I'm new to creating classes and try to solve a problem/exercise.

When I try to instantiate the toolbox, I try to add some tools to it. My approach is to create it as a list, and append new elements to it. It returns a "None" type attribute, and I don't know how to fix it.

```
class Toolbox:
    def __init__(self):
        self.toolbox = []
    def add_tool(self, tool):
        self.toolbox.append(tool)
    def remove_tool(self, tool):
        self.toolbox.remove(tool)

class Hammers:
    color = ""
    def __init__(self, color=''):
        pass
    def hammer_in(self, nail):
        pass
    def remove(self, nail):
        pass
    def paint(self, color):
        pass

class Screwdrivers:
    size = 10
    def __init__(self, size=''):
        pass
    def tighten(self, screw):
        pass
    def loosen(self, screw):
        pass

toolbox = Toolbox()
hammer = Hammers()
screwdriver = Screwdrivers()

toolbox = toolbox.add_tool(hammer)
toolbox = toolbox.add_tool(screwdriver)

```

Output :

```
Traceback (most recent call last):
  File "C:Create Classes.py", line 133, in <module>
    toolbox = toolbox.add_tool(screwdriver)
AttributeError: 'NoneType' object has no attribute 'add_tool'

```

I'm trying to :
Instantiate a toolbox, a screwdriver, and a hammer.

```
Put the hammer and screwdriver in the toolbox.

```

Thank you,