I have a class like:
class MyClass: Foo = 1 Bar = 2
Whenever MyClass.Foo
or MyClass.Bar
is invoked, I need a custom method to be invoked before the value is returned. Is it possible in Python? I know it is possible if I create an instance of the class and I can define my own __getattr__
method. But my scnenario involves using this class as such without creating any instance of it.
Also I need a custom __str__
method to be invoked when str(MyClass.Foo)
is invoked. Does Python provide such an option?
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Answer
__getattr__()
and __str__()
for an object are found on its class, so if you want to customize those things for a class, you need the class-of-a-class. A metaclass.
class FooType(type): def _foo_func(cls): return 'foo!' def _bar_func(cls): return 'bar!' def __getattr__(cls, key): if key == 'Foo': return cls._foo_func() elif key == 'Bar': return cls._bar_func() raise AttributeError(key) def __str__(cls): return 'custom str for %s' % (cls.__name__,) class MyClass(metaclass=FooType): pass # # in python 2: # class MyClass: # __metaclass__ = FooType print(MyClass.Foo) print(MyClass.Bar) print(str(MyClass))
printing:
foo! bar! custom str for MyClass
And no, an object can’t intercept a request for a stringifying one of its attributes. The object returned for the attribute must define its own __str__()
behavior.
Updated 2023-02-20 for Python 3.x default implementation (python 2 as a comment).