Python Extremely noob question

[Python] Extremely noob question - Printable Version

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Extremely noob question - SIM-2362662 - Jan 10, 2020

My code:

Code 
class A:

    def A_display(self, msg):

        print(msg)

class B:

    classA=A()

    def reveal(self):

        self.classA.A_display("Calling Inner class function from Outer class")

clB=B()

B.reveal()

Error:

Quote:Traceback (most recent call last):
File "C:/Users/ghioc/Desktop/PythonWorld/CAM4/MAIN.py", line 44, in <module>
B.reveal()
TypeError: reveal() missing 1 required positional argument: 'self'

Why is this happening and how can I avoid this error?

RE: Extremely noob question - Riԁdle - Jan 10, 2020

You're doing

Code 
B.reveal()

when you should be doing

Code 
B().reveal()

Code 
clB = B() 

clB.reveal()

RE: Extremely noob question - Nomad.Null - Jan 12, 2020

You forgot to provide your __init__() method for the class. Without an __init__() method, self is meaningless.

The __init__() method is the "instantiation" method for each class, basically the instructions that are run each time a new instance of the class is created. The first argument of the __init__() method is always the name of the instance (conventionally "self" but it could be anything. You could make it "potato" if you like.

Here is your fixed code.

Code 
class A:

    def __init__(self):

        pass

    def A_display(self, msg):

        print(msg)

class B:

    def __init__(self):

        self.classA = A()

    def reveal(self):

        self.classA.A_display("Calling Inner class function from Outer class")

b = B()

b.reveal()

And here is the potato code

Code 
class A:

    def __init__(potato):

        pass

    def A_display(potato, msg):

        print(msg)

class B:

    def __init__(potato):

        potato.classA = A()

    def reveal(potato):

        potato.classA.A_display("Calling Inner class function from Outer class")

b = B()

b.reveal()

RE: Extremely noob question - Riԁdle - Jan 12, 2020

(Jan 12, 2020 11:40 AM)True Demon Wrote: You forgot to provide your_init__() method for the class. Without an __init__() method, self is meaningless.

The __init__() method is the "instantiation" method for each class, basically the instructions that are run each time a new instance of the class is created. The first argument of the __init__() method is always the name of the instance (conventionally "self" but it could be anything. You could make it "potato" if you like.

Here is your fixed code.

-snip-

This wasn't his problem.
he defined
Code
classA=A()
outside of the __init_ method because he wanted it to be a shared global variable for the classes (probably for memory purposes), he didn't want to initialize a new

Code 
class A

object every time he initializes a new

Code 
class B

object. assuming he did it on purpose lol

so his classA variable is shared by all instances of class B

RE: Extremely noob question - Nomad.Null - Jan 12, 2020

I suppose that could be the case, but it's a rather niche use of classes that I've rarely seen in practice. I think we're both making assumptions about his use-case here.

Since he said upfront that it was a "noob" question, I assumed he wasn't terribly familiar with classes in the first place and was maybe just missing the more traditional solution. If that were the case, it would just be simpler to declare it as a static method like so:

Code 
class A:

    @staticimethod

    def A_display(msg):

        print(msg)

No need for instantiation of the class or anything, that way. ¯\_(ツ)_/¯

I personally stay away from global class variables because they have a tendency to introduce really weird behavior that can make debugging a PITA. Any time you use global variables in classes, you risk making unintended changes to the class whenever you call the class itself or an instance of it.

A static method is safer, and equally memory efficient in this case. 👍

RE: Extremely noob question - Riԁdle - Jan 12, 2020

(Jan 12, 2020 02:37 PM)True Demon Wrote: I suppose that could be the case, but it's a rather niche use of classes that I've rarely seen in practice. I think we're both making assumptions about his use-case here.

Since he said upfront that it was a "noob" question, I assumed he wasn't terribly familiar with classes in the first place and was maybe just missing the more traditional solution. If that were the case, it would just be simpler to declare it as a static method like so:

Code
class A: @staticimethod def A_display(msg): print(msg)

No need for instantiation of the class or anything, that way. ¯\_(ツ)_/¯

I personally stay away from global class variables because they have a tendency to introduce really weird behavior that can make debugging a PITA. Any time you use global variables in classes, you risk making unintended changes to the class whenever you call the class itself or an instance of it.

A static method is safer, and equally memory efficient in this case. 👍

Yeah, I just meant the error he got wasn't the result of not having an init method, i'm sure he'll learn something new from your previous post though, you explained it quite well. I doubt he made it a global variable on purpose aswell