What `for x in y` hides from you – From Scratch Code

13 days ago

A Python 'for x in y' loop actually works by using an iterator protocol: it first calls iter(...) to get an iterator, then repeatedly calls next(...) to get values, and stops when StopIteration is raised.
The for loop syntax is a wrapper that hides the underlying mechanism, which is the same for all iterable objects like lists, strings, ranges, and generators, making iteration feel unified and flexible.
Understanding this reveals that iteration is not directly over the collection but over an iterator, which can be exhausted (like in the example where a second loop prints nothing after the first uses up the iterator).
This design allows custom objects to participate in iteration by implementing __iter__() and __next__(), and explains why unpacking in loops (e.g., 'for x, y in pairs') works without special logic.

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