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In Python, a generator is a built-in object that satisfies the iterator interface. Specialized syntax such as generator expressions and the yield keyword makes generator objects a concise way of creating an iterator. A generator is lazy, producing an item only when asked for it, so it is memory efficient. As such, generators can also represent infinite sequences.

yield statement

When a function body contains a yield statement, the function will then output a generator object when called.

def double_iter():
    curr = 1
    while True:
        yield curr
        curr *= 2

Now, each time that __next__ is invoked on the generator (e.g. through the next function or a for loop), code from the function will be executed up through the yield statement. The expression next to the yield statement will then be yielded as the result of invoking __next__ on the generator object. Then, the next time __next__ is invoked, the execution starts from where it has previously left off and continues until it hits yield again. [1]

>>> a = double_iter()
>>> a
<generator object double_iter at 0x10f14e678>
>>> next(a)
>>> next(a)
>>> next(a)

To stop the generator from producing another value, one can either raise a StopIteration exception or return None. Note that returning None also happens by default when one reaches the end of the body.

Generator expression

A generator can also be created directly via a generator expression, which uses the same syntax as a list comprehension but evaluates to a generator object instead of a list.

>>> a = (i*2 for i in range(5))
>>> a
<generator object <genexpr> at 0x109d90558>
>>> next(a)
>>> next(a)
>>> for elem in a:
...     print(elem)

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