# collections
[官网文档](https://docs.python.org/3/library/collections.html#module-collections)

## [Counter](https://docs.python.org/3/library/collections.html#counter-objects)

## OrderedDict
* [官网文档](https://docs.python.org/3/library/collections.html#ordereddict-objects)
```python
od = OrderedDict()  # 注意不能直接把一个dict传入进去, 因为这个dict是没有排序的
od = OrderedDict([('key', 'value'), ('key2', 'value2')])
od['1'] = 1
a.keys()
```

## defaultdict
* [官方文档](https://docs.python.org/3/library/collections.html#collections.defaultdict)
* [参考代码]
    ```
    dd = defaultdict(int)
    dd['w']
    >>> 0
    dd['w'] = 1
    dd['w']
    >>> 1
    ```
* [方法]
    * setdefault
        ```
        dd = defaultdict(int)
        dd.setdefault('w', 2)  # dd['w'] = 2 if 'w' not in dd
        dd['w']
        >>> 2
        dd.setdefault('w', 4)
        >>> 2
        ```
* 如果要实现无限的循环
```
class MyDict(dict):
    def __getitem__(self, key):
        print("调用get")
        if key not in self:
            self[key] = MyDict()
        return super(MyDict, self).__getitem__(key)
```

## [Namedtuple](https://docs.python.org/3/library/collections.html#collections.namedtuple)
```
// 自定义不可变的类
Point = namedtuple("Point", ["x", "y"])
Point = namedtuple("Point", "x, y")
Point = namedtuple("Point", "x y")
p = Point(11, y=22)
p.x + p.y
x, y = p
```

## deque
* [官方文档](https://docs.python.org/3/library/collections.html#collections.deque)
* pop和append左右的速度很快
    ```
    a = deque('ghi')
    a.append('j')
    a.appendleft('f')
    a.pop()
    a.popleft()
    ```


# collections abc
这个是 Abstract Base Classes 可以用来确保这个Class无法被直接使用，而必须指定了特定的方法才能使用,利用的是`abc.abstractmethod`就能实现这样的效果

* #### Container

```
class A(Container):
    pass

class B(A):
    def __contains__(self, item):
        return True

b = B()  # OK
a = A()  # TypeError:  Can't instantiate abstract class A with abstract methods __contains__
```
* #### Hashable
* #### Iterable