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A Mapping is a generic container for associating key/value pairs.

This class provides concrete generic implementations of all
methods except for **getitem** , **iter** , and **len**.

## Methods

### `get`

    get(
        key, default=None
    )

Retrieve the corresponding layout by the string key.

When there isn't an exact match, all the existing keys in the layout map
will be treated as a regex and map against the input key again. The
first match will be returned, based on the key insertion order. Return
None if there isn't any match found.

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| Args ||
|---|---|
| `key` | the string key as the query for the layout. |

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| Returns ||
|---|---|
| Corresponding layout based on the query. ||

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### `items`

    items()

D.items() -\> a set-like object providing a view on D's items

### `keys`

    keys()

D.keys() -\> a set-like object providing a view on D's keys

### `values`

    values()

D.values() -\> an object providing a view on D's values

### `__contains__`

    __contains__(
        key
    )

### `__eq__`

    __eq__(
        other
    )

Return self==value.

### `__getitem__`

    __getitem__(
        key
    )

### `__iter__`

    __iter__()

### `__len__`

    __len__()