utils.py

class Try(object):
    """
    Encapsulate an operation potentially throwing an error. A try
    instance is `True` if no error was thrown, it's `False` if an
    error occured.
    """
    def __init__(self, fn):
        self.fn = fn
        self()

    @property
    def get(self):
        if self.is_success():
            return self.result
        raise self.error

    def is_success(self):
        return bool(self)

    def is_failure(self):
        return not bool(self)

    def __call__(self):
        try:
            self.result = self.fn()
        except Exception as e:
            self.error = e

    def __nonzero__(self):
        if hasattr(self, 'result'):
            return True
        else:
            return False

    def raising(self, fn = lambda err: err):
        """
        Return this try or raise `fn` applied to this try's exception.

        :: 

            soup = Try(lambda: Soup(self.get('file').read(), 'xml')).raising(
                       lambda err: forms.ValidationError("Please upload a valid XML file")).get

        """
        if self.is_failure(): raise fn(self.error)
        else: return self

    def failing(self, fn = lambda err: err):
        """
        Return this try or a new try with `fn` applied to this try's
        exception.

        ::

            Try(lambda: self.process(soup.find('Synopsis').text))).failing(
                lambda err: self.log(err))

        """
        if self.is_failure(): Try(lambda: fn(self.error))
        else: return self

    def get_or_else(self, fn):
        """
        Return the result of this try or in case of failure the result of
        `fn`

        ::

            user = Try(lambda: soup.find('User')['id'])).get_or_else(
                       lambda: None)

        """
        if self.is_success(): return self.result 
        else: return fn()

    def or_else(self, fn):
        """
        Return this try or in case of failure a new try wrapped around `fn`

        ::

            Try(lambda: self.set('category', soup.find('Category').text)).or_else(
                lambda: self.set('category', self.DEFAULT_CATEGORY))
        
        """
        if self.is_failure(): Try(fn)
        else: return self