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141 lines
3.6 KiB
Plaintext
141 lines
3.6 KiB
Plaintext
.. Copyright (C) 2001-2019 NLTK Project
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.. For license information, see LICENSE.TXT
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==========================================
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Unit tests for the nltk.utilities module
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==========================================
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overridden()
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~~~~~~~~~~~~
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>>> from nltk.internals import overridden
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The typical use case is in defining methods for an interface or
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abstract base class, in such a way that subclasses don't have to
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implement all of the methods:
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>>> class EaterI(object):
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... '''Subclass must define eat() or batch_eat().'''
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... def eat(self, food):
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... if overridden(self.batch_eat):
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... return self.batch_eat([food])[0]
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... else:
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... raise NotImplementedError()
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... def batch_eat(self, foods):
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... return [self.eat(food) for food in foods]
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As long as a subclass implements one method, it will be used to
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perform the other method:
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>>> class GoodEater1(EaterI):
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... def eat(self, food):
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... return 'yum'
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>>> GoodEater1().eat('steak')
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'yum'
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>>> GoodEater1().batch_eat(['steak', 'peas'])
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['yum', 'yum']
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>>> class GoodEater2(EaterI):
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... def batch_eat(self, foods):
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... return ['yum' for food in foods]
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>>> GoodEater2().eat('steak')
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'yum'
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>>> GoodEater2().batch_eat(['steak', 'peas'])
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['yum', 'yum']
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But if a subclass doesn't implement either one, then they'll get an
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error when they try to call them. (nb this is better than infinite
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recursion):
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>>> class BadEater1(EaterI):
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... pass
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>>> BadEater1().eat('steak')
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Traceback (most recent call last):
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. . .
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NotImplementedError
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>>> BadEater1().batch_eat(['steak', 'peas'])
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Traceback (most recent call last):
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. . .
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NotImplementedError
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Trying to use the abstract base class itself will also result in an
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error:
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>>> class EaterI(EaterI):
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... pass
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>>> EaterI().eat('steak')
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Traceback (most recent call last):
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. . .
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NotImplementedError
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>>> EaterI().batch_eat(['steak', 'peas'])
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Traceback (most recent call last):
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. . .
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NotImplementedError
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It's ok to use intermediate abstract classes:
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>>> class AbstractEater(EaterI):
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... pass
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>>> class GoodEater3(AbstractEater):
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... def eat(self, food):
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... return 'yum'
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...
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>>> GoodEater3().eat('steak')
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'yum'
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>>> GoodEater3().batch_eat(['steak', 'peas'])
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['yum', 'yum']
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>>> class GoodEater4(AbstractEater):
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... def batch_eat(self, foods):
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... return ['yum' for food in foods]
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>>> GoodEater4().eat('steak')
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'yum'
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>>> GoodEater4().batch_eat(['steak', 'peas'])
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['yum', 'yum']
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>>> class BadEater2(AbstractEater):
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... pass
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>>> BadEater2().eat('steak')
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Traceback (most recent call last):
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. . .
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NotImplementedError
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>>> BadEater2().batch_eat(['steak', 'peas'])
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Traceback (most recent call last):
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. . .
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NotImplementedError
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Here's some extra tests:
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>>> class A(object):
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... def f(x): pass
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>>> class B(A):
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... def f(x): pass
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>>> class C(A): pass
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>>> class D(B): pass
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>>> overridden(A().f)
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False
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>>> overridden(B().f)
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True
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>>> overridden(C().f)
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False
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>>> overridden(D().f)
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True
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It works for classic classes, too:
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>>> class A:
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... def f(x): pass
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>>> class B(A):
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... def f(x): pass
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>>> class C(A): pass
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>>> class D(B): pass
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>>> overridden(A().f)
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False
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>>> overridden(B().f)
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True
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>>> overridden(C().f)
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False
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>>> overridden(D().f)
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True
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