Moved

Moved. See https://slott56.github.io. All new content goes to the new site. This is a legacy, and will likely be dropped five years after the last post in Jan 2023.

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.

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.