entry_map:
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105 self.remove(task)
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106 count = next(self._counter)
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107 entry = [priority, count, task]
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108 self._entry_map[task] = entry
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109 self._push_entry(self._pq, entry)
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110
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111 def remove(self, task):
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112 """Remove a task from the priority queue. Raises :exc:`KeyError` if
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113 the *task* is absent.
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114 """
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115 entry = self._entry_map.pop(task)
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116 entry[-1] = _REMOVED
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117
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118 def _cull(self, raise_exc=True):
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119 "Remove entries marked as removed by previous :meth:`remove` calls."
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120 while self._pq:
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121 priority, count, task = self._pq[0]
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122 if task is _REMOVED:
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123 self._pop_entry(self._pq)
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124 continue
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125 return
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126 if raise_exc:
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127 raise IndexError('empty priority queue')
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128
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129 def peek(self, default=_REMOVED):
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130 """Read the next value in the queue without removing it. Returns
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131 *default* on an empty queue, or raises :exc:`KeyError` if
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132 *default* is not set.
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133 """
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134 try:
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135 self._cull()
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136 _, _, task = self._pq[0]
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137 except IndexError:
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138 if default is not _REMOVED:
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139 return default
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140 raise IndexError('peek on empty queue')
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141 return task
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142
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143 def pop(self, default=_REMOVED):
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144 """Remove and return the next value in the queue. Returns *default* on
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145 an empty queue, or raises :exc:`KeyError` if *default* is not
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146 set.
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147 """
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148 try:
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149 self._cull()
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150 _, _, task = self._pop_entry(self._pq)
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151 del self._entry_map[task]
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152 except IndexError:
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153 if default is not _REMOVED:
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154 return default
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155 raise IndexError('pop on empty queue')
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156 return task
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157
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158 def __len__(self):
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159 "Return the number of tasks in the queue."
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160 return len(self._entry_map)
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161
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162
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163 class HeapPriorityQueue(BasePriorityQueue):
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164 """A priority queue inherited from :class:`BasePriorityQueue`,
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165 backed by a list and based on the :func:`heapq.heappop` and
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166 :func:`heapq.heappush` functions in the built-in :mod:`heapq`
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167 module.
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168 """
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169 @staticmethod
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170 def _pop_entry(backend):
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171 return heappop(backend)
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172
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173 @staticmethod
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174 def _push_entry(backend, entry):
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175 heappush(backend, entry)
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176
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177
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178 class SortedPriorityQueue(BasePriorityQueue):
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179 """A priority queue inherited from :class:`BasePriorityQueue`, based
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180 on the :func:`bisect.insort` approach for in-order insertion into
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181 a sorted list.
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182 """
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183 _backend_type = BList
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184
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185 @staticmethod
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186 def _pop_entry(backend):
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187 return backend.pop(0)
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188
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189 @staticmethod
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190 def _push_entry(backend, entry):
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191 insort(backend, entry)
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192
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193
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194 PriorityQueue = SortedPriorityQueue