Understanding Meta classes in python

Question

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

i have a Base and Derived 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 extending Base class, must need to implement bar() method, so i created the meta class to hook the constuction of derived class,so now Base class looks like follow with BaseMeta 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 for Base and Derived 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 since Base not have bar, but i want to check only in the Derived class so i rewrite my metaclass 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 checking name != Base in my __new__ method.

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

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. — Arne
– Arne, Commented Jul 18, 2018 at 7:34
Theoretically, I am sure. But metaclasses are intended to achieve something different called class 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 that 99% 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. — Arne
– Arne, Commented Jul 18, 2018 at 7:58
That looks interesting, I'll try to have a look and get back if it turns out that I misunderstood your question. — Arne
– Arne, Commented Jul 18, 2018 at 8:28
Note that you could implement __init_subclass__ to verify assertions about a child without defining a metaclass. — chepner
– chepner, Commented Jul 20, 2018 at 23:18

Thtu · Accepted Answer · 2018-07-20 22:55:38Z

2

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

import abc

class Base(abc.ABC):

    @abc.abstractmethod
    def bar(self):
         pass

class 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 a bar method on your Base class that raises a NotImplementedError. The main difference is that no error is raised until you actually call something that requires bar. e.g.

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

    def bar(self):
        raise NotImplementedError

edited Jul 20, 2018 at 22:55

answered Jul 18, 2018 at 7:27

Thtu

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2 Comments

Hari Over a year ago

I want to enforce constraint for derived class without implementing bar method in my Base class, and i am confused about example 1, i think it enforce foo right? but i need to enforce bar()

Thtu Over a year ago

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

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