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# \_\_slots\_\_ magic

The special attribute **`__slots__` allows you to explicitly state which instance attributes you expect your object instances to have**, with the expected results:

1. **faster** attribute access.
2. **space savings** in memory.

The **space savings** is from

1. Storing value references in slots instead of `__dict__`.
2. Denying `__dict__` and `__weakref__` creation if parent classes deny them and you declare `__slots__`.

Every class can have instance attributes. By default Python uses a **dict to store an object’s instance attributes**. This **allows setting arbitrary new attributes at runtime**.

For small classes with known attributes **it might be a bottleneck**. The `dict` wastes a lot of RAM. Python can’t just allocate a static amount of memory at object creation to store all the attributes. Therefore it sucks a lot of RAM if you create a lot of objects (I am talking in thousands and millions). Still there is a way to circumvent this issue. It involves the **usage of `__slots__` to tell Python not to use a dict, and only allocate space for a fixed set of attributes**.

Example:

**Without** `__slots__`:

```python
class MyClass(object):
    def __init__(self, name, identifier):
        self.name = name
        self.identifier = identifier
        self.set_up()
    # ...
```

**With** `__slots__`:

```python
class MyClass(object):
    __slots__ = ['name', 'identifier']
    def __init__(self, name, identifier):
        self.name = name
        self.identifier = identifier
        self.set_up()
    # ...
```

**Using `__slots__` will reduce the burden on your RAM**.

Sidenote: you might want to try **PyPy: it does all of these optimizations by default**.