How to Avoid Explicit 'Self' in Python

How to avoid explicit 'self' in Python?

In Java terms: Python doesn't have member functions, all class functions are static, and are called with a reference to the actual class instance as first argument when invoked as member function.

This means that when your code has a class MyClass and you build an instance m = MyClass(), calling m.do_something() will be executed as MyClass.do_something(m).

Also note that this first argument can technically be called anything you want, but the convention is to use self, and you should stick to that convention if you want others (including your future self) to be able to easily read your code.

The result is there's never any confusion over what's a member and what's not, even without the full class definition visible. This leads to useful properties, such as: you can't add members which accidentally shadow non-members and thereby break code.

One extreme example: you can write a class without any knowledge of what base classes it might have, and always know whether you are accessing a member or not:

class A(some_function()):
  def f(self):
    self.member = 42
    self.method()

That's the complete code! (some_function returns the type used as a base.)

Another, where the methods of a class are dynamically composed:

class B(object):
  pass

print B()
# <__main__.B object at 0xb7e4082c>

def B_init(self):
  self.answer = 42
def B_str(self):
  return "<The answer is %s.>" % self.answer
# notice these functions require no knowledge of the actual class
# how hard are they to read and realize that "members" are used?

B.__init__ = B_init
B.__str__ = B_str

print B()
# <The answer is 42.>

Remember, both of these examples are extreme and you won't see them every day, nor am I suggesting you should often write code like this, but they do clearly show aspects of self being explicitly required.

How do I avoid the self.x = x; self.y = y; self.z = z pattern in __init__?

Edit:
If you have python 3.7+ just use dataclasses

A decorator solution that keeps the signature:

import decorator
import inspect
import sys

@decorator.decorator
def simple_init(func, self, *args, **kws):
    """
    @simple_init
    def __init__(self,a,b,...,z)
        dosomething()

    behaves like

    def __init__(self,a,b,...,z)
        self.a = a
        self.b = b
        ...
        self.z = z
        dosomething()
    """

    #init_argumentnames_without_self = ['a','b',...,'z']
    if sys.version_info.major == 2:
        init_argumentnames_without_self = inspect.getargspec(func).args[1:]
    else:
        init_argumentnames_without_self = tuple(inspect.signature(func).parameters.keys())[1:]

    positional_values = args
    keyword_values_in_correct_order = tuple(kws[key] for key in init_argumentnames_without_self if key in kws)
    attribute_values = positional_values + keyword_values_in_correct_order

    for attribute_name,attribute_value in zip(init_argumentnames_without_self,attribute_values):
        setattr(self,attribute_name,attribute_value)

    # call the original __init__
    func(self, *args, **kws)

class Test():
    @simple_init
    def __init__(self,a,b,c,d=4):
        print(self.a,self.b,self.c,self.d)

#prints 1 3 2 4
t = Test(1,c=2,b=3)
#keeps signature
#prints ['self', 'a', 'b', 'c', 'd']
if sys.version_info.major == 2:
    print(inspect.getargspec(Test.__init__).args)
else:
    print(inspect.signature(Test.__init__))

In python, when self can be omitted?

To make a long story short: you MUST use self. whenever you want to access an attribute (data attribute, property or method) of the current instance.

For a somewhat more detailed answer: when using def in a class statement, what you create is not a "method" but a plain function. When looked up a class or instance attribute (Duck.about or duck.about) this function will be wrapped (together with the class and instance on which it's looked up) in a callable method object that will take care of automagically inject the instance (or class) object as first parameter to the function call. You can read the whole details here : https://wiki.python.org/moin/FromFunctionToMethod

As other already mentionned, your code snippet only accidentally "works" because it ends up looking global names that happens to be defined, and breaks as soon as these names are not defined:

# tail = Tail('long')
# bill = Bill('wide orange') 
duck = Duck(Bill('wide orange'), Tail('long'))
duck.about()

=> crashes with a NameError

You'd also get unexpected results if rebinding these global names ie:

tail = Tail('long')
bill = Bill('wide orange') 
duck1 = Duck(bill, tail)

tail = Tail('short')
bill = Bill('norvegian blue') 
duck2 = Duck(bill, tail)

duck1.about()

=> Duh, why does it prints "short norvegian blue" ???

Why do you need explicitly have the self argument in a Python method?

I like to quote Peters' Zen of Python. "Explicit is better than implicit."

In Java and C++, 'this.' can be deduced, except when you have variable names that make it impossible to deduce. So you sometimes need it and sometimes don't.

Python elects to make things like this explicit rather than based on a rule.

Additionally, since nothing is implied or assumed, parts of the implementation are exposed. self.__class__, self.__dict__ and other "internal" structures are available in an obvious way.

Python class methods: when is self not needed

What is self?

In Python, every normal method is forced to accept a parameter commonly named self. This is an instance of class - an object. This is how Python methods interact with a class's state.

You are allowed to rename this parameter whatever you please. but it will always have the same value:

>>> class Class:
    def method(foo): # 
        print(foo)

        
>>> cls = Class()
>>> cls.method()
<__main__.F object at 0x03E41D90>
>>> 

But then why does my example work?

However, what you are probably confused about is how this code works differently:

>>> class Class:
    def method(foo):
        print(foo)

    methods = {'method': method}
    
    def __init__(self):
        self.run = self.methods['method']

        
>>> cls = Class()
>>> cls.run(3)
3
>>> 

This is because of the distinction between bound, and unbound methods in Python.

When we do this in __init__():

self.run = self.methods['method']

We are referring to the unbound method method. That means that our reference to method is not bound to any specific instance of Class, and thus, Python will not force method to accept an object instance. because it does not have one to give.

The above code would be the same as doing this:

>>> class Class:
    def method(foo):
        print(foo)

        
>>> Class.method(3)
3
>>> 

In both examples, we are calling the method method of the class object Class , and not an instance of the Class object.

We can further see this distinction by examining the repr for a bound and unbound method:

>>> class Class:
    def method(foo):
        print(foo)

        
>>> Class.method
<function Class.method at 0x03E43D68>
>>> cls = Class()
>>> cls.method
<bound method Class.method of <__main__.Class object at 0x03BD2FB0>>
>>> 

As you can see, in the first example when we do Class.method, Python shows:
<function Class.method at 0x03E43D68>. I've lied to you a little bit. When we have an unbound method of a class, Python treats them as plain functions. So method is simply a function that is not bound to any instance of `Class.

However in the second example, when we create an instance of Class, and then access the method object of it, we see printed: <bound method Class.method of <__main__.Class object at 0x03BD2FB0>>.

The key part to notice is bound method Class.method. That means method is **bound** to cls - a specfic an instance of Class.

General remarks

As @jonshapre mentioned, writing code like in your example leads to confusion (as proof by this question), and bugs. It would be a better idea if you simply defined nonLinearBipolarStep() outside of Activation, and reference that from inside of Activation.activation_functions:

def nonLinearBipolarStep(self,x,string=None):
        if not string: return (-1 if x<0 else 1 )
        else: return ('-' if x<0 else '1')

class Activation:

    activation_functions = {
        'bipolar': nonLinearBipolarStep,
    }

    ...

I guess a more specific question would be: what should I pay attention to on that code in order to become evident that ag.run(x) would be a call to an unbound function?

If you'd still like to let nonLinearBipolarStep be unbound, then I recommend simply being carefully. If you think your method would make for the cleanest code then go for it, but make sure you know what you are doing and the behavior your code will have.

If you still wanted to make is clear to users of your class that ag.run() would be static, you could document it in a docstring somewhere, but that is something the user really shouldn't even have to be concerned with at all.

What is the purpose of the `self` parameter? Why is it needed?

The reason you need to use self. is because Python does not use special syntax to refer to instance attributes. Python decided to do methods in a way that makes the instance to which the method belongs be passed automatically, but not received automatically: the first parameter of methods is the instance the method is called on. That makes methods entirely the same as functions, and leaves the actual name to use up to you (although self is the convention, and people will generally frown at you when you use something else.) self is not special to the code, it's just another object.

Python could have done something else to distinguish normal names from attributes -- special syntax like Ruby has, or requiring declarations like C++ and Java do, or perhaps something yet more different -- but it didn't. Python's all for making things explicit, making it obvious what's what, and although it doesn't do it entirely everywhere, it does do it for instance attributes. That's why assigning to an instance attribute needs to know what instance to assign to, and that's why it needs self..

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