217. Metaclass Conflict Resolution

🔹 1. Understanding Metaclass Conflict

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class MetaA(type):
    pass

class MetaB(type):
    pass

class A(metaclass=MetaA):
    pass

class B(metaclass=MetaB):
    pass

# Trying to inherit from both A and B causes a metaclass conflict
class C(A, B):
    pass  # ❌ TypeError: metaclass conflict

🔍 Issue: Python does not know whether to use MetaA or MetaB.

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🔹 2. Using a Common Metaclass

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class MetaCommon(type):
    pass

class A(metaclass=MetaCommon):
    pass

class B(metaclass=MetaCommon):
    pass

# No conflict since both use MetaCommon
class C(A, B):
    pass

✅ Fix: Ensure all base classes use the same metaclass.

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🔹 3. Manually Defining a Compatible Metaclass

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class MetaA(type):
    pass

class MetaB(type):
    pass

# Custom metaclass that resolves conflicts
class MetaC(MetaA, MetaB):
    pass

class A(metaclass=MetaA):
    pass

class B(metaclass=MetaB):
    pass

class C(A, B, metaclass=MetaC):  # Explicitly specify the new metaclass
    pass

✅ Fix: Create a new metaclass inheriting from both conflicting ones.

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🔹 4. Using type to Dynamically Resolve Conflict

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class MetaA(type):
    pass

class MetaB(type):
    pass

class A(metaclass=MetaA):
    pass

class B(metaclass=MetaB):
    pass

# Dynamically resolve metaclass conflict
MetaC = type('MetaC', (MetaA, MetaB), {})

class C(A, B, metaclass=MetaC):
    pass

✅ Fix: Use type() to generate a metaclass dynamically.

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🔹 5. Using __new__ to Merge Metaclasses Dynamically

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def metaclass_resolver(*metaclasses):
    class MetaResolved(*metaclasses):
        pass
    return MetaResolved

class MetaA(type):
    pass

class MetaB(type):
    pass

class A(metaclass=MetaA):
    pass

class B(metaclass=MetaB):
    pass

class C(A, B, metaclass=metaclass_resolver(MetaA, MetaB)):
    pass

✅ Fix: Use a function to dynamically merge metaclasses.

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🔹 6. Using ABCMeta for Compatibility

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from abc import ABCMeta

class A(metaclass=ABCMeta):
    pass

class B(metaclass=ABCMeta):
    pass

class C(A, B):
    pass  # No conflict because ABCMeta is a common metaclass

✅ Fix: Use ABCMeta (from abc module) as a common metaclass.

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🔹 7. Enforcing a Single Metaclass with __metaclass__ (Python 2)

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class MetaBase(type):
    pass

class Base(object):
    __metaclass__ = MetaBase  # Works in Python 2

✅ Fix: In Python 2, explicitly set __metaclass__.

(Not needed in Python 3, where metaclass= is used inside class definitions.)

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🔹 8. Using six.with_metaclass for Python 2 & 3 Compatibility

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import six

class MetaA(type):
    pass

class BaseA(six.with_metaclass(MetaA, object)):
    pass

class BaseB(six.with_metaclass(MetaA, object)):  # Same metaclass
    pass

class C(BaseA, BaseB):
    pass

✅ Fix: Use six.with_metaclass for Python 2 & 3 compatibility.

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🔹 9. Implementing __prepare__ to Control Metaclass Behavior

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class MetaA(type):
    @classmethod
    def __prepare__(cls, name, bases, **kwargs):
        print(f"Preparing {name}")
        return super().__prepare__(name, bases, **kwargs)

class A(metaclass=MetaA):
    pass

class B(metaclass=MetaA):
    pass

class C(A, B):
    pass  # No conflict since both use MetaA

✅ Fix: __prepare__ allows fine-grained control over metaclass creation.

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🔹 10. Using __mro_entries__ to Adjust Class Resolution

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class MetaA(type):
    pass

class MetaB(type):
    pass

class A(metaclass=MetaA):
    pass

class B(metaclass=MetaB):
    def __mro_entries__(self, bases):
        return (A,)  # Redirects MRO resolution to avoid conflict

class C(A, B):
    pass

✅ Fix: Use __mro_entries__ (Python 3.6+) to adjust method resolution order (MRO) dynamically.

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🚀 Summary: Strategies for Resolving Metaclass Conflicts

Method

When to Use?

Use a Common Metaclass

When all classes can share the same metaclass.

Create a New Metaclass

When conflicting metaclasses exist.

Use type() to Merge Metaclasses

When resolving conflicts dynamically.

Use ABCMeta

When using abstract base classes.

Use six.with_metaclass

For Python 2 & 3 compatibility.

Override __mro_entries__

When fine-grained MRO control is needed.

Use __prepare__ in Metaclasses

When you need metaclass customization.