When calling a function without parametter the context is preserved and can be used

@api.multi
def button_print(self):
    _logger.info('Context : %s: ', self.env.context)

Calling this function from a button we get a filled context dictionnary.

But calling the same function with a  defined parametter we get an empty context {}

@api.multi
def button_print(self, demo=False):
    _logger.info('Context : %s: ', self.env.context)