Iterator & Iterable

iterator

• get next item (__next__)
• no indexes needed (Don’t need to be Sequence type)
• consumable

iterable

• collections that implement iterator

Protocal

Python need to count on certain funcionality: __next__、__iter__、StopIteration

compare to sequence type

iteration can be more general than sequential indexing, we only need:

• a bucket of items: collection, container
• a way to get the next item, no need to care about ordering
• an exception to raise if there is no next item

try to custom an iterator ourselfs:

Why re-create?

Seperate the Collection from the iterator

Iterable object

• Maintaining the data of the collection is one object
• Created once
• implements __iter__, return a new iterator instance

Iterator object

• Iterating over that data should be another object
• throw away the iterator but don’t throw away the collection
• Created every time
• implements __iter__, return itself
• implements __next__, return next item

iterable can be lazy

Caculate the next itme in an iterable until it’s actually requested

lazy evaluation

• often used in class properties
• properties of classes may not always populated when the object is created
• value of property only becomes known when the property is requested/deferred

infnite iterables

• itertools.cycle

Python Built-ins

• range: return iterable
• zip: return iterator
• enumerate: return iterator
• open: return iterator
• reversed: return iterator

The type is important. Iterator object can be only iter over once.

iter()

when iter is called:

• Python first looks for __iter__, if not then:
• look for __getitem__ and create an iterator, if not then:
• raise TypeError

Test it:

The __iter__ must return an iterator!

Iterating callable

iterator delegation

Example 1

Example 2