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Python Classes, Objects and Special Methods, what does print(l.x, l.y) print after class Lazy?
Block 3, Classes, Objects and Special Methods, question 5 of the Python sample
class Lazy:
def __getattr__(self, name):
return f'missing:{name}'
l = Lazy()
l.x = 1
print(l.x, l.y)What does this print?
Show the answer and why each option is right or wrong
Answer, B. 1 missing:y
- A. 1 1
- Assumes the fallback method also somehow affects the value of x, making both reads return the same number. x was assigned the value 1 directly and is read back unchanged, y is the only name missing from the instance.
- B. 1 missing:y
- Correct. __getattr__ only runs for attributes that normal lookup fails to find, so the explicitly set x is returned directly and only y triggers the fallback.
- C. AttributeError
- Assumes accessing an attribute that does not exist on the instance always raises, even when __getattr__ is defined. Defining __getattr__ specifically replaces the usual AttributeError with whatever the method decides to return.
- D. missing:x missing:y
- Assumes __getattr__ intercepts every attribute access, even ones that were explicitly set. __getattr__ only fires when normal lookup fails to find the attribute, x was set directly on the instance so it is found without ever calling __getattr__.
__getattr__ is only called as a fallback, after Python has already checked the instance dictionary and the class hierarchy and found nothing. Attributes that already exist, like an explicitly assigned x, are returned by normal lookup and never reach __getattr__ at all.
The answer above is what the CPython interpreter printed when the snippet was run, not what someone expected it to do.
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