__getattr__ method in python

Question

i meet this code today, and it seems confusing

class ClassOne:
    def __init__(self,some_object):
        self.not_important_attribute=some_object   
class ClassTwo:
    def __init__(self,some_object):
        self.tmp=ClassOne(some_object)
    def __getattr__(self,attr):
        return getattr(self.tmp,attr)
a=ClassTwo('not_important_string')
print(getattr(a,'undefined_attribute',3))

When we use getattr at the last line we trigger the __getattr__ method in the SubClass , then delegate to the getattr(self.tmp,attr) function which will raise an exception if the attribute is undefined and no default value was given. But how the value 3 at the last line still go through all the process, and eventually return to getattr(a,'undefined_attribute',3) function ?. Because we didn't have a slot for the default value when we delegate getattr(self.tmp,attr), how is that possible ?

Answer 1

In your case

getattr(self.tmp,attr)

raises AttributeError and if getattr has third argument(default value) then it return default value instead of raising AttributeError

Answer 2

Step by step why 3 value is shown:

the super class has has an atribute not_important_attribute and it is set when the constructor is called

class ClassOne:
    def __init__(self,some_object):
        self.not_important_attribute=some_object   

Here, in the ClassTwo contructor, you create an instance of ClassOne and save it into tmp variable. Meaning that when you overrides __getattr__ you will be asking for the value of attribute of ClassOne

print(getattr(a,'not_important_attribute',3))
not_important_string # founds the method
print(getattr(a,'any',3))
3 #doesn't found method, returns default

That is the same of directly do:

b = ClassOne("not_important_string")
print(getattr(b,'not_important_attribute',3))
not_important_string # founds the method
print(getattr(b,'any',3))
3 # doesn't found method, returns default