Track a value throughout a program

track.py

import sys
from bdb import Bdb
from traceback import print_stack


class Tracker(Bdb):
    """
    Debugger which looks for a given value by identity in local variables of all frames
    it comes across. When it finds it and the given condition function called with the value
    is true, prints out a stack trace and stops tracing.
    
    Usage example:
    
        Tracker(x, lambda: x.y == 0).set_trace()
    """
    def __init__(self, value, condition):
        super().__init__()
        self.condition = condition
        self.value = value

    def do_clear(self, arg):
        pass

    def user_line(self, frame):
        for name, v in frame.f_locals.items():
            if v is self.value:
                break
        else:
            return
        if self.condition():
            print('Variable:', name, file=sys.stderr)
            print_stack(f=frame)
            sys.settrace(None)