PyNash October 2015 - What's an iterable?

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# What are iterables?
# How do I use them?

PyNash October 2015

Brian Dailey

[@byeliad](http://twitter.com/byeliad)

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# What's an iterable?

An iterable is anything over which you can _iterate_.

If you can loop over it, it's probably an iterable.

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.left-column[
  ## Example
]
.right-column[
```terminal
In [1]: iterable_list = [1,2,3,4]

In [2]: for i in iterable_list:
   ...:     print(i)
   ...:
1
2
3
4
```
]

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.left-column[
  ## Example
]
.right-column[
```python
iterable_list = [1,2,3,4]

for i in iterable_list:
  print(i)
```

### Output
```terminal
1
2
3
4
```
]

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## How does it iterate?

# `object.__iter__`

---

`l.__iter__()` returns an `iterator`

```terminal
In [1]: l = [1,2,3,4]

In [5]: pointer = l.__iter__()

In [6]: pointer
Out[6]: <list_iterator at 0x1116642e8>

In [8]: next(pointer)
Out[8]: 1

In [9]: next(pointer)
Out[9]: 2

In [10]: next(pointer)
Out[10]: 3

In [11]: next(pointer)
Out[11]: 4
```

An iterator has a `__next__` method.

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## What happens if I call
# `next(pointer)`
## on this iterator again?

---

```terminal
In [11]: next(pointer)
Out[11]: 4

In [12]: next(pointer)
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-12-ebf92218e69a> in <module>()
----> 1 next(pointer)

StopIteration:
```

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## `StopIteration`

This is how a loop knows to stop!

---

This is an iterable, too!

```python
s = 'cookies'
```

It has a `__getitem__` method that allows us to lookup by index.

```terminal
In [13]: s = 'cookies'

In [14]: s[0]
Out[14]: 'c'

In [15]: s[-1]
Out[15]: 's'
```

---

We can also loop over the string.

```terminal
In [16]: for i in s:
   ....:     print(i)
   ....:
c
o
o
k
i
e
s
```

---

Or, I can get clever and return an iterator.

```terminal
In [22]: i = iter(s)

In [23]: next(i)
Out[23]: 'c'

In [24]: next(i)
Out[24]: 'o'

In [25]: i.__next__()
Out[25]: 'o'

In [26]: i.__next__()
Out[26]: 'k'
```

---

You can also cast this to a list or a tuple.

```terminal
In [32]: s = 'cookies'

In [33]: list(s)
Out[33]: ['c', 'o', 'o', 'k', 'i', 'e', 's']
```

---

What do you think will happen?

```terminal
In [34]: s = 'cookies'

In [35]: i = iter(s)

In [36]: next(i)
Out[36]: 'c'

In [37]: next(i)
Out[37]: 'o'

In [38]: list(i)
```

```terminal
Out[38]: ['o', 'k', 'i', 'e', 's']
```

---
## Remember we said that an _iterable_ is an object with `__getitem__` or `__iter__`

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## Whereas an _iterator_ is something that has a `__next__` method.

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# So what can we do with iterables?

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# Built-in methods!

---

```terminal
In [4]: l = [True, True, False]

In [5]: any(l)
Out[5]: True

In [6]: all(l)
Out[6]: False

In [11]: l = [('flavor', 'chocolate chip'),
  ('count', 'too many')]

In [12]: dict(l)
Out[12]: {'count': 'too many',
  'flavor': 'chocolate chip'}
```

---
```terminal
In [14]: for index, detail in enumerate(l):
    print(index, detail)
   ....:
0 ('flavor', 'chocolate chip')
1 ('count', 'too many')
```

---
```terminal
In [32]: l1 = [1,2,3,4]

In [33]: l2 = [5,6,7,8]

In [34]: l3 = zip(l1, l2)

In [35]: print(l3)
```

What will happen?
--

```terminal
In [35]: print(l3)
<zip object at 0x10e69b848>
```

Python3 thing here, `zip` returns an iterator!

We can, however, cast to a list.

```terminal
In [36]: print(list(l3))
[(1, 5), (2, 6), (3, 7), (4, 8)]
```

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# List Comprehension!

---

```terminal
In [15]: l = [1,2,3,4]

In [16]: [i + 1 for i in l]
Out[16]: [2, 3, 4, 5]
```
--
```terminal
In [18]: [i for i in l if i > 2]
Out[18]: [3, 4]
```

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# Generators!

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### `raise OutOfScopeError`

---

# `itertools`!

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```terminal
In [37]: import itertools

In [38]: itertools.chain(l1, l2)
Out[38]: <itertools.chain at 0x10e6b9470>

In [39]: list(itertools.chain(l1, l2))
Out[39]: [1, 2, 3, 4, 5, 6, 7, 8]
```

... and more!

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# Bonus Questions!

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# How do you learn?

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# How do you avoid burnout?

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## Try not to always look at glowing rectangles.

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## Have hobbies not involving computers.

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![Image](./garden.1024.jpg)

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# Thank you!

@byeliad