I was also typing a similar reply and Steve explained it better :) Just to add there is a note on implicit next call on a for loop in the documentation.

https://docs.python.org/3/reference/expressions.html#generator.__next__

> Starts the execution of a generator function or resumes it at the last executed yield expression. When a generator function is resumed with a __next__() method, the current yield expression always evaluates to None. The execution then continues to the next yield expression, where the generator is suspended again, and the value of the expression_list is returned to __next__()’s caller. If the generator exits without yielding another value, a StopIteration exception is raised.

> This method is normally called implicitly, e.g. by a for loop, or by the built-in next() function.