`From` predicate does not use watermark argument correctly

[davidkhess]

Unless I'm missing something there's a bug at https://github.com/ionelmc/python-hunter/blob/master/src/hunter/predicates.py#L389C1-L390C1

            if delta_depth < self.watermark:

I believe the polarity of this check is reversed and should be:

            if delta_depth > self.watermark:

As it is, for any positive value of watermark, the From will correctly trigger on condition but on the next event it will immediately disable regardless of the actual depth requested by watermark.

[ionelmc]

I am not sure why you would want that. The watermark is supposed to "drop" events that would have the delta less than the watermark. It's more of a fine tuning mechanism in case this predicate would get events with negative deltas (it can happen in some strange situations).

If you invert that check you essentially turn it into a depth limit (eg: drop events that have delta too big - and for that you can just add a Q(depth_lt=123) in the predicate).

[davidkhess]

That explanation makes sense and fills in the gaps in the documentation for watermark.

It's just that in my usage, with the default value of 0, the From fires once and then immediately disables.

It might help to have a cookbook section that covers how to properly use a From?

[ionelmc]

Here's an example:

From(Q(function="render",module="rest_framework.renderers"),Q(stdlib=0,depth_lt=4))

This will start producing events whenever the interpreter calls any render function from rest_framework.renderers up to a max call depth of 4 while ignoring stdlib stuff. Basically it's a "show me what is going in those render methods, but don't show me too much".

[davidkhess]

I was using kind="call" on the condition as that seems to make more sense for a "From" concept especially since the implementation seems to have some sort of logic to stay active based on stack depth.

Seems it needs to be a predicate that keeps matching continuously in that region of code. Thanks for the example.