Understanding Meta classes in python

Question 1

I am trying to learnmetaclasses in python, from my research i found a example like follow.

i have aBase andDerived classes like follow

class Base():    def foo(self):        return self.bar() class Derived(Base):    def foo2(self):        return "i am foo2"

now, when i want to make sure that whoever extendingBase class, must need to implementbar() method, so i created the meta class to hook the constuction of derived class,so nowBase class looks like follow withBaseMeta meta class.

class BaseMeta(type):  def __new__(cls, name, bases, body):    if not "bar" in body:      raise TypeError("bar not implemented")    return super().__new__(cls, name, bases, body)class Base(metaclass=BaseMeta):   def foo(self):        return self.bar()

The problem is when i get looks into body it returns 2 records forBase andDerived class, like follow.

 {'__module__': '__main__', '__qualname__': 'Base', 'foo': <function  Base.foo at 0x7ffbaae436a8>} {'__module__': '__main__', '__qualname__': 'Derived', 'bar': <function  Derived.bar at 0x7ffbaae437b8>}

my code in__new__ breaks sinceBase not havebar, but i want to check only in the Derived class so i rewrite mymetaclass like follow.

def __new__(cls, name, bases, body):    if name !="Base" and not "bar" in body:      raise TypeError("bar not implemented")    return super().__new__(cls, name, bases, body)

I am checkingname != Base in my__new__ method.

Is that the right way to do it or we can use some other best way?

Question 2

fyi, what you are talking about is not a metaclass but a baseclass.Metaclass has a very specific meaning and refers to something completely different.

Question 3

We can also usemetaclass to achieve this right?

Question 4

Theoretically, I am sure. But metaclasses are intended to achieve something different calledclass alteration. Sorry that I am so vague, but I don't understand their use very well myself and just trust the top-voted answer from the question I linked which says that99% of the time you need class alteration, you are better off using [monkey patching or decorators]. But 98% of the time, you don't need class alteration at all and go about my day ignoring that metaclasses exist.

Question 5

That looks interesting, I'll try to have a look and get back if it turns out that I misunderstood your question.

Question 6

Note that you could implement__init_subclass__ to verify assertions about a child without defining a metaclass.

Question 7

You can use theabc module in the stdlib, which has tools for doing exactly this.https://docs.python.org/3/library/abc.html

import abcclass Base(abc.ABC):    @abc.abstractmethod    def bar(self):         passclass Derived(Base):    pass# This will raise an error because foo is not implemented# >>> Derived() # TypeError: Can't instantiate abstract class Derived with abstract methods bar

Another strategy would be to have abar method on your Base class that raises a NotImplementedError. The main difference is that no error is raised until you actually call something that requiresbar. e.g.

class Base():    def foo(self):        return self.bar()     def bar(self):        raise NotImplementedError

Question 8

I want to enforce constraint forderived class without implementingbar method in myBase class, and i am confused about example 1, i think it enforcefoo right? but i need to enforcebar()

Question 9

To enforcebar for a derived class, declare an abstract method on your base class for that method. Edited response to reflect this.

Thtu 2,03217 silver badges21 bronze badges · Accepted Answer · 2018-07-20 22:55:38Z

You can use theabc module in the stdlib, which has tools for doing exactly this.https://docs.python.org/3/library/abc.html

import abcclass Base(abc.ABC):    @abc.abstractmethod    def bar(self):         passclass Derived(Base):    pass# This will raise an error because foo is not implemented# >>> Derived() # TypeError: Can't instantiate abstract class Derived with abstract methods bar

Another strategy would be to have abar method on your Base class that raises a NotImplementedError. The main difference is that no error is raised until you actually call something that requiresbar. e.g.

class Base():    def foo(self):        return self.bar()     def bar(self):        raise NotImplementedError

I want to enforce constraint forderived class without implementingbar method in myBase class, and i am confused about example 1, i think it enforcefoo right? but i need to enforcebar()
To enforcebar for a derived class, declare an abstract method on your base class for that method. Edited response to reflect this.

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Understanding Meta classes in python

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