Iterable objects are those that implement __iter__ function, which returns an iterator object, i.e. and object providing the functions __iter__ and __next__ and behaving correctly. Usually the size of the iterable object is not known beforehand, and iterable object is not expected to know how long the iteration will last; however, there are some cases in which knowing the length of the iterable is valuable, for example, when creating an array. list(x for x in range(1000000)), for example, creates an initial array of small size, copies it after it is full, and repeats for many times as explained here. Of course, it is not that important in this example, but it explains the point.
Is there a protocol in use for those iterable objects who know their length beforehand? That is, is there a protocol extending Sized and Iterable but not Collection or Reversible? It seems like there is no such protocol in language features, is there such a protocol for well-known third-party libraries? How this discussion relates to generators?
from Iterables / generators with specified length
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