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POSame iterator object yields different result in for loop?
 

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  1. POSame iterator object yields different result in for loop?
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    <p>I came across a very strange behaviour in Python. Using a class derived from <code>UserDict</code>, the iterator <code>a.items()</code> behaves differently in a for loop than <code>a.data.items()</code>, even though the two are <strong>identical</strong>:</p> <pre><code>Python 3.3.1 (default, Apr 17 2013, 22:32:14) [GCC 4.7.3] on linux Type "help", "copyright", "credits" or "license" for more information. &gt;&gt;&gt; from datastruct import QueueDict &gt;&gt;&gt; a=QueueDict(maxsize=1700) &gt;&gt;&gt; for i in range(1000): ... a[str(i)]=1/(i+1) ... &gt;&gt;&gt; a.items() ItemsView(OrderedDict([('991', 0.0010080645161290322), ('992', 0.0010070493454179255), ('993', 0.001006036217303823), ('994', 0.0010050251256281408), ('995', 0.001004016064257028), ('996', 0.0010030090270812437), ('997', 0.001002004008016032), ('998', 0.001001001001001001), ('999', 0.001)])) &gt;&gt;&gt; a.data.items() ItemsView(OrderedDict([('991', 0.0010080645161290322), ('992', 0.0010070493454179255), ('993', 0.001006036217303823), ('994', 0.0010050251256281408), ('995', 0.001004016064257028), ('996', 0.0010030090270812437), ('997', 0.001002004008016032), ('998', 0.001001001001001001), ('999', 0.001)])) &gt;&gt;&gt; a.items()==a.data.items() True &gt;&gt;&gt; # nevertheless: ... &gt;&gt;&gt; for item in a.items(): print(item) ... ('992', 0.0010070493454179255) &gt;&gt;&gt; for item in a.data.items(): print(item) ... ('993', 0.001006036217303823) ('994', 0.0010050251256281408) ('995', 0.001004016064257028) ('996', 0.0010030090270812437) ('997', 0.001002004008016032) ('998', 0.001001001001001001) ('999', 0.001) ('991', 0.0010080645161290322) ('992', 0.0010070493454179255) &gt;&gt;&gt; </code></pre> <p>The class definition is as follows:</p> <pre><code>import collections, sys class QueueDict(collections.UserDict): def __init__(self, maxsize=1*((2**10)**2), *args, **kwargs ): self._maxsize=maxsize super().__init__(*args, **kwargs) self.data=collections.OrderedDict(self.data) def __getitem__(self, key): self.data.move_to_end(key) return super().__getitem__(key) def __setitem__(self, key, value): super().__setitem__(key, value) self._purge() def _purge(self): while sys.getsizeof(self.data) &gt; self._maxsize: self.data.popitem(last=False) </code></pre> <p>This is quite disturbing. Any ideas how the same object [by "visual" inspection, and also by <code>(a.items()==a.data.items()) == True</code>] can, and why it does, behave differently in the for loop?</p> <p>Thanks for your help and ideas!</p>
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    1. USA Sz
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    1. COWhat exactly is the difference you are seeing here?
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      1. USDaniel Roseman
    2. COExactly what is the difference between using the two objects in a for loop?
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    3. CO@DanielRoseman: the first only iteratoes over the first element, the second over all!
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