The problem may still be reproduced by running the attached pdb_refleak.py script. Enter three times 'c' (short for the pdb 'continue' command) and the following line is printed:

    pdb 3 -> pdb 2 -> pdb 1

This shows that the third pdb instance 'pdb 3', owns indirectly a reference to the second and first pdb instances. As the _signal extension module owns a reference to 'pdb 3' through its sigint_handler() method, then it also owns indirectly a reference to 'pdb 1'.