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Using the ContractsMeta meta-class

The ContractsMeta meta-class can be used as a drop-in replacement for ABCMeta. It allows you to declare contracts for a superclass and then have those contracts automatically enforced for any class that derives from it.

For example, let us define a “timer” interface whose start method requires a positive number:

from contracts import ContractsMeta, contract
from abc import abstractmethod

class TimerInterface():
    __metaclass__ = ContractsMeta

    @abstractmethod
    @contract(interval='(float|int),>0')
    def start(self, interval):
        pass

Now we can subclass TimerInterface and all contracts will be automatically inherited:

class Timer(TimerInterface):

    def start(self, interval):
        time.sleep()


t = Timer()
t.start(-1) # raises ContractNotRespected

# contracts.interface.ContractNotRespected: Breach for argument 'interval' to Timer:start().
# Condition -1 >= 0 not respected
# checking: >=0               for value: Instance of int: -1
# checking: (float|int),>=0   for value: Instance of int: -1

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