Lecture 21

Dispatch tables

MCS 260 Fall 2021
Emily Dumas


  • Project 2 solution will be posted Friday
  • Project 3 to be announced next week; due Nov 5
  • Homework 7 due tomorrow

Common situation

Chain of of/elif/elif/else checking the same variable, taking a different action for each possible value.

        if s == "exit":
        elif s == "help":
        elif s == "next":
            x = f(x)

If we put the body of each if/elif into a function, this would look like:

        if s == "exit":
        elif s == "help":
        elif s == "next":

This is ok, but the similarity of all the elif blocks is suspicious. Is there a shorter way?

We can reduce duplication by storing the mapping from values to functions in a dict.

        handlers = {
            "exit": exit,
            "help": do_help,
            "next": do_next
        if s in handlers:
            handlers[s]() # replaces all the if/elif bodies

The dictionary handlers is an example of a dispatch table.

Dispatch tables

A mapping from values to actions, so looking up the value associated to a key and calling it replaces a long chain of if/elif.


  • Possible actions are stored in an actual data structure, rather than implicitly described by code.
  • Make introspection possible (program can list, examing, modify the table)
  • Late extensibility: Program doesn't necessarily need to know the entire table when it starts!


Let's refactor our mini-terminal to perform each command through a function, and to use a dispatch table to decide which one to call.


We covered dispatch tables in detail because it provided a way to demonstrate important ideas from Lecture 20 (functions are values, variadic functions, argument unpacking) in a realistic example. Dispatch tables are not covered directly in any of the optional texts.

When we discuss object-oriented programming, we'll revisit this idea.

Revision history

  • 2021-10-10 Initial publication