## mutable_default()


Append a marker value to a list and return it.


Usage


``` python
mutable_default(items=None)
```


If no list is provided, an internal default list is used. Appends the string `"added"` to the list and returns it.


## Parameters


`items: list = None`  
A list to append to. If `None`, a new empty list is created for each call.


## Returns


`list`  
The list with `"added"` appended.


## Warnings

If you modify this function to use a mutable default argument (e.g., `items=[]`), the default empty list would be shared across all calls that omit the argument. Each call would mutate the same list object, leading to surprising accumulation of values.

Always use `None` as the default and create a new list inside the function body to avoid this pitfall.


## Examples

``` python
>>> mutable_default()
```

\['added'\]

``` python
>>> mutable_default(["existing"])
```

\['existing', 'added'\]