Python Forum

I am trying to call the method of the class having no arguments but it is failing.

class ab:
        def a():
                print("in a")
def b():
        print("in b")

if __name__ == '__main__':
        b()
        ab.a()

Error:
in b
Traceback (most recent call last):
  File "p.py", line 11, in <module>
    ab.a()
TypeError: unbound method a() must be called with ab instance as first argument (got nothing instead)

I have also tried to modify the code as below but not successful

class ab:
        def a():
                print("in a")
def b():
        print("in b")

if __name__ == '__main__':
        b()
        c=ab()
        c.a()

Error:
in b
Traceback (most recent call last):
  File "p.py", line 12, in <module>
    c.a()
TypeError: a() takes no arguments (1 given)

# Could be wrong but

if __name__ == "__main__"

# is looking for the def main

Methods need to be declared with self as the first parameter, e.g.

class Foo:
    def bar(self):
        pass

f = Foo()
f.bar()

Alternatively you can define it as a static method:

>>> class Foo(object):
...    @staticmethod
...    def bar():
...       print('bar')
...
>>> f = Foo()
>>> f.bar()
bar

Two issues. As above, need self in the function definition. Next, you need an ab object in order to call the function. The class is just a template. So, in the example below, we create the cab object of type ab, then call the function:

class ab:
        def a(self):
                print("in a")
def b():
        print("in b")
 
if __name__ == '__main__':
        b()
        cab = ab()
        cab.a()

Output:
in b
in a

(Oct-30-2019, 06:24 PM)ndc85430 Wrote: [ -> ]Methods need to be declared with self as the first parameter, e.g.

 class Foo: def bar(self): pass f = Foo() f.bar() 

Yes, with self as argument it is working. That being said, Is it necessary to have all the class methods should have at least 1 argument (self) in our case. Can't we have no arguments ?

(Oct-31-2019, 12:03 PM)dataplumber Wrote: [ -> ]Is it necessary to have all the class methods should have at least 1 argument (self) in our case

all instance and class methods should have one argument. For instance methods, by convention, we call that argument self. For class methods we use, again by convention, cls.
And there are staticmethods that answer to your next question:

(Oct-31-2019, 12:03 PM)dataplumber Wrote: [ -> ]Can't we have no arguments ?

As ichabood explained you need to declare it staticmethod. However in this case the question is does it really belong to a class or can simply be a function, outside class. i.e. if it doesn't need to access instance or the class

Google the difference between instance, class, static methods and simple function

(Oct-31-2019, 12:03 PM)dataplumber Wrote: [ -> ]

(Oct-30-2019, 06:24 PM)ndc85430 Wrote: [ -> ]Methods need to be declared with self as the first parameter, e.g.

 class Foo: def bar(self): pass f = Foo() f.bar() 

Yes, with self as argument it is working. That being said, Is it necessary to have all the class methods should have at least 1 argument (self) in our case. Can't we have no arguments ?

Thank you. That Answers my question.