Exocet makes code reloading easy

Wednesday, July 20 2011

One of the big philosophical "pillars" I’ve been building my tinker Python MUD on is that cold restarts (restart the process, clients are disconnected) should be exceedingly rare. Code re-loading in Python can be challenging, but Allen Short has nailed it with his Exocet module.

The coolest part for me (as it pertains to my game) is as follows:

"Exocet is a new way to load Python modules. It separates the act of naming a dependency from the act of creating a module object. As a result, more than one instance of a module can be created from the same source file."

As a result of this, we can deal with Python modules as objects, and can easily replace them. This is how it ends up looking for me (lots of things removed for the sake of brevity):

import exocet
# Load the general commands module, stuff it in a variable.
general_cmds = exocet.loadNamed(
   'src.game.commands.general',
   exocet.pep302Mapper
)
# Add a command to the command table.
self.add_command(general_cmds.CmdLook())

This is an excerpt from my game’s global command table. The general command module is loaded by exocet instead of Python’s regular import statement. Since we’ve tossed the module reference in a variable, the normal rules of Python garbage collection apply.

While my game does this a little differently, running the line that populates general_cmds again would mean that the old general command module’s reference count would drop to zero, hence it would be garbage collected. In its place, you have the newly loaded general commands module with any new code updates. Here is all I’d need to do to re-load the general commands module after some modifications:

import exocet
# Re-load the general commands module, replacing the old one.    general_cmds = exocet.loadNamed(
   'src.game.commands.general',
   exocet.pep302Mapper
)

Some disclaimers

Of course there had to be a catch, right? I’m still learning how to best use this, but here are a few pointers:

  • Your Exocet-based imports will probably always look strange to you, because they are. However, the power you get from them is well worth the "different" look to them. This is a pretty silly point, but I am pretty silly about code cosmetics.
  • Be careful about storing references to exocet-loaded modules. If you reference an exocet module from other modules/classes, you may find yourself in a situation where the old module’s reference count never drops to zero, and it isn’t garbage collected. I get around this by using properties to replace things that were previous instance attributes. I probably did a bad job explaining this, sorry!
  • Exocet is still in development. It is only available on Launchpad (sadface). However, some pip+bzr magic can make that less of a problem.
  • Documentation is limited to a series of blog posts by the author. They are pretty useful, but you’ll have to wade through them if you’re looking for something.


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