Ivan Zakharyaschev <i...@altlinux.org> added the comment:

Actually, Alexey shared this problem with me orally first, and then asked to 
have a look at his report, and I felt that just describing the technical 
details about what is going on is not enough, and suggested to include a brief 
sentence which would state what kind of assumptions seemed to be broken to 
Alexey. And it was:

> a list contains elements that have never been appended.

And this statement actually appeared quite helpful for me: in a few hours after 
this statement was made, I got an idea that this statement might actually be 
not true in this case. And this quickly lead me to the demonstration what 
really happens here, and that it was correct Python behavior.

Without this brief formal statement of the broken assumptions, I had simply had 
no ideas (because I supposed that something was going wrong according to 
Alexey's words, and didn't question whether it is really wrong or we might 
expect these results under some conditions as correct results).

(Actually, I'm a fan of type-checking, and what is more, of languages with 
expressive types like Agda or Idris where the programmer is to make provable 
statements of the expected properties written down as types; sometimes, they 
can be derived automatically, but nevertheless my point is the attitude to 
programming, even without formal tools: think how the expected properties would 
be proved, how they would be decomposed into simpler statements during the 
proof. Well, this is offtopic here, so, Alexey, let's not continue this 
discussion here.)

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<https://bugs.python.org/issue33186>
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