> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sourcery.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Default Mutable Arguments

#### Sourcery refactoring id: `default-mutable-arg`

#### Description:

Replaces use of default mutable arguments in a function

#### Before:

```python
def func(hats: list = []):
    change(hats)
```

#### After:

```python
def func(hats: list = None):
    if hats is None:
        hats = []
    change(hats)
```

#### Explanation:

A common gotcha in Python involves default argument values which are mutable.
These are evaluated only once at function definition time, so only one list, set
or dictionary instance will be created. This means that if this list is mutated
in one call to a function, those changes will show up in subsequent calls of
that function. This is usually unintended behaviour, though it can be useful in
limited circumstances for writing caches.