Why do I get "TypeError: Missing 1 required positional argument: 'self'" trying to call a method from the class directly?

Question

I have some code like:

class Pump:    
    def __init__(self):
        print("init")

    def getPumps(self):
        pass

p = Pump.getPumps()
print(p)

But I get an error like:

Traceback (most recent call last):
  File "C:\Users\Dom\Desktop\test\test.py", line 7, in <module>
    p = Pump.getPumps()
TypeError: getPumps() missing 1 required positional argument: 'self'

Why doesn't __init__ seem to be called, and what does this exception mean? My understanding is that self is passed to the constructor and methods automatically. What am I doing wrong here?

Answer 1

To use the class, first create an instance, like so:

p = Pump()
p.getPumps()

A full example:

>>> class TestClass:
...     def __init__(self):
...         print("init")
...     def testFunc(self):
...         print("Test Func")
... 
>>> testInstance = TestClass()
init
>>> testInstance.testFunc()
Test Func

Answer 2

You need to initialize it first:

p = Pump().getPumps()

Answer 3

Works and is simpler than every other solution I see here :

Pump().getPumps()

This is great if you don't need to reuse a class instance. Tested on Python 3.7.3.

Answer 4

Adding a @classmethod decorator to the method allows for calling it like Pump.getPumps().

A class method receives the class as the implicit first argument, just like an instance method receives the instance.

class Pump:
    def __init__(self):
        print("init")

    @classmethod
    def getPumps(cls):
        pass

Answer 5

The self keyword in Python is analogous to this keyword in C++ / Java / C#.

In Python 2 it is done implicitly by the compiler (yes Python does compilation internally). It's just that in Python 3 you need to mention it explicitly in the constructor and member functions. example:

class Pump():
    # member variable
    # account_holder
    # balance_amount

    # constructor
    def __init__(self,ah,bal):
        self.account_holder = ah
        self.balance_amount = bal

    def getPumps(self):
        print("The details of your account are:"+self.account_number + self.balance_amount)

# object = class(*passing values to constructor*)
p = Pump("Tahir",12000)
p.getPumps()

Answer 6

You can also get this error by prematurely taking PyCharm's advice to annotate a method @staticmethod. Remove the annotation.

Answer 7

I got the same error below:

TypeError: test() missing 1 required positional argument: 'self'

When an instance method had self, then I called it directly by class name as shown below:

class Person:
    def test(self): # <- With "self" 
        print("Test")

Person.test() # Here

And, when a static method had self, then I called it by object or directly by class name as shown below:

class Person:
    @staticmethod
    def test(self): # <- With "self" 
        print("Test")

obj = Person()
obj.test() # Here

# Or

Person.test() # Here

So, I called the instance method with object as shown below:

class Person:
    def test(self): # <- With "self" 
        print("Test")

obj = Person()
obj.test() # Here

And, I removed self from the static method as shown below:

class Person:
    @staticmethod
    def test(): # <- "self" removed 
        print("Test")

obj = Person()
obj.test() # Here

# Or

Person.test() # Here

Then, the error was solved:

Test

In detail, I explain about instance method in my answer for What is an "instance method" in Python? and also explain about @staticmethod and @classmethod in my answer for @classmethod vs @staticmethod in Python.

Answer 8

If skipping parentheses for the object declaration (typo), then exactly this error occurs.

# WRONG! will result in TypeError: getPumps() missing 1 required positional argument: 'self'
p = Pump
p.getPumps()

Do not forget the parentheses for the Pump object

# CORRECT!
p = Pump()
p.getPumps()

Answer 9

Remember 2 points

1. While defining the class do not enclose the class name by () bracket.

   class Pump:    
   def __init__(self):
   

2. Also while instantiating class by object do not forget to use () bracket. Because only then above error is displayed.

• If you want to call method without instantiating the object then inline instantiating Pump().getPumps()

• But ideally best practice is to instantiate the object with short key and then use that object instantiated to call the methods of the class, like

   p = Pump()
   p.getPumps()