Class instance throws exception when __getattr__ is added

[kc64]

I'm trying to evaluate the best way to write surrogate classes in CPython that then act on the uPython target. When I added getattr to trap unknown attribute access, belay threw an exception. I'm not able to determine what I broke when I did this. Has anyone seen this before?

The exception:

Traceback (most recent call last):
  File "C:\My Software\x\src\x\x.py", line 127, in <module>
    x= X('COM5')
  File "C:\My Software\x\src\x\x.py", line 34, in __init__
    super().__init__(*args, **kwargs)
  File "C:\My Software\x\.venv\lib\site-packages\belay\device.py", line 152, in __init__
    executer_name = metadata.executer.__registry__.name
AttributeError: 'NoneType' object has no attribute 'executer'

Sample code:

from belay import Device
class Y:
    ...
class X(Device):
    def __init__(self, *args, **kwargs)
        super().__init__(*args, **kwargs)
    y = Y()
x = X('COM5')

If Y has a __getattr__ method, the exception occurs.

Maybe the premise of the exercise is faulty. What other abstraction techniques are being used?

[BrianPugh]

hmmm, can you post a complete working example?

[kc64]

from belay import Device

class Y:
    def __init__(self):
        print('got here')
    def __getattr__(self, name):    # take this out,
        print(name)                 # and no traceback

class X(Device):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
    y = Y()

x = X('COM33')

The traceback:

got here
__belay__
__isabstractmethod__
__belay__
Traceback (most recent call last):
  File "C:\My Software\aim2\src\aim\x.py", line 15, in <module>
    x = X('COM33')
  File "C:\My Software\aim2\src\aim\x.py", line 12, in __init__
    super().__init__(*args, **kwargs)
  File "C:\My Software\aim2\.venv\lib\site-packages\belay\device.py", line 152, in __init__
    executer_name = metadata.executer.__registry__.name
AttributeError: 'NoneType' object has no attribute 'executer'

[BrianPugh]

So here's what's happening:

1. In Device.__init__, belay scans all attributes of the current class for @Device.task (or similar) decorators. These decorators put a "secret" attribute __belay__ onto the decorated function. So, for example, consider the following declaration:

   class Foo(Device):
       @Device.task
       def bar(self):
           pass

   The @Device.task decorator stores data at Foo.bar.__belay__.
2. When the __belay__ field cannot be accessed, Device.__init__ will skip attempting to handle that attribute.
3. In your example, we try to access X.y.__belay__. This doesn't exist, but your Y.__getattr__ handles that case. However, the returned value (in this case, None) isn't what Belay code expects (we expect a belay.device_support.MethodMetadata class).

Does this make sense?

[kc64]

It makes perfect sense. A couple questions.

1. I'm only ever using the call method of the device and there are no decorators. Why does the belay scanner still introspect my X.y instance for __belay__?
2. What workaround should I use to make the scanner pass over such occurrences?

[kc64]

I added this and it skips over. Many thanks for the excellent guidance.

def __getattr__(self, name):
    if 'belay' in name:
        raise AttributeError
    ...

[BrianPugh]

glad you got it working!

1. I'm only ever using the call method of the device and there are no decorators. Why does the belay scanner still introspect my X.y instance for __belay__?

The class can only detect the decorations via the secret __belay__ attribute, which is why it has to scan through all attributes. I do wonder if I change this code to instead do a hasattr check (instead of catching the AttributeError) if it would play nicer to your code (and I think be more correct). I'll try and give that a shot when I get a chance!

[BrianPugh]

So I don't think I can "fix" this on my side, hasattr really just calls getattr, which is exactly where we are at right now.