Safe function cannot raise any exceptions. Alias for raises without arguments.
def divide(a, b):
    return a / b

divide(1, 2)
# 0.5

divide(1, 0)
# ZeroDivisionError: division by zero
# The above exception was the direct cause of the following exception:
# RaisesContractError:


Can you be sure that some function never raises exceptions? Make promise about this, and will help you to control it.