Why do I get "TypeError: Missing 1 required positional argument: 'self'"?

问题描述：

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:UsersDomDesktop    est    est.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?

See Why do I get 'takes exactly 1 argument (2 given)' when trying to call a method? for the opposite problem.

解决方案 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

解决方案 2：

You need to initialize it first:

p = Pump().getPumps()

解决方案 3：

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

解决方案 4：

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()

解决方案 5：

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

解决方案 6：

Remember 2 points

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

class Pump:    
def __init__(self):

1. 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()

解决方案 7：

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()

解决方案 8：

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.

解决方案 9：

This error may also be raised if a required positional argument for a function is not passed. For example the following triggers the error.

def func(value, *args, **kwargs):
    pass

func()            # <--- TypeError: func() missing 1 required positional argument: 'value'
func(a=1, b=2)    # <--- TypeError: func() missing 1 required positional argument: 'value'

# passing the required arg solves the issue
func(30)          # <--- OK
func(3, a=1, b=2) # <--- OK

The above example is very simple but if func() was imported from a module, it would be less obvious. For example, numpy module's allclose function (which checks if two arrays are close enough) expects two positional arguments. If you pass the two arrays as a tuple/list, you'll get the TypeError in the title.

import numpy as np
np.allclose(([1, 2], [1, 1]))  # <--- TypeError
np.allclose([1, 2], [1, 1])    # <--- OK

解决方案 10：

BEFORE

 class Person:
        def __init__(self, full_name, uuid, user_type, working_hours=None):
            self.full_name = full_name
            self.uuid = uuid
    
        def load_data(filename):
            print("Data")
           
        
        def load_readers(self):
            return self.load_data("readers.txt")

   

AFTER

class Person:
        def __init__(self, full_name, uuid, user_type):
            self.full_name = full_name
            self.uuid = uuid
            self.user_type = user_type
    
        def load_data(self, filename):
            print("Data")
           
        
        def load_readers(self):
            return self.load_data("readers.txt")

In my situation, the error was caused by a missing self keyword in the load_data() function, which was used in the load_readers() function.

Try to check if all your functions have the self keyword

HOW I CALLED THE CLASS

if __name__ == '__main__':
        person = Person("Foo", 1222, "READER")
        print(person.load_readers())

So instead of using the class name, first initiate the class object to call the function.