Defying the rejection of PEP 377

conditional_with.py

import sys

def dummy_trace(frame, event, *args):
	pass

class CancelExecutionException(Exception):
	pass


class alternative_execution:
	def __init__(self, demo_skip):
		self.demo_skip = demo_skip


	def __enter__(self):
		if self.demo_skip:
			f = sys._getframe(1)

			#Enable fine grained tracing but tracing function will throw an exception
				#First we must initialize the tracing machinery, we do this by installing a dummy function that will just uninstall itself each call (no return value)
				#see https://docs.python.org/3/library/sys.html?highlight=fine-grained#sys.settrace for more information about this.

			sys.settrace(dummy_trace)
			f.f_trace = self.cancel_execution
			f.f_trace_opcodes = True

	def cancel_execution(self, frame, event, *args):
		raise CancelExecutionException()

	def __exit__(self, et, ev, tb):
		f = sys._getframe(1)

		#If the error type is CancelExecutionException we return True to suppress pending exceptions
		if et is CancelExecutionException:
			return True



#Demonstration

with alternative_execution(False):
	print('This statement is always executed')	#Prints

with alternative_execution(True):
	print('This statement is never executed')	#Does not print

print('This is the end')