Python Optional

optional.py

"""
Optional.
"""

from __future__ import print_function

_novalue = object()



class NoValuePresentError(ValueError):
    """
    No value present in optional.
    """



class Optional(object):
    """
    Object that either contains a value or is empty.
    """

    def __init__(self, value=_novalue):
        """
        Initialize an optional with the given value.  If no value is given, the
        optional is empty.
        """
        self._value = value


    def __repr__(self):
        if self._value is _novalue:
            value = ""
        else:
            value = repr(self._value)

        return "{}({})".format(self.__class__.__name__, value)


    def __eq__(self, other):
        if isinstance(other, Optional):
            self._value == other._value

        return False


    def __hash__(self):
        return hash(self._value)


    def present(self):
        """
        Return L{True} if this optional contains a value, L{False} otherwise.
        """
        return self._value is not _novalue


    def value(self, default=_novalue):
        """
        Return the contained value, if present.  If no value is present, return
        C{default} if provided by the caller, otherwise raise
        L{NoValuePresentError}.
        """
        if self._value is _novalue:
            if default is _novalue:
                raise NoValuePresentError()
            else:
                return default
        else:
            return self._value


    def valueElse(self, f, *args, **kwargs):
        """
        Return the contained value, if present.  If no value is present, return
        the result of calling C{f} with the given C{args} and C{kwargs}.
        """
        if self._value is _novalue:
            return f(*args, **kwargs)
        else:
            return self._value



if __name__ == "__main__":
    empty = Optional()
    none  = Optional(None)
    foo   = Optional("foo")

    for optional in (empty, none, foo):
        print(optional)
        print("present:", optional.present())
        try:
            print("value:", optional.value())
        except:
            print("value: (raised exception)")
        print("value default:", optional.value("(default)"))
        print("value else:", optional.valueElse(lambda: "(else)"))
        print()