Making a mockery of open!

somecode.py

def test(filename, message):
	with open(filename, 'w') as f:
		f.write(message)

sometest.py

#!/usr/bin/env python

from somecode import func

if __name__ == '__main__':
	from cStringIO import StringIO
	from contextlib import contextmanager
	buf = StringIO()
	def open(*args, **kwargs):
		@contextmanager
		def wrapper():
			try: yield buf
			finally: pass
		return wrapper()
	
	# mock it	
	import somecode
	somecode.open = open
	
	# run it
	filename = 'test'
	message = 'hello!'
	func(filename, message)
	assert buf.getvalue() == message