How to watch for a variable change in python without dunder setattr or pdb

Question

There is large python project where one attribute of one class just have wrong value in some place.

It should be sqlalchemy.orm.attributes.InstrumentedAttribute, but

Answer 1

A simpler way to watch for an object's attribute change (which can also be a module-level variable or anything accessible with getattr) would be to leverage hunter library, a flexible code tracing toolkit. To detect state changes we need a predicate which can look like the following:

import traceback


class MutationWatcher:

    def __init__(self, target, attrs):
        self.target = target
        self.state = {k: getattr(target, k) for k in attrs}

    def __call__(self, event):
        result = False
        for k, v in self.state.items():
            current_value = getattr(self.target, k)
            if v != current_value:
                result = True
                self.state[k] = current_value
                print('Value of attribute {} has chaned from {!r} to {!r}'.format(
                    k, v, current_value))

        if result:
            traceback.print_stack(event.frame)

        return result

Then given a sample code:

class TargetThatChangesWeirdly:
    attr_name = 1


def some_nested_function_that_does_the_nasty_mutation(obj):
    obj.attr_name = 2


def some_public_api(obj):
    some_nested_function_that_does_the_nasty_mutation(obj)

We can instrument it with hunter like:

# or any other entry point that calls the public API of interest
if __name__ == '__main__':
    obj = TargetThatChangesWeirdly()

    import hunter
    watcher = MutationWatcher(obj, ['attr_name'])
    hunter.trace(watcher, stdlib=False, action=hunter.CodePrinter)

    some_public_api(obj)

Running the module produces:

Value of attribute attr_name has chaned from 1 to 2
  File "test.py", line 44, in 
    some_public_api(obj)
  File "test.py", line 10, in some_public_api
    some_nested_function_that_does_the_nasty_mutation(obj)
  File "test.py", line 6, in some_nested_function_that_does_the_nasty_mutation
    obj.attr_name = 2
                                 test.py:6     return        obj.attr_name = 2
                                               ...       return value: None

You can also use other actions that hunter supports. For instance, Debugger which breaks into pdb (debugger on an attribute change).