A context manager that can skip its body.

unpack.py

import sys
import inspect

class _UnpackMarker(Exception): pass

class unpack:
	def __init__(self, pred):
		self.pred = pred
	
	def __enter__(self):
		if self.pred:
			return self.pred
		# else skip the with block’s body
		sys.settrace(lambda *args, **kw: None)
		frame = inspect.currentframe().f_back
		frame.f_trace = self.trace
	
	def trace(self, frame, event, arg):
		raise _UnpackMarker
	
	def __exit__(self, type, value, traceback):
		if type is _UnpackMarker:
			return True  # suppress the exception

if __name__ == '__main__':
	with unpack(True) as result:
		assert result is True
		print('This should print')
	
	with unpack(False) as result:
		print('This should not print')
	
	try:
		with unpack(True) as result:
			raise RuntimeError('This should raise')
	except RuntimeError as e:
		print(str(e))
	else:
		print('This should not display')
	
	try:
		raise RuntimeError('This should also raise')
	except RuntimeError as e:
		print(str(e))
	else:
		print('This should also not display')