Specialized variable watcher

[ionelmc]

There's VarsPrinter and VarsSnooper but they can't handle well this usecase: find out when a specific field or variable changes, perhaps even to a certain value.

Some ideas for a predicate:

Watcher(attr="foobar", type="MyClass", value=lambda value: value.startswith("stuff"))
Watcher(name="self", value=lambda value: value.field.startswith("stuff"))
Watcher(name="self", attr="field", type="MyClass", value=lambda value: value.startswith("stuff"))
Watcher(name=lambda name: name in ["self", "instance", "obj"], attr="field", type="MyClass", value=lambda value: value.startswith("stuff"))

Usecases like this one https://stackoverflow.com/a/54536941/23658 could be supported like this:

inst = ...

trace(Watcher(object=inst, attr="field", type="MyClass", value=lambda value: value.startswith("stuff")))

inst.field = "stuff123"

All the options and alternative names:

• name, variable
• attr, field
• type, klass
• value
• object, instance, inst

There is a question of disallowing name and object at the same time. Perhaps allow it but make it mean "if object is object and variable name is name".

Another problem is if type is a string or the actual type.

[ionelmc]

Idea 1 - implement an ID tracker (don't poke around in objects):

• name - state[event.code, name] = id(event.locals[name])
• name + attr - state[event.code, name, attr] = id(event.locals[name].__dict__[attr])
• klass + attr - for name in event.locals: if type(event.locals[name]).__name__ == klass: state[klass, attr] = id(event.locals[name].__dict__[attr])
• klass - for name in event.locals: state[event.code, name] = id(event.locals[name])
• inst + attr - state[id(inst), attr] = id(inst.__dict__[attr])
• value - doesn't affect storage

Trigger condition: state[<criteria>] != new_id and (value(new_value) if callable(value) else value == new_value). Need to note that value == new_value is a potentially dangerous side-effect incurring operation.