Zero to Py by Michael Green

Author:Michael Green [Michael Green]
Language: eng
Format: epub
Publisher: leanpub.com
Published: 2023-02-20T00:00:00+00:00

Metaclasses can also be used to enforce from the library constraints or expectations on user code. Consider another example where a library expects users to define certain methods on a derived classes. You can use a metaclass to catch type errors at the class instantiation, instead of later during runtime.

1 >>> class LibraryMetaclass(type): 2 ... def __new__(cls, name, bases, attrs): 3 ... if "my_method" not in attrs: 4 ... raise AttributeError( 5 ... "derived class must define method my_method()" 6 ... ) 7 ... return type.__new__(cls, name, bases, attrs) 8 ... 9 >>> class BaseClass(metaclass=LibraryMetaclass): 10 ... my_method = None 11 ... 12 >>> class DerivedClass(BaseClass): 13 ... def my_method(self): 14 ... pass 15 ... 16 >>> class BadClass(BaseClass): 17 ... pass 18 Traceback (most recent call last): 19 File "<stdin>", line 1, in <module> 20 File "<stdin>", line 4, in __new__ 21 AttributeError: derived class must define method my_method()

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