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?
__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
# in python 3:
# class MyClass(metaclass=FooType):
# pass
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.