Iterator & Generator

Iterator

The iterator objects themselves are required to support the following two methods, which together form the iterator protocol:

iterator.__iter__()

Return the iterator object itself. This is required to allow both containers and iterators to be used with the forand in statements. This method corresponds to the tp_iter slot of the type structure for Python objects in the Python/C API.

iterator.__next__()

Return the next item from the container. If there are no further items, raise the StopIteration exception. This method corresponds to the tp_iternext slot of the type structure for Python objects in the Python/C API.

Generator

Generator 是一种特殊的 iterator。

Generator Expressions

a = [1,2,3]
gen = (x for x in a)

Yield Expressions

yield 表达式可以拥有值。

generator.__next__()

Starts the execution of a generator function or resumes it at the last executed yield expression.

generator.send(value)

Resumes the execution and “sends” a value into the generator function. The value argument becomes the result of the current yield expression. The send() method returns the next value yielded by the generator, or raises StopIteration if the generator exits without yielding another value.

# This function returns a generator object.
def infinite_sequence():
    num = 0
    while True:
        yield num
        num += 1

If a container object’s __iter__() method is implemented as a generator, it will automatically return an iterator object (technically, a generator object) supplying the __iter__() and __next__() methods.

When yield from <expr> is used, the supplied expression must be an iterable. The values produced by iterating that iterable are passed directly to the caller of the current generator’s methods.

>>> def genfrom():
...     a = [1,2,3]
...     yield from a
...
>>> b = genfrom()
>>> b.__next__()
1
>>> b.__next__()
2

Reference

[1] https://docs.python.org/3/library/stdtypes.html#iterator-types

[2] https://docs.python.org/3/reference/expressions.html#generator-expressions