Thursday, March 13, 2014

The Visitor Design Pattern and Python

Epiphany.

In Python, with iterators, the Visitor design pattern is useless. And a strongly-ingrained habit. Which I'm trying to break.

Here's a common Visitor approach:

class Visitor:
    def __init__( self ): ...
    def visit( self, some_target_thing ): ...
    def all_done( self ): ...

v = Visitor()
for thing in some_iterator():
    v.visit(thing)
v.all_done()

If we refactor the for statement into the Visitor, then it's just a Command or something.

Here's the refactored Iterating Visitor:

class Command:
    def __init__( self ): ...
    def process_all( self, iterable ):
        for thing in iterable:
            self.visit( thing )
    def visit( self, thing ): ...
    def all_done( self ): ...

c=Command()
c.process_all( some_iterator() )
c.all_done()

Possible Objection

The one possible objection is this: "What if our data structure is so hellishly complex that we can't reduce it to a simple iterator?"

That's perfectly silly. Any hyper-complex algorithm to walk any hyper-complex data structure, no matter how hyper complex, can always be recast into a generator function which uses yield to iterate over the objects.

Better Design

Once we start down this road, we can generally simplify processing into a kind of Command that looks something like this.


class Command:
    def __init__( self ): ...
    def run( self ): 
        for thing in self.iterable:
            ....

c= Command()
c.iterable= some_iterator()
c.run()


I find that this interface is somewhat easier to deal with when composing large commands from individual small commands. It follows a Create-Configure-Run pattern that seems to work out well. I just wish I would start with this rather than start with a Visitor, refactor, and end up with this.