Adding Methods to a Python Class

I took your advice and looked around the room. I saw a box of popcorn, so here is what I came up with:

class Popcorn:
    status = 0

    def __init__(self, flavor):
        self.flavor = flavor

    def description(self):
        if self.status == 0:
            return f"This is a bag of un-popped {self.flavor} popcorn."
        elif self.status == 1:
            return f"This is a bag of freshly popped {self.flavor} popcorn."
        else:
            return f"You overcooked your {self.flavor} popcorn by cooking it too much."

    def pop(self):
        self.status += 1

If you’re going through this course like me, grab some popcorn and keep on going! It comes in any flavor you like (just don’t pop it too much)!

Why the below does not work?

def birthday(self):
    return self.age += 1

@sangeeth2kin The code looks fine, but it would help to clarify what you mean by saying it doesn’t work 😊

Are you getting a runtime exception? If so, would you mind sharing the error message, please? Does the program work but gives incorrect results? Or maybe nothing happens at all?

Without knowing the context and elaborating on the expected vs. actual outcome, we can only guess what went wrong. My wild guess is that the .age property might not have been initialized in the class.

Hello Bartosz,

I get the below error message

File "<ipython-input-42-527d42b7a18b>", line 18
    return self.age += 1
                    ^
SyntaxError: invalid syntax

This is the code:

class dog:
    species = 'mammal'

    def __init__(self,name,age):
        self.name = name
        self.age = age

    def description(self):
        return "{} is {} ".format(self.name,self.age)

    def speak(self,sound):
        return "{} says {} ".format(self.name,sound)

 #   def birthday(self):
 #        self.age = self.age+1

    def birthday(self):
        return self.age += 1

tiger = dog("tiger",3)

print(tiger.description())

print(tiger.speak("woh woh"))

tiger.birthday()

print(tiger.description())

Ok the below works:

def birthday(self):
    self.age += 1
    print(self.age)

@sangeeth2kin Oh, actually, I haven’t noticed that your previous code wasn’t okay. You were trying to return an assignment statement instead of an expression. Pardon me 🤦

Can I declare and define a function in a class; that is a method without self-parameter?

class dog():
    ......

    def goof(goofy):
        print(goofy)

Hi @Andrew, yes you can do that, but you’ll have to prefix it with the @staticmethod decorator:

@staticmethod
def goof(goofy):
    print(goofy)

You can read more about it in Python’s Instance, Class, and Static Methods Demystified.

for the class methods “description” and “speak”, is there any reason you shouldn’t just use the print() function inside of the definition instead of return?

This way you wouldn’t have to call the method inside of the print function in the main program.

@gilpinbmchs you’ll generally want to return values from functions instead of using print().

This gives you the advantage that you can re-use the values that your function produces in other parts of your code. If you use print(), which is also just a Python function that writes to your console and returns None, you can’t do anything besides showing a value on the console.

Makes sense. Thanks for your help!

@orinax I’m not sure that in your Popcorn class is a good idea using self to access the class member status. I think would be better to use Popcorn.status instead of self.status. It’s not an instance member. Is it right?

@Leonardo the problem with using Popcorn.status is that it would change the status of the whole class, which would change the popped satus for all instances of the class - in other words, if you wrote Popcorn.status += 1 for the last line, calling .pop on one bag of popcorn would result in all the bags of popcorn being popped.