也许Python中的"善良"monad

Question

试图找到一种方法来清理我的一些代码.

所以,我的Python代码中有这样的东西:

company = None
country = None

person = Person.find(id=12345)
if person is not None: # found        
    company = Company.find(person.companyId)

    if company is not None:
         country = Country.find(company.countryId)

return (person, company, country)

在阅读了关于Haskell的monad(特别是Maybe)的教程后,我想知道是否可以用另一种方式编写它.

Answer 1

company = country = None
try:
    person  =  Person.find(id=12345)
    company = Company.find(person.companyId)
    country = Country.find(company.countryId)
except AttributeError:
    pass # `person` or `company` might be None

EAFP

Answer 2

利用短路行为,默认情况下自定义对象为true,None为false:

person  = Person.find(id=12345)
company = person and person.company
country = company and company.country

Answer 3

Python对monad没有特别好的语法.话虽如此,如果你想限制自己使用像Maybemonad 这样的东西(意思是你只能使用Maybe;你将无法制作处理任何monad的泛型函数),你可以使用以下做法:

class Maybe():
    def andThen(self, action): # equivalent to Haskell's >>=
        if self.__class__ == _Maybe__Nothing:
            return Nothing
        elif self.__class__ == Just:
            return action(self.value)

    def followedBy(self, action): # equivalent to Haskell's >>
        return self.andThen(lambda _: action)

class _Maybe__Nothing(Maybe):
    def __repr__(self):
        return "Nothing"

Nothing = _Maybe__Nothing()

class Just(Maybe):
    def __init__(self, v):
        self.value = v
    def __repr__(self):
        return "Just(%r)" % self.value

然后,使当前返回的所有方法None返回Just(value)或Nothing替代.这允许您编写此代码:

Person.find(id=12345).andThen(lambda person: Company.find(person.companyId)).andThen(lambda company: Country.find(company.countryId))

您当然可以调整lambdas以将中间结果存储在变量中; 这取决于你如何正确地做到这一点.

Answer 4

我认为这是一个完美的情况getattr(object, name[, default])：

person  = Person.find(id=12345)
company = getattr(person, 'company', None)
country = getattr(company, 'country', None)

Answer 5

您检查了PyMonad吗？

https://pypi.python.org/pypi/PyMonad/

它不仅包括Maybe monad，还包括列表monad，Functor和Applicative functor类。Monoids等。

在您的情况下，它将类似于：

country = Person.find(id=12345)          >> (lambda person: 
          Company.find(person.companyId) >> (lambda company: 
          Country.find(company.countryId))

比EAFP更容易理解和清洁。