# Python Collections Module

The `collections` module implements specialized container datatypes providing alternatives to Python’s general purpose built-in containers, `dict`, `list`, `set`, and `tuple`.

## Importing the Module

```python
import collections
```

## `Counter`

A `Counter` is a dict subclass for counting hashable objects.

```python
from collections import Counter

cnt = Counter(['red', 'blue', 'red', 'green', 'blue', 'blue'])
print(cnt)
# Output: Counter({'blue': 3, 'red': 2, 'green': 1})

# Most common elements
print(cnt.most_common(2))
```

## `defaultdict`

A `defaultdict` works exactly like a dictionary, but it is initialized with a function ("default factory") that takes no arguments and provides the default value for a nonexistent key.

```python
from collections import defaultdict

# Default value is an empty list
d = defaultdict(list)

d['python'].append('awesome')
d['java'].append('verbose')

print(d['python']) # ['awesome']
print(d['c++'])    # [] (No KeyError, returns default)
```

## `namedtuple`

Factory function for creating tuple subclasses with named fields.

```python
from collections import namedtuple

Point = namedtuple('Point', ['x', 'y'])
p = Point(11, y=22)

print(p.x, p.y) # Access by name
print(p[0], p[1]) # Access by index
```

## `deque`

A `deque` (double-ended queue) is a list-like container with fast appends and pops on either end.

```python
from collections import deque

d = deque(['a', 'b', 'c'])

d.append('d')      # Add to right
d.appendleft('z')  # Add to left

d.pop()            # Remove from right
d.popleft()        # Remove from left
```

## `OrderedDict`

A dictionary subclass that remembers the order in which entries were added.
*Note: As of Python 3.7, standard dicts also preserve insertion order, but `OrderedDict` has some extra methods like `move_to_end`.*

```python
from collections import OrderedDict

od = OrderedDict()
od['a'] = 1
od['b'] = 2
od.move_to_end('a')
print(list(od.keys())) # ['b', 'a']
```

[[programming/python/python]]