Beginner Python: AttributeError: 'list' object has no attribute

The error says:

AttributeError: 'list' object has no attribute 'cost' 

I am trying to get a simple profit calculation to work using the following class to handle a dictionary of bicycles:

class Bike(object): def __init__(self, name, weight, cost): self.name = name self.weight = weight self.cost = cost bikes = { # Bike designed for children" "Trike": ["Trike", 20, 100], # Bike designed for everyone" "Kruzer": ["Kruzer", 50, 165] } 

When I try to calculate profit with my for statement, I get the attribute error.

# Markup of 20% on all sales margin = .2 # Revenue minus cost after sale for bike in bikes.values(): profit = bike.cost * margin 

First, I don't know why it is referring to a list, and everything seems to be defined, no?

3 Answers

Consider:

class Bike(object): def __init__(self, name, weight, cost): self.name = name self.weight = weight self.cost = cost bikes = { # Bike designed for children" "Trike": Bike("Trike", 20, 100), # <-- # Bike designed for everyone" "Kruzer": Bike("Kruzer", 50, 165), # <-- } # Markup of 20% on all sales margin = .2 # Revenue minus cost after sale for bike in bikes.values(): profit = bike.cost * margin print(profit) 

Output:

 33.0 20.0 

The difference is that in your bikes dictionary, you're initializing the values as lists [...]. Instead, it looks like the rest of your code wants Bike instances. So create Bike instances: Bike(...).

As for your error

AttributeError: 'list' object has no attribute 'cost' 

this will occur when you try to call .cost on a list object. Pretty straightforward, but we can figure out what happened by looking at where you call .cost -- in this line:

profit = bike.cost * margin 

This indicates that at least one bike (that is, a member of bikes.values() is a list). If you look at where you defined bikes you can see that the values were, in fact, lists. So this error makes sense.

But since your class has a cost attribute, it looked like you were trying to use Bike instances as values, so I made that little change:

[...] -> Bike(...) 

and you're all set.

They are lists because you type them as lists in the dictionary:

bikes = { # Bike designed for children" "Trike": ["Trike", 20, 100], # Bike designed for everyone" "Kruzer": ["Kruzer", 50, 165] } 

You should use the bike-class instead:

bikes = { # Bike designed for children" "Trike": Bike("Trike", 20, 100), # Bike designed for everyone" "Kruzer": Bike("Kruzer", 50, 165) } 

This will allow you to get the cost of the bikes with bike.cost as you were trying to.

for bike in bikes.values(): profit = bike.cost * margin print(bike.name + " : " + str(profit)) 

This will now print:

Kruzer : 33.0 Trike : 20.0 

You need to pass the values of the dict into the Bike constructor before using like that. Or, see the namedtuple -- seems more in line with what you're trying to do.