Passing argument between two classes

Question

I have a python program with this structure:

import sys

class A:
    def __init__(self):
        ...

    def func(self, other, args):
        z = something
        n = B.start(z)
        print n

    def other_funcs(self, some, args):
        ...

class B:
    def __init__(self):
         self.start(z)

    def start(self, z)
         k = something
         return k

if __name__ == '__main__'
    A()

When I generate z I want to give it to B class and then B returns k for me again.

But the error exists:

TypeError: unbound method start() must be called with B instance as first argument (got list instance instead)

shouldn't `self.start(z)` say `z` is not defined? – OneCricketeer Feb 09 '16 at 06:17 — OneCricketeer, Feb 09 '16 at 06:17

score 2 · Answer 1 · edited May 23 '17 at 12:23

IIUC, what you're looking for here is classmethod.

The problem is that you don't have a a B object, but rather just the B class. You need a method of B that takes a class, not an instance. Define start like so:

@classmethod
def start(cls, z):

For example, this runs fine:

class A:                                                                                                                                                                                               
     def func(self):                                                          
          n = B.start(0)                                                          


class B:    
     @classmethod                                                                                                                                               
     def start(cls, z):                                                          
          pass                                                                 

if __name__ == '__main__':                                                   
     A().func()

@MLSC The above runs fine by me. By the way, it's pretty clear the code you posted is not exactly the one you're using, as it has other errors (missing `:`, for example). — Ami Tavory, Feb 09 '16 at 06:13

score 2 · Answer 2 · answered Feb 09 '16 at 06:04

2

Alternatively you can initialize a B object:

n = B().start(z)

However your __init__ method calls start with argument z which may not work as z is not defined yet.

answered Feb 09 '16 at 06:04

Selcuk

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Now the error is: `NameError: global name 'B' is not defined` – MLSC Feb 09 '16 at 06:06
Switch places of class B and A in the source (B must be defined before A) – Selcuk Feb 09 '16 at 06:06
Again I have that errro. Shouldn;t I import somthing? – MLSC Feb 09 '16 at 06:08
You don't need to import anything as both classes are defined in the same module (ie. same .py file) – Selcuk Feb 09 '16 at 06:09
I have done it in same .py and switched the classes. but problem exists – MLSC Feb 09 '16 at 06:10
1

Please update your question with your new code, and new error message including full stack trace; or post another question. – Selcuk Feb 09 '16 at 06:13

score 1 · Answer 3 · answered Feb 09 '16 at 07:20

You can modify __init__ of class B to take an argument which passes z to it.

class B:
    __init__(self, z): #Pass 'z' when you create an object a 'class B' in 'class A'

There are some problems with your code.

You need to understand what a constructor is and when it gets called. You have

if __name__ == '__main__': # Fixed missing colon here
    A()

This only calls the __init__ function of class A.

You need to use something like

A().func() # Pass required arguments here

The proper way to use methods in classes is to create objects (however not required for class methods). In your class A you have

n = B.start(z) # Line in func() of class A

This doesn't work.

You need to call B() with required arguments for __init__ in class B and not simply B.

Example code passing message from class A and printing it from class B:

class foo:
    def __init__(self):
        self.var1 = "I'm from class A"
        bar(self.var1)


class bar:
    def __init__(self, var):
        print(var)

if __name__ == '__main__':
    foo()

Passing argument between two classes

3 Answers3