Iteration

Describing Iterators and Iterables

  • An iterable is an object with a __iter__ method
  • An iterator is an object with a __next__ method
  • An iterator is created by calling the iter() on an iterable:
iter(some iterable)→iterator\text{iter(some iterable)} \to \text{iterator}

  • The following are examples of iterables:

    • String
    • List
    • Set

  • The following is an example of iterating through a string:

    >>> s = 'foo'
>>> type(s)  # iterable
<class 'str'>
>>> next(s)  # cant call next()
TypeError: 'str' object is not an iterator

siter = iter(s) # can call iter() type(siter) # new iterator object <class 'str_iterator'> next(s) f

### Why Iterators are Relevant to For Loops

- Let's say we have an iterable called **mylist**
- This iterable is a list
- Suppose we want to loop over our list like:

```python
for x in mylist:
    ...loop body...

  • Then, Python performs the following steps during for loops:

    1. Gets an iterable mylist

    2. Call iter(mylist)

      • This returns an iterator object
      • An iterator object should have the __next__() method

    3. Use the iterator to loop over items

      • This involves calling the __next__() method
      • This output is assigned to x
      • Then, the loop body is executed
      • The loop is exited for any StopIteration
  • Python always performs the above steps for any looping
  • Meaning, Python performs thse steps for while and for loops

Example of Iterables and Iterators

>>> class PowTwo:
...     def __init__(self, max):
...         self.max = max
...     def __iter__(self):
...         return PowIter(self.max)

>>> class PowIter:
...     def __init__(self, max):
...         self.max = max
...         self.n = 0
...     def __next__(self):
...         if self.n <= self.max:
...             result = 2 ** self.n
...             self.n += 1
...             return result
...         else:
...             raise StopIteration

>>> a = PowTwo(4)
>>> i = iter(a)
>>> type(a)
__main__.PowTwo
>>> type(i)
__main__.PowIter
>>> next(i)
1

Example of an Object that is an Iterable and Iterator

>>> class PowTwo:
...     def __init__(self, max = 0):
...         self.max = max
...     def __iter__(self):
...         self.n = 0
...         return self
...     def __next__(self):
...         if self.n <= self.max:
...             result = 2 ** self.n
...             self.n += 1
...             return result
...         else:
...             raise StopIteration

>>> a = PowTwo(4)
>>> i = iter(a)
>>> type(a)
__main__.PowTwo
>>> type(i)
__main__.PowTwo
>>> next(i)
1

Summarizing Special Methods

Method Description
__iter__(self) Returns an iterator
__next__(self) Returns the next element of the iteration

References

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Sequence and Mapping Methods

Mathematical Operations

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