Pretty Damn awful, PyEVM Virtual Machine debugger. Using a custom opdcode set in Vyper ;)

pre_vdb_ipdb.py

def debug_opcode(computation):
    print('YOUR ARE HERE!')
    import ipdb; ipdb.set_trace()
    
import evm
from evm.vm.opcode import as_opcode
from vyper.opcodes import opcodes as vyper_opcodes
from evm.vm.forks.byzantium.computation import ByzantiumComputation
opcodes = ByzantiumComputation.opcodes.copy()
opcodes[vyper_opcodes['DEBUG'][0]] = as_opcode(
    logic_fn=debug_opcode,
    mnemonic="DEBUG",
    gas_cost=0
)
setattr(evm.vm.forks.byzantium.computation.ByzantiumComputation, 'opcodes', opcodes)

How to use it in contracts?

Checkout https://github.com/status-im/vyper-debug, has a vyper-run command wich one can use to run a contract.
One just places vdb in the contract to create a breakpoint.

This works only with Vyper? I use PyEVM to test Solidity contracts. Looks like your snipped is related only to PyEVM, and not Vyper. Maybe combination of your snippet with inline assembler with this instruction could work?

Yes this is for vyper, for solidity checkout https://github.com/iamdefinitelyahuman/brownie - has pretty good debugger that uses PyEVM underlying.
The above snippet uses a custom opcode that the vyper compiler injects into the bytecode - which was a quick hack, in favour of using the program counter.

Thanks for replies. I will check it out.