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Python - Iterators

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• Python - Iterators
  • Iterators
    • Create an Iterator
    • StopIteration

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Iterators

• An iterator is an object that contains a countable number of values.

  • An iterator is an object that can be iterated upon, meaning that it can traverse through all the values.

• Technically, in Python, an iterator is an object which implements the iterator protocol, which consist of the methods __iter__() and __next__().

• Iterator vs Iterable

  • Lists, tuples, dictionaries, and sets are all iterable objects. They are iterable containers which you can get an iterator from.
  • All these objects have a iter() method which is used to get an iterator
  • strings are iterable objects, and can return an iterator.

• Looping Through an Iterator

  • The for loop actually creates an iterator object and executes the next() method for each loop.

print("\n--------Iterator--------\n")
xTuple = ("apple", "banana", "cherry")
yIter = iter(xTuple)

print(next(yIter))          # apple
print(next(yIter))          # banana
print(next(yIter))          # cherry


xStr = "banana"
xIter = iter(xStr)

print(next(xIter))          # b
print(next(xIter))          # a
print(next(xIter))          # n
print(next(xIter))          # a
print(next(xIter))          # n
print(next(xIter))          # a


print("\n--------Loop--------\n")
xlist=[1,2,3,4]
iter = iter(xlist)      # get an iterator
for x in iter:          # executes the `next()` method for each loop
    print (x, end=" ")

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Create an Iterator

• implement the methods __iter__() and __next__() to an object.

• __iter__(): do initialization, and always return the iterator object itself.

• ``next()`: do operations, and must return the next item in the sequence.

print("\n--------Create an Iterator--------\n")


class XIterator:
    def __iter__(self):
        self.a = 0
        return self

    def __next__(self):
        x = self.a
        self.a += 1
        return x


xIter = XIterator()
iter = iter(xIter)      # get an iterator

print(next(iter))
print(next(iter))
print(next(iter))
print(next(iter))
print(next(iter))
print(next(iter))
print(next(iter))
print(next(iter))
print(next(iter))

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StopIteration

• To prevent the iteration to go on forever, we can use the StopIteration statement, without which the iteration keep going on forever.

  • StopIteration 异常用于标识迭代的完成，防止出现无限循环的情况，

• In the next() method, we can add a terminating condition to raise an error if the iteration is done a specified number of times.

print("\n--------Create an Iterator--------\n")


class XIterator:
    def __iter__(self):
        self.a = 1
        return self

    def __next__(self):
        if self.a <= 20:
            x = self.a
            self.a += 1
            return x
        else:
            raise StopIteration


xIter = XIterator()
iter = iter(xIter)

for x in iter:
    print(x)        # 1-20

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