Python "good enough error handling"

utils.py

#
# ErrorMonad and its fmap, bind and m_pipe
# inspired by
# http://www.slideshare.net/eldariof/monads-in-python
#

ErrorMonad = namedtuple('ErrorMonad', ['error', 'value'])


def fmap(func):
    """
    fmap wraps func in a function so that:
    - it receives ErrorMonad's value as a parameter
    - its output set as a new ErrorMonad's value
    - it is skipped entirely in case of error
    """

    def fmap_func(error_monad):
        if error_monad.error:
            return ErrorMonad(error=error_monad.error, value=None)

        try:
            return ErrorMonad(error=None, value=func(error_monad.value))
        except Exception as e:
            return ErrorMonad(error=e, value=None)

    return fmap_func


def bind(error_monad, func):
    """
    Pass the error_monad to an fmap wrapped func if the error_monad has a value
    otherwise skip execution of func
    """
    return fmap(func)(error_monad) if error_monad.value is not None else error_monad


def m_pipe(value, *fns) -> ErrorMonad:
    """
    Pass the first value to the first function and its output to the next until
    the last function.

    Returns the output of the last functions as an ErrorMonad (err, val) tuple.

    Similar to the thread macro in Lisps: (-> param1 param2 param3 ...)
    """
    error_monad = ErrorMonad(value=value, error=None)
    for f in fns:
        error_monad = bind(error_monad, f)
    return error_monad