Attribute Error creating Classes with Python

Question

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,

• juanpa.arrivillaga: toolbox.add_tool returns None. You assign that to toolbox. Hence the error when you try to do toolbox.add_tool again. Just don't assign to toolbox

Answer 1

As Arseniy says, your problem comes from reassigning toolbox to a new object:

First, a Toolbox object is assigned to the toolbox reference

toolbox = Toolbox()

Then, the result of toolbox.add_tool, which is the object None is assigned to the toolbox reference, thus toolbox now no longer points to a instantiated Toolbox object:

toolbox = toolbox.add_tool(hammer) 

So, what you should do instead is simply remove the re-assignment:

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

And this should result in:

>>> print (toolbox.toolbox)
[<__main__.Hammers object at 0x...>, <__main__.Screwdrivers object at 0x...>]

• m3.b: Thanks for this! So quick question - why does it return None? And I don't understand the result... Why does it not just result in a simple list?
• esocrats: Hi @m3.b. The issue here comes from how Python works internally. In Python everything is handled as an object (including non instantiated classes). So when you make reference assignments to a variable, what you are doing is assigning it an object. Thus, this assignment must always be possible. But what happens when you are not assigning anything in a variable assignment? Well, in fact that variable will reference the object None. Since the method toolbox.add_tool(hammer) does not have any return value, assigning it to toolbox will make toolbox reference the object None.
• m3.b: Hi @esocrats . Thanks for this comment. However, the add.tool() method has a function to append an element...doesn't it? Why would it return nothing? Or should I write return self.toolbox.append()instead?
• esocrats: Indeed, as long you do not write return [object] ([object] is a placeholder for the object you want to return) your method will return None. It might be useful for you to also take a look at the Python Typing module. Adding type hints to your functions, will help you keep track of what Python is (or should be) doing.
• m3.b: thank you. If I call >>> toolbox = Toolbox() >>> toolbox <__main__.Toolbox object at 0x0000025831C3DF70> . I do not understand this line : why does it not show me an empty list at this point?

Answer 2

add_tool return nothing (None), and you assign it to toolbox

toolbox = toolbox.add_tool(hammer) <--- problem here 
toolbox = toolbox.add_tool(screwdriver)

So, toolbox become None and 'NoneType' object has no attribute 'add_tool'

Answer 3

add_tool and remove_tool return implicitly None. You should not assign this return value to toolbox.

You are using class-Attributes wrongly. color should be a instance attribute. The default value of size is wrong, since size is not a empty string.

The class Hammers represent one Hammer. Why can you paint with a hammer?

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 Hammer:
 def __init__(self, color=''):
 self.color = color
 def hammer_in(self, nail):
 pass
 def remove(self, nail):
 pass
 def paint(self, color):
 pass
class Screwdriver:
 def __init__(self, size=10):
 self.size = size
 def tighten(self, screw):
 pass
 def loosen(self, screw):
 pass
toolbox = Toolbox()
hammer = Hammer()
screwdriver = Screwdriver()
toolbox.add_tool(hammer)
toolbox.add_tool(screwdriver)