回答編集履歴
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追記
test
CHANGED
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@@ -17,3 +17,159 @@
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listのサイズが0、すなわち`シーケンスが空`である場合は**即座に**ループは終了します。
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すなわちループ内の処理は実行されません。
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# 追記
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むしろ以下コードで示すような変態クラスも想定すると`len`なりでチェックする**べきではなく**
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単純に`for in`に任せた方がよいかもしれません。
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参考：[Pythonはどうやってlen関数で長さを手にいれているの？](https://www.slideshare.net/shimizukawa/how-does-python-get-the-length-with-the-len-function)
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```Python
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class MyIterator:
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def __init__(self,obj):
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self.obj = obj
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self.c = 0
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# ここが変態
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def __len__(self):
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print('__len__')
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return 0
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def __next__(self):
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print('__next__')
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try:
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r = self.obj[self.c]
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self.c += 1
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return r
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except IndexError:
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self.c = 0
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raise StopIteration
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def __iter__(self):
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print('__iter__')
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return self
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def disp(it):
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print('disp')
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for e in it:
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print(e)
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for l in [[],[1,2]]:
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print(l)
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it = MyIterator(l)
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print('just disp')
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disp(it)
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print('check len')
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if len(it) > 0:
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disp(it) # 呼ばれない
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print('check if')
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if it:
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disp(it)# 呼ばれない
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"""
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[]
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just disp
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disp
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__iter__
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__next__
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check len
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__len__
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check if
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__len__
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[1, 2]
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just disp
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disp
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__iter__
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__next__
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1
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__next__
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2
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__next__
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check len
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__len__
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check if
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__len__
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"""
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```
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