Do not raise when an instance already exists (fixes issue #607)

[souliane]

Do not raise when an instance already exists (fixes issue #607)

I assume that this is a dirty fix, yet it works for me.
Please let me know what is the good way to fix the issue, I would like to tackle it.
How to reproduce is described here.

[inducer]

The problem with this is that various parts of the code assume that only one of these objects ever exists, e.g. here:

pudb/pudb/__init__.py

Lines 74 to 90 in b9c1948

	def _get_debugger(**kwargs):
	from pudb.debugger import Debugger
	if not Debugger._current_debugger:
	tty_path = _tty_override()
	if tty_path and ("stdin" not in kwargs or "stdout" not in kwargs):
	tty_file, term_size = _open_tty(tty_path)
	kwargs.setdefault("stdin", tty_file)
	kwargs.setdefault("stdout", tty_file)
	kwargs.setdefault("term_size", term_size)
	tty_file.close()
	from pudb.debugger import Debugger
	dbg = Debugger(**kwargs)
	return dbg
	else:
	return Debugger._current_debugger[0]

[souliane]

I see... can you think of way to fix the issue without messing with those parts of the code? Maybe we can destroy the pudb instance when c is pressed?

[asmeurer]

This seems like the wrong fix. If the debugger is already running then further breakpoints should just break the debugger. If that doesn't happen we should try to investigate why. Having multiple debugger classes implies recursive debugging, which sounds really complicated and probably not worth supporting.

[hwalinga]

@asmeurer Hello

I did an analysis of this. If you use pytest --pdbcls pudb.debugger:Debugger --pdb --capture=no test.py, pytest instantiates a new Debugger class (1) whenever a new breakpoint is encountered (!! with PYTHONBREAKPOINT not set !!) and (2) on post-mortem analysis.

Making Debugger a singleton using metaclasses should solve this. HOWEVER, an old bug from #52 resurfaced then. I tried to fix this in #667

What is more, pytest-pudb also has exhibits the bug in #52 when using below code:

def test_foo():
    breakpoint()
    raise Exception