Unit tests for the nltk.utilities module
overridden()
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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
abstract base class, in such a way that subclasses don't have to
implement all of the methods:
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>>> class EaterI(object):
... '''Subclass must define eat() or batch_eat().'''
... def eat(self, food):
... if overridden(self.batch_eat):
... return self.batch_eat([food])[0]
... else:
... raise NotImplementedError()
... def batch_eat(self, foods):
... 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
perform the other method:
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>>> class GoodEater1(EaterI):
... def eat(self, food):
... return 'yum'
>>> GoodEater1().eat('steak')
'yum'
>>> GoodEater1().batch_eat(['steak', 'peas'])
['yum', 'yum']
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>>> class GoodEater2(EaterI):
... def batch_eat(self, foods):
... return ['yum' for food in foods]
>>> GoodEater2().eat('steak')
'yum'
>>> GoodEater2().batch_eat(['steak', 'peas'])
['yum', 'yum']
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But if a subclass doesn't implement either one, then they'll get an
error when they try to call them. (nb this is better than infinite
recursion):
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>>> class BadEater1(EaterI):
... pass
>>> BadEater1().eat('steak')
Traceback (most recent call last):
. . .
NotImplementedError
>>> BadEater1().batch_eat(['steak', 'peas'])
Traceback (most recent call last):
. . .
NotImplementedError
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Trying to use the abstract base class itself will also result in an
error:
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>>> class EaterI(EaterI):
... pass
>>> EaterI().eat('steak')
Traceback (most recent call last):
. . .
NotImplementedError
>>> EaterI().batch_eat(['steak', 'peas'])
Traceback (most recent call last):
. . .
NotImplementedError
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It's ok to use intermediate abstract classes:
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>>> class AbstractEater(EaterI):
... pass
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>>> class GoodEater3(AbstractEater):
... def eat(self, food):
... return 'yum'
...
>>> GoodEater3().eat('steak')
'yum'
>>> GoodEater3().batch_eat(['steak', 'peas'])
['yum', 'yum']
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>>> class GoodEater4(AbstractEater):
... def batch_eat(self, foods):
... return ['yum' for food in foods]
>>> GoodEater4().eat('steak')
'yum'
>>> GoodEater4().batch_eat(['steak', 'peas'])
['yum', 'yum']
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>>> class BadEater2(AbstractEater):
... pass
>>> BadEater2().eat('steak')
Traceback (most recent call last):
. . .
NotImplementedError
>>> BadEater2().batch_eat(['steak', 'peas'])
Traceback (most recent call last):
. . .
NotImplementedError
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Here's some extra tests:
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>>> class A(object):
... def f(x): pass
>>> class B(A):
... def f(x): pass
>>> class C(A): pass
>>> class D(B): pass
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>>> overridden(A().f)
False
>>> overridden(B().f)
True
>>> overridden(C().f)
False
>>> overridden(D().f)
True
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overridden() can be called on unbound methods as well:
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>>> overridden(A.f)
False
>>> overridden(B.f)
True
>>> overridden(C.f)
False
>>> overridden(D.f)
True
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It works for classic classes, too:
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>>> class A:
... def f(x): pass
>>> class B(A):
... def f(x): pass
>>> class C(A): pass
>>> class D(B): pass
>>> overridden(A().f)
False
>>> overridden(B().f)
True
>>> overridden(C().f)
False
>>> overridden(D().f)
True
>>> overridden(A.f)
False
>>> overridden(B.f)
True
>>> overridden(C.f)
False
>>> overridden(D.f)
True
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