Methon chaining inspired by $q / Q in Javascript

q_chain.py

# Inspired by $q/Q in javascript to chain methods
class Q:
    def __init__(self, d):
        self.d = d
        self.error = None

    def then(self, fn):
        if not self.error:
            try:
                self.d = fn(self.d)
            except Exception as e:
                self.error = e

        return self

    def catch(self, fn):
        if self.error:
            fn(self.error)

        return self

    def done(self):
        return self.d

if __name__=='__main__':

    input = 123
    def add(d):
        d+=999
        return d

    def insert(d):
        d+=102
        raise Exception('hello')
        return d

    def abc(e):
        print ('error', e)


    print(Q(input).then(add).then(insert).catch(abc).then(add).done())