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    Just to be explicit, this means that the one-level flatten function can be defined as lambda z: [x for y in z for x in y]. Commented Apr 16, 2015 at 9:24
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    "A general purpose flattener needs some way to be told what is atomic and what can be further subdivided.": This sounds like a problem that can be solved using OOP: each object could have a flatten method. The implementation of this method should recursively call flatten on its subcomponent, if the object is a composite. Unfortunately, AFAIK not every value is an object in Python. In Ruby it should work though. Commented Sep 28, 2015 at 11:54
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    a flatten helper for a one-level flatten rather than a continued "for in for in" is already a good enough case IMO. easily readable Commented Jun 15, 2016 at 22:01
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    @Giorgio Python shies away from such methods. Protocols are preferred, and I find they're vastly smoother to work with than an OOP design since you often don't even need to implement very much at all. Commented Dec 13, 2017 at 22:58
  • @Giorgio Having a flatten method only moves the problem, it does not solve it. What is and isn't atomic depends on the use-case, not the types. Same for traversal order. In one case, you may want to flatten a list of lists of tuples completely, in another case those tuples are coordinates and should be preserved. Commented Jun 11, 2020 at 7:52